CHAPTER 1
INTRODUCTION
TO RESEARCH QUESTION
Introduction
The
use of fire in human prehistory has undoubtedly had a profound influence on the
evolutionary success of humans. Based on
archaeological evidence, the earliest use of fire by hominids has been suggested
to extent over one million years into prehistory, perhaps longer (Brain and
Sillen 1988; see also Barbetti 1986; James 1989). Throughout prehistory, fire has served a
variety of uses including, heating, cooking, heat treatment of chert, metallurgy,
cremation, hunting, defense from predators, warfare, and land management. However, the signature of fire in the
archaeological record may in some instances be confounded due to the occurrence
of natural fires. Conner and Cannon
(1991) have shown that burned logs from forest fire contexts can generate soil
profiles that roughly approximate basin shaped hearths commonly recorded by
archaeologists. Bellomo (1991; 1993; see
also Bellomo and Harris 1990) has demonstrated that archaeologically derived
hearths may be differentiated from more anomalous feature-like manifestations
produced by natural fire via the documentation of basin-shaped configurations
of oxidized sediment, and the use of magnetic and archaeomagnetic analyses.
Unfortunately,
extensive laboratory analysis of sediments associated with hearth features is
generally not within the research parameters of most archaeological
investigations. Nonetheless, it is of
primary importance that archaeologists are able to differentiate between
transformations in the archaeological record generated by natural fire and
those produced by human activity. The
accurate analysis of the fire-related archaeological phenomena is not limited
to the interpretation of hearth features alone.
For example, does the occurrence of burned bone from an archaeological
context necessarily suggest evidence of cooking; is burned human bone an
indication of cremation, warfare, or cannibalism; does thermally altered chert
suggest intentional heat treatment; do burned architectural features
necessarily suggest intentional abandonment or warfare? Perhaps not, this is the impetus underlying
the effort to understand the transformations affected on the archaeological
record by natural fire.
Natural
fire has far-reaching implications for the analysis and preservation of
cultural materials deposited in the archaeological record. As such, investigating the impact of natural
fire on archaeological materials has important implications for the interpretation
of the archaeological record in general as well as the more specific
preservation issues important to cultural resource management. The impact of wildland fire on the
archaeological record is an important but often-overlooked site formation process. Schiffer (1987) makes little reference to the
role of fire as a natural or cultural site formation process. Schiffer briefly addresses fire-related
issues such as refuse burning, fire and site abandonment, charring as a wood
preservative, and burned structures as a source of variability in the overall
deterioration process of structures.
Interestingly, wildland fire is entirely absent from his discussion of
regional environmental site formation processes. Research conducted by Conner et al. (1989)
and Conner and Cannon (1991) represent one of the few instances in which
archaeologists have explicitly addressed the potential implications of natural
fire as a site formation process.
Wildland
fire is a ubiquitous phenomenon affecting landscapes on a global scale. Prior to 20th Century fire
suppression activities in the Western United States, major wildland fires are
estimated to have occurred every 5-10 years in grasslands (Collins 1990); 5-20
years in Sierra Nevadan forests (Skinner and Chang 1996); 10-30 years in
ponderosa pine communities (Brown and Sieg 1996; Veblen et al. 2000); 20-150
years in the Northern Rocky Mountains (Arno 1980); 25-100 years in the Pacific
Northwest (Agee 1990); 100 years in southwestern oak shrubland (Floyd et al.
2000); 300-400 years in southwestern piñon-juniper communities (Floyd et al.
2000); and 300-400 years in subalpine forests of the Yellowstone Plateau (Romme
1982). In addition to the common
occurrence of wildland fire in
Project
Purpose and Research Questions
The
general purpose of the present dissertation project is to investigate the
impact of wildland and prescribed fire on archaeological resources. This project was funded by the Canon National
Parks Science Scholars Program, a cooperative program between the Canon Corporation,
National Park Service, and the American Association for the Advancement of
Science. This program has an expressed
interest in supporting research relevant to the interests of the National Park
Service. The potential effects of
prescribed burn programs and wildland fires on cultural resources within the
parks-system have been an enduring concern for several years. As a result, a research design explicitly
focused on addressing this topic was developed and implemented over the course
of the project. The goals of the project
are to identify the conditions under which selected artifact classes are
significantly affected under prescribed and wildland fire conditions, and to
assess the more long-term site formation process concerns related to burning of
archaeological sites during wildland fires.
To
address these issues, the overall project into three major research components:
1) Field-based Prescribed Burn Experiments
Field
experiments were performed in conjunction with prescribed burn programs at several
National Parks as well as Forest Service and Bureau of Land Management
land. The experiments encompassed a
variety of fuel types common to the
2) Laboratory Muffle Furnace Heating Trials and Laboratory Wildland
Fire
Simulation
The
laboratory muffle furnace heating trials were conducted in order to document
the temperature ranges at which specific forms of thermal alteration affect
common archaeological material types.
During the experiment, artifacts representing a wide range of common
material types, were heating during trials ranging from 100-1000°C. The experimental design and results of this
experiment are presented in Chapter 3.
The
wildland fire simulations were conducted at the USDA Intermountain Fire
Sciences Laboratory,
3) Fire Effects Sampling of Burned sites at
During
this phase of the project, a sampling strategy was developed to assess the
immediate and long-term effect of wildland fire on Ancestral Pueblo sites at
Previous
Research
The
majority of published literature surrounding the effects of fire on
archaeological resources originates from research that has been conducted in
the form of post-facto observations of the aftermath of wildland fires (Conner
et al. 1989; Conner and Cannon 1991; Eininger 1990; Johnson et al. 1991; Jones
and Euler 1986; Lentz et al. 1996; Noxon and Marcus 1983; Racine and Racine
1979; Romme et al. 1993; Switzer 1974; Traylor et al. 1990; Wettstaed
1992). In general, these studies consist
of immediate post-fire inventories of affected archaeological sites that range
in scope from general observation to comprehensive documentation of fire
effects. Unfortunately, the majority of
existing research has been limited to general post-fire observations that lack
any systematic methodology to specifically document fire effects at the site or
artifact level. Lentz et al. (1996) and
Traylor et al. (1990) provide the most in-depth research on the subject of
wildland fire and its impact on archaeological resources, and Conner et al.
(1989) and Conner and Canon (1991) effectively place wildland fire within the
conceptual framework of a regional site formation process. These studies are briefly summarized
below.
The
Traylor et al. (1990) investigation of the 1977 La Mesa Fire at Bandelier
National Monument is an important contribution to the study of fire effects on
archaeological resources due to the extensiveness of work conducted, which
included post-fire inventory, excavation and sampling of four burned sites, as
well as dendrochronlogy, ethnobotanical, palynological, soil, obsidian
hydration, thermoluminescence, archaeomagnetic, and radiocarbon analyses of
materials from sampled sites. In brief,
the results of the study showed that the fire impact at selected sites was variable
and largely dependent on fuel type, fuel load, and fire intensity. Significant thermal alteration of
archaeological materials was generally limited to surface contexts at severely
burned sites. Surface specimens from
light to moderately burned sites were minimally affected, and materials from
all subsurface contexts were unaffected regardless of fire severity.
The
most prevalent fire effect observed by during the project was thermal spalling,
cracking, and increased friability of tuff masonry elements associated with
architectural features, particularly in severely burned areas. Ceramic materials from surface contexts
exhibited combustive residue deposits, limited oxidation/color alteration of
slips, possible thermal spalling of surface paint and slips, and possible
increased friability of utilitarian sherds.
Thermal alteration of lithic was limited to combustive residue deposits,
no thermal fracturing was observed.
Overall, significant thermal alteration of artifacts was not pervasive
and was limited to surface specimens from severely burned sites. Ethnobotanical and palynological analyses
conducted during the study indicated that subsurface heating generated by the
fire was insufficient to affect pollen and plant remains from subsurface archeological
contexts (Ford 1990; Scott 1990).
Similarly, the fire did not affect archeomagnetic dates from selected
subsurface hearth features, nor did it affect tree ring samples from selected
sites within the study area (DuBois 1990; Robinson 1990). Results of the radiocarbon analysis were reported
as inconclusive due to the lack of adequate control samples; however,
theoretically contamination of modern charcoal with archaeological deposits has
the potential to produce erroneous dates (Stehli 1990). Analysis of surface basalt and ceramic artifacts
from surface context show that thermoluminescence dating may produce
erroneously young dates; however, subsurface materials are unlikely to be
affected (Rowlett and Johannessen 1990).
Similarly, obsidian hydration analysis of obsidian artifacts from
surface contexts showed a high prevalence of damaged hydration bands, whereas
subsurface specimens were generally unaffected.
Lentz et al. (1996) conducted a similar
investigation during a post-fire assessment of the 1991 Henry Fire that impacted
several archaeological sites located in the
The
results of the study showed that the degree of thermal alteration observed was
variable at intra-site and inter-site levels and largely dependent on the fuel
load and burn severity encompassing each site.
In addition, subsurface materials were generally unaffected by the
penetration of heat energy into the mineral soil, with the exception of a few
instances where downed logs had burned for an extended period. These observations is consistent with that
reported by Traylor et al. (1990), and further illustrate the important
relationship between fuel load and fire severity as they condition the
potential for significant thermal alteration of archaeological resources.
Significant
thermal alteration of tuff architectural elements included thermal spalling,
fracturing, and increased friability, the extent of which was directly related
to fire severity. Artifact classes
collected during sampling included ceramics (black-on-white, utility), lithic
(chert, rhyolite quartzite and obsidian), and ground-stone. Post-fire analyses of these materials showed
that combustive residue deposition was the most pervasive fire effect
observed. Thermal spalling, oxidation,
and pigment alteration of ceramics was observed at low frequencies, lithics
exhibited low incidences of potlid fracturing and crazing, and thermal
alteration of ground-stone specimens was generally limited to combustive
residue deposition. Obsidian hydration
analysis of 10 specimens showed that the fire did damage hydration rinds;
however, extent of damage could not be specifically determined due to the lack
of pre-burn comparative samples (Origer 1996).
Regarding
the thermal alteration of archaeological materials in general, the researchers
illuminate the importance of making the distinction between “fire effect” and
“damage.” Wildland fires will
undoubtedly generate fire effects that impact archaeological resources;
however, the extent to which materials sustain significant structural damage is
variable. Combustive residue deposits,
defined by Lentz et al. (1996:48) as “sooting” (“carbonized particles clinging
to the surface of the item”) and “adhesions” (“sticky black substance of
unknown origin”) can be referred to as a fire effect that does not necessarily
damage archaeological resources. What
the authors refer to, as “adhesions” consists of a highly nitrogenous condensate
tar that forms on cooler surfaces (i.e., archaeological materials) below
combusting organic fuels (Yokelson et al. 1997). “Sooting” is most likely the byproduct of the
pyrolysis and combustion of organic fuels, referred to as “char” by DeBano et
al. (1998:23), a substance that is neither an intact organic compound nor pure
carbon. The combustive residue deposits
should be considered a fire effect; however, they do not generally constitute
damage of archaeological resources subjected to wildland or prescribed fire
conditions. These deposits do weather
from the surfaces of artifacts with time (see Chapter 4). Conversely, significant forms of thermal
alteration such as thermal spalling, thermal fracturing, and heat-induced
deterioration among various artifact classes does, with certainty, constitute
permanent damage since the structural integrity of affected materials has been
altered or destroyed indefinently.
Clearly, wildland fire can impact archaeological resources to the extent
that the interpretive value of the archaeological record has been significantly
altered.
Other
relevant work on the subject of wildland fire and archaeological resources
includes research conducted by Conner et al. (1989) and Conner and Canon (1991)
in the aftermath of the 1988 fires at
Regarding
the impact of prescribed burning on archaeological resources, several
researchers have performed experiments in conjunction with prescribed fires to
assess the immediate effect of burning on archaeological resources (Benson
2002; Brunswig et al. 1995; Deal and McLemore 2002; Green et al. 1997; Halford
and Halford 2002; Hanson 2001; Kelly and Mayberry 1980; Sayler et al. 1989 (see
also Picha et al. 1991); Smith 2002; Solomon 2002). However, the majority of this research has
been performed with the purpose of assessing the impact of prescribed fire on
obsidian, particularly as it affects obsidian hydration rinds. With the exception of Brunswig et al. 1995
and Sayler et al. 1989, few researchers have adequately assessed the impact of
prescribed fire on a wide range of common archaeological materials, and none
have done so within the contexts of variable fuel types. Brief summaries of each of the studies cited
above are provided in Chapter 2.
Overall, the combined body of literature on the subject offers important
information on the impact of fire on archaeological resources; however, more
extensive and systematic research needs to be conducted to fully develop a base
from which to assess fire impact.
In
addition to the limited archaeological literature that addresses the impact of
wildland fire and prescribed fire on archaeological resources in general, some
researchers have specifically assessed the potential for thermal alteration of
obsidian and prehistoric ceramics with in the context of natural fire. Moreover, there exists additional ancillary
information on the thermal alteration of specific material types can also be
gleaned from additional sources. These
include sources from the archaeological literature that pertain to the thermal
alteration of chert, thermal alteration of bone, and ceramic studies. Brief summaries of research most relevant to
studying the impact of wildland and prescribed fire are provided in the
following sections.
Thermal Alteration of Chert
The majority of archaeological literature
surrounding the thermal alteration of chert is concerned with the intentional
heat treatment of lithic raw materials during prehistory. In attempting to replicate the production of
prehistoric lithic artifacts, Crabtree and
Purdy (1974) conducted the most
comprehensive study involving the thermal alteration of chert. When chert is heated to temperatures between
100-150˚C, free water is evaporated from pores and cracks (0.4-2.0% weight
loss) (Purdy 1974). At temperatures
between 350-500˚C the chemically bound water within the chert is driven
off, and sulfur and iron compounds begin to oxidize (Purdy 1974; Griffiths et
al. 1987; Shepard 1971). Schindler et al. (1982) have also observed that
goethite oxidizes to hematite at temperatures between 200-300˚C. At temperatures above 500˚C, carbon and
other non-silica materials begin to oxidize, decompose, dehydrate, and
potentially fuse (Shepard 1971). The
observable effects of thermal alteration of cherts include; color change,
increased luster, reduced tensile strength, fracturing (blocky/angular),
fracturing (potlid), and crazing (internal fracturing) (Ahler 1983; Griffiths
et al. 1987; Schindler et al. 1982; Purdy 1974). Depending on the variety of chert, the color
change associated with heating is the result of changes within its internal
mineral structure. In the instance of
thermal alteration of cherts from
The lustrous quality in chert results from
light being reflected from the surface of the material, and is largely
dependent on mineralogy and surface characteristics (Luedtke 1992). Thermally altered cherts tend to exhibit an
increased luster or gloss (on newly flaked surfaces after the material has been
heat treated). Three explanations for
increased luster with heating have been suggested; 1) Light is increasingly
reflected off of fractured quartz grains (Purdy and Brooks 1971); 2) An
increase in the number of fluid inclusions that reflect light occurs with
heating (Griffiths et al. 1987); and 3) Altered hematite crystals increase
light reflection (Schindler et al. 1982).
These explanations are specific to the particular raw materials and
experimental conditions used by each researcher; however, there is a general
consensus that lustrous qualities are related to a change in microcrystalline
structure and subsequent change in the refraction of light.
The reduction of tensile strength in lithic
materials is often attributed to heat treatment. Indeed, the heat treatment of particular
varieties of raw materials does enhance their workability (Bleed and Meier
1980; Crabtree and Bulter 1964; Rick 1978; Rick and Chappell 1983). The most widely excepted explanation for this
occurrence is that it heat treatment allows a fracture to propagate across the
microcrystalline quartz grains within the lithic material as opposed to around
them (Purdy and Brooks 1971; Purdy 1974).
In addition, heated cherts have a smoother fracture surface topography
under SEM magnification as compared to unheated cherts (Luedtke 1992). Two explanations for this have been
suggested; 1) Heating results in the fusion of silica leading to a denser
structure; 2) Heating results in cracking which in turn increases
fracturability (see Luedtke
1992:95-96). Regardless of a specific
explanation, the heating of chert does affect its tensile strength and fracturability. Researchers have also shown that it is
important to heat treat material slowly over a long period of time, and at
temperatures below certain critical thresholds, which can range from between
250-450˚C depending on the variety of lithic material (Ahler 1983;
Griffiths et al. 1987; Purdy 1974; Schindler et al. 1982). Researchers point out that if materials are
heated too rapidly, or above their critical maximum temperature, thermal shock
and fracturing will occur. These are
precisely the parameters that characterize heating of surface materials during
wildland and prescribed fire. That is,
surface heating during wildland or prescribed fires is not uniform, with the
potential for temperatures to rise and fall sharply depending on fire behavior
and fuel type/loading.
Fracturing of materials subjected to heat
is generally the result of thermal stress.
Thermal stress occurs when a portion of the material becomes
differentially warmer or colder than another resulting in an uneven rate of
contraction or expansion resulting in heat-induced fracturing (Luedtke
1992). Quartz has a high coefficient of
thermal expansion, experiencing a 3.76% expansion in volume when heated to
570˚C (Winkler 1973). Since chert
is composed primarily of microcrystalline quartz, it is susceptible to thermal
stress (Luedtke 1992). Heat induced
fracturing in lithics can take the form of large blocky/angular fragments,
potlid fracturing, or surface crazing.
Blocky and angular fracturing is often the
result of rapid heating in which the original piece of raw material explodes
into multiple fragments (Purdy 1974).
The release of pressure within lithics is related to presence of water
within the material’s internal structure.
Water is turned to gas at 100˚C; however, under pressure water will
remain in liquid form until its critical temperature (365˚C, the
temperature at which gas cannot be liquefied) is reached (Luedtke 1992;
Weymouth and Williamson 1951). If water
is present deep within the material when it is heated, it may be transformed to
steam as it approaches the critical temperature for water. Steam can produce internal pressure within
the material that is capable of generating an explosion and subsequent
shattering of the material (Luedtke 1992).
Potlid fracturing is attributed to the rapid heating and cooling of raw
materials (Ahler 1983). Potlid fractures
result from differential heating and pressure release probably due to steam
buildup in areas of the material that has impurities or high moisture
content. The fracture is characterized
by a circular pit on the surface of the specimen. Crazing is the result of internal fracturing
and takes the form of very fine non-linear cracks, similar to a spider web
pattern, on the surface of a specimen (Ahler 1983). Crazing also occurs as the result of
differential heating and pressure release.
These forms of thermal fracturing have been observed in the field during
post-fire inventories and prescribed fire experiments (Benson 2002; Lentz 1996;
Lentz et al. 1996; Rondeau 1995; Sayler et al. 1989).
Thermal Alteration
of Obsidian
As a volcanic glass, obsidian has
excellent flaking qualities requisite for tool manufacture, and as such, was
commonly utilized during prehistory. Archaeologists commonly use obsidian
hydration analysis to effectively date obsidian artifacts via quantification of
moisture uptake measured in hydration rinds that has occurred since artifacts
were flaked (see Anovitz et al. 1999; Friedman and Long 1976; Friedman and
Smith 1960; Friedman and Trembour 1983).
In addition, geochemical trace element analysis of obsidian artifacts is
often used to source the locations from which the raw material was initially
quarried (Glascock et al. 1999; Hatch et al. 1990; Hughes 1988; 1994). While it remains relatively clear that
thermal alteration of obsidian does not affect its geochemical composition
(Shackley and Dilian 2002); it is quite clear that exposure to heat does
negatively affect hydration rinds. The
findings of several researchers suggest that obsidian hydration bands begin to
be altered by heating at approximately 250°C, become significantly affected at
approximately 400°C, and may be completely destroyed at temperatures exceeding
700°C (Benson 2002; Deal and McLemore 2002; Green et al. 1997; Findlow and
Garrison 1982; Halford and Halford 2002; Mazer et al. 1991; Origer 1996;
Ridings 1991; Smith 2002; Solomon 2002; Steffen 2002; Trembour 1990). As such, diffuse or destroyed hydration rinds
render a specimen unsuitable for obsidian hydration analysis. Thermal alteration of obsidian and subsequent
hydration rind damage is likely to occur where obsidian artifacts are directly
exposed to prescribed and wildland fire conditions.
In addition, some researchers have
documented heat-induced morphological change of obsidian during laboratory
heating experimentation (Bennett and Kunzmann 1985; Nakazawa 2002; Steffen
2002; Trembour 1990). Most researchers
are in general agreement regarding types of morphological change associated
with the thermal alteration of obsidian.
Steffen (2002:163) provides the most comprehensive description of
various forms of thermal alteration observed for obsidian. These definitions
are summarized as follows:
Matte Finish: Surface dulling similar to
weathered or lusterless patina.
Surface Sheen: metallic-like surface luster,
cause uncertain, but may be due to organic carbon buildup and/or bubble
formation and shallow microscopic crazing.
Fine Crazing: Network of shallow cracks forming
closed polygons on fresh fractures and flaked surfaces, likely due to
differential thermal expansion/contraction.
Deep Surface Cracking: Shallow
crevices formed on artifact surfaces, observed in conjunction with deformation,
caused by expansion of surface crazing.
Vesiculation: Formation of interconnected
bubbles within obsidian specimen due to release of volatiles, specimen
metamorphism to a foam-like mass.
Fire Fracture (field observation): Rapid thermal
fracture of specimen (presumably due to differential thermal stress), may
initiate near an inclusion.
With the exception of vesiculation which
occurs at 800°C +, specific temperatures ranges are not provided for each of
the associated types of thermal alteration defined above. However, several of the researchers have
observed that appreciable thermal alteration of obsidian occurs at temperature
in the 450-550°C range. Overall,
research pertaining to the thermal alteration of obsidian suggests the exposure
to heat can significantly affect the morphological integrity of obsidian
artifacts as well as the potential to derive obsidian hydration dates from
affected specimens. As such the integrity of the archaeological record as it
pertains to obsidian artifacts may be significantly affected during prescribe
and wildland fires.
Thermal
Alteration of Bone
Bone consists of two major components, an organic
phase and an inorganic phase. The
organic phase (35% of dry bone mass) consists largely of protein, mostly in the
form of collagen (Posner and Belts 1975).
The inorganic phase is composed of mineral, mostly hydroxyapatite in
microcrystalline form (Ortner et al. 1972).
Thermal alteration of bone can significantly affect both components
through chemical reaction and physical transformations. Shipman et al. (1984) have
summarized the major observable changes that occur in bone when it is exposed
to heat: 1) change in bone color; 2) change in the microscopic morphology of
bone surfaces; 3) changes in the cystalline structure of bone; and 4) bone
shrinkage. The degree of change in bone
due to thermal alteration is dependent on the temperature at which bone is
exposed, the duration of exposure, position of bone in relation to heat source,
bone composition, and bone size (Brain 1993; Herrmann 1977; McCutcheon 1992; Nicholson
1993; Shipman et al. 1984; Sillen and Hoering 1993; Stiner et al. 1995; Von
Endt and Ortner 1984).
Based on controlled experimentation, thermal
alteration of bone can be generalized into three basic processes; water loss,
carbonate loss, and mineral sintering (Bonucci and Graziani 1975; Kizzely 1973;
Shipman et al. 1984). In general, there
exists a relationship between temperature and degree of thermal alteration as
they relate to each of the two major components of bone. At lower temperatures (~ 100-600oC),
the initial effect of thermal alteration is concentrated in the organic
phase. As temperature increases into the
600oC range the organic phase is effectively burned away, and as
temperatures approach 700oC and beyond the inorganic phase is affected
through the recystallization of hydroxyapatite and the eventual fusion of those
crystals.
Thermal alteration of the organic phase is generally
characterized by distinct alterations in bone color. Chemical reactions are accelerated two fold
for each 10oC rise in temperature at which bone is exposed (Von Endt and Ortner 1984). Heat-induced
color changes in bone are attributed to alterations in the chemical composition
of bone, the oxidation of organics, and thin films of carbon layers deposited
on bone mineral (Bonucci and Graziani 1975; Grupe and Hummel 1991; Shipman et
al. 1984). Based on experimental work
several authors have made observations concerning the relationship between
temperature and observed color change in bone (Brain 1981, 1993; McCutcheon
1992; Nicholson 1993; Richter 1986; Shipman et al. 1984). These studies indicate that the color of bone
changes progressively with increased temperature, and that the color of
thermally altered bone can provide a rough index of the temperature range that
bone reached as a result of exposure to heat.
Shipman et al. (1984) provide the most systematic and widely cited
assessment of heat-induce color alteration of bone. The researchers observed
that heat-induced color alteration can be divided into five stages: stage 1
(20- <285oC), neutral white/pale yellow; stage 2 (285- <525oC),
red brown, very dark grey-brown, neutral dark grey, and reddish yellow; stage 3
(525- <645oC), neutral black dominant with some medium blue and
reddish-yellow; stage 4 (645- <940oC), neutral white dominant
with some light blue-grey and light grey; stage 5 (940oC+), neutral
white with minimal medium grey and reddish-yellow. The range of colors reported in this study
and others reflects the differential combustion of the organic component within
a particular temperature range. Dark
colors, particularly black, are related to the carbonization of collagen. Carbonized bone with a blackened or charred
appearance is likely to have reached a temperature in the 250-550oC
range. Grey is associated with the final
stages of organic component combustion. At temperatures of 600oC and
beyond the organic component is completely burned away resulting in calcination
and a neutral white, chalky appearance.
Changes in
the microscopic morphology of bone surfaces, its crystalline structure, and
bone shrinkage due to thermal alteration are largely related to observable
changes in the mineral phase of bone.
Heat-induced change in the microscopic surface morphology of bone has
typically been observed using Scanning Electron Microscopy (SEM) (Bonucci and
Graziani 1975; McCutcheon 1992; Nicholson 1993, 1995; Shipman et al. 1984), and
standard light microscopy (Brain 1993; Herrmann 1977; Nicholson 1993; Richter
1986). Shipman et al. (1984) provide the most succinct summary temperature
dependent micro-morphological change: stage 1 stage 1 (20- <185o C),
normal bone texture observed, surface undulating but intact; stage 2 (185-
<285o C), surface increasingly irregular, tiny pore and fissures
present, but surface intact; stage 3 (285- <440o C), bone surface
becomes glassy and smooth, patterned cracking appears; stage 4 (440- <800o
C), bone surface highly particularized; stage 5 (800- <940o),
particles melt and form larger polygonal structures. The results of each of the studies cited
indicate that heat-induced change in the surface morphology of bone may be
summarized into three general temperature dependent processes; carbonization,
cracking, and eventual recrystallization.
The
conditions under which the recrystallization of hydroxyapatite occurs have been
investigated through the use of x-ray diffraction (XRD) (Bonucci and Gaziani
1975; McCutcheon 1992; Shipman et al. 1984).
These researchers have shown that heat causes hydroxyapatite crystals to
increase in size as temperature increases.
The major change in crystal size is observed between 525-645oC,
and at temperatures above 645oC larger hydroxyapatite crystals begin
to expand at the expense of the smaller crystals until there is eventual fusion
at temperatures above 800o C.
Heat-induced changes in hydroxyapatite crystal size can also affect
macro-level morphological change through bone shrinkage and deformation
(Buikstra and Swegle 1989; Herrmann 1977; Shipman et al. 1984). These studies suggest that observable changes
in metric values and overall morphology of thermally altered bone will occur at
temperatures above 700oC.
Overall,
archaeology’s interest in thermally altered bone encompasses a broad range of
archaeological phenomena from cremations to post-depositional processes. The most common issue related to the thermal
alteration of archaeological bone relates to the desire to differentiate
whether or not bone has been heated as a result of human intention. Experimental research directed at addressing
these issues can be divided into three general categories; controlled
laboratory experimentation, actualistic field research, or a combination of
both. Laboratory experiments are
generally characterized by making systematic observations of the effects of
differential heating on the various structural and biological components of
bone (Nicholson 1993, 1995; McCutcheon 1992; Richter 1986; Shipman et al. 1984;
Taylor et al. 1995). Field experiments
are also generally concerned with these questions, but the address them within
the context of the replicated prehistoric campfires (Bellomo 1991; Bellomo and
Harris 1990; Brain 1993; David 1990; De Graaff 1961; Gilchrist and Mytum 1986;
Nicholson 1995; Robins and Stock 1990; Spennemann and Colley 1989).
In addition
to an interest in the cultural thermal alteration of bone, archaeologists have
also become interested in studying the how burned bone and shell are affected
by taphonomic processes (Knight 1985; Pearce and Luff 1994; Nicholson 1992;
Robins and Stock 1990; Stiner et al. 1995).
The main topic of interest here generally surrounds the survivorship
potential of remains after thermal alteration.
Most researchers are interested in the potential for differential
destruction/survivorship between species and experimental conditions. In sum, these studies suggest that thermal
alteration does significantly affect bone strength. Reduced bone strength was shown to have
differential effect between species and skeletal element. Heat-induced change is an important
taphonomic process that archaeologists need to consider when making inferences
about site formation processes and human paleoecology.
Experimental
research aimed at addressing issues related to cremation of human remains have
generally been concerned with establishing the condition of bone prior to
cremation (dry or fresh), and accounting for the effect of shrinkage when
determining population estimates from cremated human remains (Baby 1954;
Binford 1963; Buikstra and Herrmann 1977; Krogman 1939; Merbs 1967; Mckinley
1983; Ramrakhiani and Datta 1980; Swegle 1989; Thurman and Willmore 1981). Through experimentation, these researchers
have shown that, when burned, dry bone will generally exhibit superficial checking
and deep longitudinal cracking, and fresh bone will exhibit deep checking, deep
transverse cracking, and warping.
While there
is a considerable body of literature surrounding the thermal alteration of bone
from cultural context, research concerning the post-depositional thermal alteration
of bone is more limited (e.g., Bellomo 1991; Bellomo and Harris 1990; Bennett
1999; David 1990; De Graaff 1961; Sayler et al. 1989). David (1990) has shown that bone burned
during an Australian bush fire (20-30 seconds flaming combustion, no temperature
data) became carbonized and showed superficial cracking and longitudinal
collapsing of the mid-shaft. Bellomo and
Harris 1991 and Sayler et al. 1989 have shown that grassfire will generate
minimal thermal alteration of surface bone generally consisting of carbonized
color alteration and minor charring.
Thermal alteration of subsurface bone as the result campfires can also
be demonstrated (Bennett 1999; De Graaff 1961; Stiner et al. 1995). Campfires can reach temperatures in excess of
800o C, and it has been shown that bone buried as deep as 10cm below
campfire can show significant thermal alteration. The degree of thermal alteration is dependent
on the depth of burial beneath the heat source and the heat transfer and
retention properties of the sediment.
Other important variables include the pre-burn condition of bone as well
as species and skeletal element represented.
It is unlikely that significant thermal alteration of subsurface bone
will occur during natural fires. Sayler
et al. (1989) have shown that while surface bone is moderately affected by
prairie fire, bones buried as shallow as 2cm are not likely to be susceptible
to significant thermal alteration. No
data exists on subsurface thermal alteration of bone during natural fire in
heavily fueled area such as forest.
Thermal
Alteration of Ceramics
Simply by
nature of their manufacture and use, archaeological ceramic materials are
generally resistant to thermal alteration under moderate thermal gradient. At the time of manufacture, most prehistoric
pottery was fired at temperatures in the 400-600°C range (Cogswell et al. 1997;
Colten 1951; Feathers et al. 1998; Goodyear 1971; Heimann and Franklin 1979;
Kaiser and Lucius 1989; Rice 1987; Roberts 1963; Rye 1981; Shepard 1956; Tite
1969; Ziad and Roussan 1999). Under
prescribed and wildland fire conditions, prehistoric ceramic materials should
remain relatively stable until temperatures climb beyond original firing
temperature and/or the temperature gradient becomes excessive and induces significant
thermal stress. Historic ceramic
materials such as porcelain and china are fired at temperatures of 1280-1400°C,
and beyond (Rice 1987). However, these
materials are manufactured to withstand significant thermal stress due to
quality controlled manufacture and high firing temperature. Although, thermal stress in modern ceramics
has received attention from several researchers in the ceramic industry (e.g.,
Amberg and Hartsook 1946; Buessem 1955; Chandler 1981; Coble and Kingery 1955;
Crandall and Ging 1955; Davidge and Tapppin 1967; Grimshaw 1971; Hasselman
1969; 1970; 1983; Kingery 1955; Salmang 1961), it is unlikely that prescribed
and wildland fire conditions would significantly affect these material in
archaeological contexts.
The archaeological literature does offer
some useful information regarding the thermal alteration of prehistoric ceramic
materials within both the context of laboratory experimentation and from
post-fire field observation. Materials
science investigations into the thermal properties and thermal failure
properties of prehistoric pottery offer important background information
regarding the potential for thermal damage of ceramic materials (Bronitsky
1986; Bronitsky and Hamer 1986; Schiffer 1990; Schiffer et al. 1994; Young and
Stone 1990). In general, thermal
fracture of pottery occurs as the result of tensile or compressive stress
generated by thermal gradient that exceeds the strength of the ceramic body. Essentially, differential thermal expansion
or contraction within the body will elicit thermal fracture if the thermal
stress is sufficiently greater than that which the structural components of the
body can withstand. Important variables
affecting this process include the coefficient of thermal expansion, thermal
conductivity, temperature differential, and temperature gradient (Hasselman
1970; Kingery 1955; Rice 1987). In
addition, Schiffer et al. (1994) have reported that thermal spalling occurs
were steam pressure produces sufficient stress to exfoliate elliptical portions
of ceramic surfaces, particularly where irregularities in the surface are
present.
Forms of thermal alteration observed for
ceramic materials that have been burned over during wildland fire include;
thermal fracture, thermal spalling, combustion of organic paint, and oxidation
or re-firing resulting in subsequent color alteration (Burgh 1960; Lentz et al.
1996; Switzer 1974). For the various
types of Southwestern pottery sherds (particularly black-on-white), some
researchers have attempted to document the conditions under which these forms
of thermal alteration occur using laboratory and field experimentation. Through field experimentation, Oppelt and
Oliverius (1993) have demonstrated that the application of fire retardant foam
during burning will induce thermal shock among Mesa Verde Pottery sherds. However, the experiment did not document the
potential for thermal fracture or spalling of sherds during combustion of
natural fuels alone. Through laboratory
experimentation using a muffle furnace, Bennett and Kunzmann (1985) observed
heat-induced color alteration of ceramics at temperatures above 500°C in which
redware varieties can change to a darker hue, and black-on-white wares will
change to a “slightly buff” color due to the oxidation of iron minerals. Under laboratory conditions, other
researchers have also observed that black-on-white sherds can oxidize to
resemble redware (Burgh 1950; Colten 1953; Shepard 1956). Bennett and Kunzmann (1985) also suggest that
the pigmented design on black-on-white sherds is stable under high temperature
if the paint is mineral based, but may combust and fade significantly if the
paint is organic based. This experiment
produced only one occurrence of thermal spalling on a black-on-white sherd at a
maximum temperature of 500°C, and one incidence of surface cracking on a
redware specimen at 600°C. This is
likely due to the limited potential for extreme thermal gradient produced
within the muffle furnace as compared to a natural fire where the rate of
heating and cooling would likely be more severe.
In addition to potential physical
alteration of pottery sherds during natural fire, thermoluminescence dating of
affected sherds may also be confounded as a result of heating during the
fire. Luminescence signals in pottery
are generally not affected by heating where the temperature is less than 250°C
(Aitken 1985). However, the potential
for wildland fire to generate soil surface temperatures above this threshold is
quite probable. Theoretically, then, the
TL signal of sherds burned during wildland fires may be altered depending on
temperature, rate and duration of heating experienced by the specimen during
the fire. Rowlett (1991) and Rowlett and
Johannessen (1990) demonstrated that wildland fire can change the TL reaction
for pottery sherds. Here the researchers
sampled specimens burned during a wildland fire at
Thermal Alteration of Metals and Glass
The literature surrounding the thermal
alteration of metals under wildland fire and prescribed fire conditions is
quite limited. Traylor (1990) refers to
anecdotal accounts of automotive steel undergoing thermal deformation during
severe wildland fire. In addition,
Sayler et al. (1989) observed the partial melting and deformation of lead
during prescribed burns in mixed grass prairie fuels. Metals common to the archaeological record
such as iron, steel, tin, brass, copper, and lead do have established melting
points at which significant damage could be predicted given sufficient fire
intensity and energy output. The
established melting points for these metals are as follows: iron (1275-1535°C),
steel (1250-1480°C), tin (232°C), brass (900-930°C), copper (1083°C), lead
(327°C) (Perry and Green 1984). Based on
this information, tin and lead artifacts will likely be the most susceptible to
melting during natural fire, other metals with higher melting points could be
significantly affected only under conditions of severe fire intensity.
Archaeological metals, are of course,
generally in a corrosive state in which electrochemical processes and ions
create chemical compounds that adhere and bond to metal surfaces, and over
time, may penetrate deep within the structure of metal objects (e.g., rust) (Hoff 1970; Organ 1976; Waite
1976). The extent to which exposure to
fire affects corroded metal is not well documented; however, Engle and Weir
(1998; 2000) have demonstrated that breaking strength, elongation, and
ductility of new and corroded barbed wire is not significantly affected by
exposure to grass fire. Schiffer (1987)
suggests that corrosion on metal surfaces can serve as a protective film in
archaeological contexts. Of particular
interest, Schiffer also notes that if the coefficients of thermal expansion of
the corrosive file and the metal are significantly different, thermal cycling
could initiate cracking that could result in perpetuation of corrosion within
the metal object. It is possible the archaeological
metals subject to natural fire conditions could be affected heat-induced
cracking and subsequent internal corrosion.
In sum, then, archaeological metals subjected to wildland fire may be
impacted by natural fire immediately due to melting and deformation, and
over-time due to the potential increased internal corrosion.
Glass consists of three primary elements,
silica, soda or potash, and lime or magnesia (Goffer 1980). The formation of glass is accomplished
through fusion and cooling of its major constituents (Goffer 1980; Havlác
1983). The established melting point of
glass is 750-870°C. Glass is essentially
a supercooled liquid that performs as a solid, and is therefore, subject to
thermal stress (DeHann 1997). Thermal
stress caused by uneven heating will induce fracturing of glass when internal
stresses exceed the tensile strength limit of glass (DeHann 1997; Lentini
1992). Thermally altered glass may
exhibit straight fracture lines, crazing, shattering, and melting depending on
fire conditions (DeHann 1997). As such,
archaeological glass will be highly susceptible to thermal alteration under
natural fire conditions given that the heat energy released by the fire is
sufficient to induce significant thermal stress and/or melting.
Fuels, Combustion, and Heat Transfer
The
ignition and combustion of organic fuels is largely a function of the fuel’s
surface to volume ratio and mass (DeBano et al. 1998). Fine fuels ignite, combust, and produce heat
quickly (high combustion efficiency), whereas, heavier fuels tend to ignite,
combust, and produce heat more slowly and uniformly low/moderate combustion
efficiency) (DeBano et al. 1998).
Important variables associated with fuels are loading, size and arrangement,
composition, and moisture content (Rothermel 1972). These variables are also conditioned by
environmental variables occurring during combustion such as temperature, wind
speed and direction, relative humidity, slope and aspect, fire behavior, etc.
(Chandler et al. 1983; DeBano et al. 1998; Pyne et al. 1996; Rothermel 1972;
Whelan 1995; Wright and Bailey 1982).
Concerning the impact
of wildland fire on archaeological resources, some important variables to
consider are the total amount to heat (energy) produced, and the proportion of
that energy which is transmitted to the mineral soil surface and within the
mineral soil. Heat transfer can take the
form of radiation (transfer via electromagnetic waves), convection (via the
mixing of fluids), or conduction (via molecular activity within or between
substances in contact) (Drysdale 1985).
The dominant form of heat transfer downward from surface fuels to the
duff layer (if present) and eventually the mineral soil is radiation (DeBano et
al 1998). DeBano (1974) estimates that
only 10-15% of energy released during the combustion of surface fuels is
transmitted downward into the duff and/or mineral soil. If the duff is ignited and begins to combust
(smoldering combustion), 40-73% of the heat may be transferred to the mineral
soil (Hungerford and Ryan 1996). Burning
duff can produce mineral soil temperatures of >350˚C for periods
extending over several hours (DeBano et al. 1998). When no duff is present, heat energy is
transferred directly from surface fuels to the mineral soil via radiation. The amount of heat transfer is variable and
dependent on fuel loading, type, size and arrangement, etc. (Rothermel
1972). Within the mineral soil, heat is
transferred via conduction and convection of hot gasses through the soil matrix
(after water is vaporized ~100˚C) (Campbell et al. 1995; Campbell et al.
1994; DeBano et al. 1998).
However, significant surface heating does not necessarily elicit high
levels of soil heating due to thermal conductivity characteristics of soils
(Frandsen and Ryan 1986; Hartford and Frandsen 1992; Nidal et al. 2000).
The proportion of heat energy that is
transferred from organic fuels (surface or duff) to archaeological materials is
dependent on the proximity of the artifact to the fuel as well as the multitude
of variables associated with fuels and fire behavior. An artifact deposited on the contact between
the mineral soil surface and combusted fuels will be subjected to a greater
transmission of heat energy as compared to an artifact deposited 5cm beneath
the mineral soil. Mineral soil and duff
(non-combusted) insulate against heat transfer of combusted surface fuels. However, if there is a heavy duff
concentration that ignites and begins to combust the heat energy transfer
downward to the mineral soil surface can be significant. Similarly, if heavy combusted surface fuels
(logs, etc.) are present, the transfer of heat energy towards the mineral soil
may also be significant regardless of the presence of a duff layer. Artifact deposited greater than 5cm beneath
the soil surface will likely be well insulated against radiant and conductive
heat energy under most natural fire conditions.
The manner in which an artifact absorbs
and is affected by heat energy is largely dependent on the physical composition
of the artifact, and the thermal conductivity associated with its
composition. Obviously, certain
artifacts classes will be affected more significantly compared to others. For example, it requires a substantial amount
of heat energy to significantly affect lithic artifacts, whereas, archaeological
materials with organic composition such as bone may be affected by much lower
levels of heat energy. Thus, the
important variables to consider when assessing the impact or potential impact
of wildland fire or prescribed fire on archaeological materials are; 1) fuel
type, 2) fuel load, 3) fire behavior, 4) proximity of artifacts to fuels, and
5) artifact class. In the following chapters, these variables are studied under
a variety of controlled experimental conditions as well as during field
recording of burned archaeological sites.
CHAPTER 2
THE
IMPACT OF PRESCRIBED BURNING ON ARCHAEOLOGICAL RESOURCES
Introduction
Prescribed fire is an increasingly common
land management practice utilized by public land agencies to reduce hazardous
fuels and manage vegetative and wildlife communities. Moreover, given the severity of wildland
fires in the
Kelly
and Mayberry (1980) conducted one of the first research projects involving
field experimentation to investigate the effects of prescribed fire on
archaeological materials. The report
itself is quite limited; however, the basic methodology consisted of establishing
5x5m test plots in which “artifact clusters” were placed on the sediment
surface. The plots, six in total, were
burned during prescribed fires in mixed conifer and sequoia-white fir
environments (sparse understory and light ground fuels). Temperature measurement was attempted using
temperature sensitive pellets and pyrometric cones; however, neither method was
successful. Thermal alteration of the
experimental artifacts was limited to “carbon smudging”, and the diagnostic
attributes of each specimen were not significantly altered during the
experiment. The authors conclude that,
cool burning prescribed fires are likely to have a limited impact on the
diagnostic characteristics of surface artifacts. The incomplete nature of this report makes it
difficult to ascertain specific information regarding the research methodology
(e.g., the type of artifacts used, method of artifact analysis, fire behavior,
etc.). The only specific information
regarding artifact type was the inclusion of obsidian flakes that were
submitted for obsidian hydration testing post-fire. The results of the obsidian hydration
analysis were reported to be “inconclusive.”
The report, however limited, is nonetheless, a seminal contribution in the
area of field-based experimentation and the potential impact of prescribed fire
on archaeological resources.
Sayler
et al. (1989) (see also Picha et al. 1991) conducted a more systematic
investigation focusing on the impact of prescribed fire in prairie fuels on a
range of archaeological materials common to the Knife River Indian Villages
National Historic Site, North Dakota.
Their research methodology consisted of placing experimental artifacts
in 10x10m burn plots, both at the sediment surface and 2cm subsurface. Experimental artifacts included non-flint
cobbles,
The
results of the experiment showed that the most pervasive form of thermal
alteration observed was “scorching”, which refers to the presence of a
combustive residue deposits on the surfaces of artifacts. Artifacts with organic components such as
bone, shell, and wood, exhibited a combination of “scorching” and “charring”,
the later refers to the partial combustion of organic component of the
specimen. These two forms of thermal
alteration resulted in shifts to darker Munsell values for most specimens,
which the researchers define as “color change”.
Significant forms of thermal alteration such as thermal fracturing and
deformation/melting were observed at low frequencies. One flint cobble exhibited thermal spalling,
approximately 11% of flint flakes exhibited potlid fracturing, two glass beads
sustained partial melting, seven lead sinkers exhibited melting, and shell
specimens sustained structural and morphological alteration. In general, the greatest degree of thermal
alteration was observed for artifacts from Plot 4, which was characterized by the
greatest fuel density. Maximum
temperatures recorded at 2cm and 10cm above the soil surface for all plots
ranged between 316-399°C, and flaming combustion within the plots was estimated
at between 30-60 seconds. Subsurface
artifacts were largely unaffected during the experiments. Soil temperatures, measured with digital
thermometer prior to ignition and immediately following cessation of flaming
combustion, were elevated only 2-4°C over baseline values.
The
authors conclude that the impact of prescribed fires in prairie fuels will be
insignificant for subsurface archaeological materials buried greater than 1cm
beneath the mineral soil surface. They
also predict that all artifacts positioned on the soil surface will become
blackened due to scorching (combustive residue deposit) with organic materials
sustaining the greatest degree of thermal alteration due to a combination of
combustive residue deposition and partial combustion. Moreover, significant forms of thermal
alteration such as thermal fracturing and structural change will be
significantly less severe for lithic materials, pottery sherds, and other
inorganic artifacts types as compared to organic materials. They also conclude that prescribed fire in
prairie environments is unlikely to produce thermally altered artifacts that
might be confused with materials from archaeological contexts such as
fire-cracked rock and calcined bone.
This
experiment is an important contribution to the study of the impact of
prescribed fire on archaeological resources.
The methodology used is the most systematic and most comprehensive
available in the fire effects literature.
However, one weak point in the experimental design relates to the method
of temperature measurements. Maximum
temperatures were recorded using temperature sensitive crayons wrapped in
aluminum foil. This method produced only
two maximum temperatures for each plot, one at 2cm above the soil surface and
the other at 10cm above the soil surface.
These temperature data are quite limited in that the method does not
provide sufficient data on rate of heating and duration of heating. Moreover, temperature data were taken only at
one point within each of the four plots, which does not account for variation in
fire intensity within and between the plots.
The use of a data logger and thermocouple system would have provided
more detailed temperature data, which in turn could have been correlated to
variability in artifact thermal alteration between each plot. Nonetheless, this experiment generated important
information regarding the impact of prescribed fire in prairie fuels on various
artifact classes, and remains one of the most comprehensive studies available
on the subject.
Brunswig
et al. (1995) conducted an experiment that essentially replicated the study
performed by Sayler et al. (1989). Using
the same research questions and similar methodology, Brunswig et al. (1995)
carried out an experiment focused on assessing the impact of prescribed fire on
archaeological resources in a high plains short-grass prairie environment. Two 4x4m burn plots “salted” with
experimental artifacts were used in the study as well as two 4x4m plots at
actual archaeological sites located within the prescribed burn area. Experimental artifacts included materials common
to archaeological sites in the area such as quartzite and chert projectile
points, quartzite and agate scrapers, quartzite flakes and cores, Plains Kansas
pottery, deer antler, and cow bone. The
researchers hypothesized that prescribed burning in a heavily grazed
short-grass prairie environment would have a “minimal and short-lived” impact
on the experimental artifacts and actual archaeological materials located
within the burn area. It should be noted
that no method of temperature measurement was implemented during the
experiment.
The
results of the experiments fully supported the “minimal-impact” hypothesis put
forth by the researchers. Thermal
alteration of artifacts was predominantly limited to combustive residue
deposition on artifact surfaces. The
only exceptions were limited evidence of partial charring and slight cracking
of antler and bone specimens. Overall,
no significant forms of thermal alteration such as thermal fracturing was
observed. The authors conclude that the
light fuel load compounded by cool and moist burning conditions were the most
significant variables affecting the minimal impact of the burn on
archaeological materials. They further
suggest that, in general, prescribed fires in grassland environments are likely
to have a limited impact on archaeological resources.
The
results of this experiment are roughly consistent with those reported by Sayler
et al. (1989). The major exception being
the potlid fracturing of
Several
research papers exclusively focused on studying the effects of prescribed fire
on obsidian, particularly obsidian hydration, are included in a recent volume
compiled by Loyd et al. (2002). Deal and
McLemore (2002) conducted two prescribed fire experiments in a Sierran yellow
pine / black oak forest environment with the purpose of assessing the effects of
prescribed fire on obsidian hydration bands.
Research suggests that obsidian hydration bands begin to be affected by
heating at approximately 260°C, become significantly affected at 427°C, and may
be completely destroyed at temperatures exceeding 700°C (Benson 2002; Brunswig
et al. 1995; Deal and McLemore 2002; Green et al. 1997; Halford and Halford
2002; Origer 1996; Smith 2002; Solomon 2002; Steffen 2002; Trembour 1990). The research design consisted of placing
obsidian artifacts with previously established hydration bands in burn plots
with variable fuel load (“light”, “woody”, and “log”). Temperatures were recorded at the soil
surface and subsurface (-5-7cm) using a thermocouple and data logger
system. This is an important component
of the experimental design since most studies have only incorporated crude
indicators of maximum temperature such as temperature sensitive crayons,
pellets, and paints. Use of a
thermocouple / data logger system allowed the researchers to generate coarse
time and temperature curves from which heating rate, duration, and maximum
temperature could be derived.
The
first experiment was conducted in an environment characterized by a
considerable build up of hazardous fuels with an estimated fuel load of
approximately 16-31 tons/acre (5888-11,409 kg/ha). Due to a thick duff accumulation, surface
temperatures reached maximum levels over a protracted period of 2.5 hours. The peak surface temperature associated with
log fuels was approximately 520°C, woody fuels 310°C, and light fuels
305°C. Subsurface temperature reached
maximum levels of 73-67°C over an extended period of 6.5 hours. The results of this experiment showed that in
lighter fuels hydration bands were altered for 56% of the sample, woody fuels
67% of the sample, log fuels 78% of the sample, and subsurface 44% of the
sample (N=27).
The
second experiment was conducted during the spring in an area where hazardous
fuels had been reduced by periodic prescribed burning (estimated fuel load 4
tons/acre, 1472 kg/ha). The same
experimental method used during the previous experiment was also implemented
here. Peak temperatures recorded during
the experiment ranged from approximately 475°C for log fuels to 137°C and 79°C
for woody and light fuels respectively.
Temperatures peaked rapidly within minutes and were sustained at high
levels for 2-4 hours. The results of the
experiment showed that 44% of the log fuel specimens, 11% of the woody fuel
specimens, and 44% of the light fuel specimens exhibited altered hydration
bands. Due to the lighter fuel load,
alteration of hydration bands during this experiment were diminished during
this experiment as compared to the previous where fuel load was heavier. In addition to alteration of hydration bands,
74% of the specimens from both experiments exhibited additional forms of
thermal alteration such as surface sheen, pitting, and combustive residue
deposition. Overall, the experiments
demonstrate that fuel load, burn duration, and peak temperature are important
variables affecting the alteration of obsidian hydration bands.
Solomon
(2002) conducted a similar experiment in a Ponderosa pine / mixed conifer
environment with an estimated fuel load of 3.5 tons/acre (1288 kg/ha). The experimental design consisted of placing
obsidian artifacts, with previously determined hydration bands, at the soil
surface within designated burn plots.
Fuel compositions within the burn areas were variable to include a slash
pile, log fuels, woody fuel, and light fuels.
Maximum temperature data were recorded using heat sensitive temperature
pellets placed beneath each artifact.
This method is unlikely to produce accurate temperature readings since
there is often a significant temperature differential between the upper and
lower surfaces of an artifact during burning (see dissertation chapter 4). The results of the experiment show that
temperatures at the soil surface did not exceed 101°C with the exception of the
surface beneath the slash pile in which surface temperatures were estimated to
have reached the 400-500°C range. Two
obsidian specimens from the slash pile slot exhibited diffused hydration bands;
however, none of the remaining specimens included in the study exhibited
altered hydration bands. The author
concludes that low intensity prescribed burns in Ponderosa pine / mixed conifer
environments are unlikely to negatively affect obsidian hydration bands. Conversely, the author suggests that burning
under slash pile fuel loads has a potentially greater probability of negatively
affecting hydration bands. Although the
method of temperature measurement used during this study is insufficient, the
results further reiterate the important relationship between fuel load / fire
intensity and potential thermal alteration of obsidian hydration bands.
Halford
and Halford (2002) conducted a prescribed burn experiment in sagebrush fuels to
assess the impact of burning in this fuel model on obsidian hydration
bands. The experiment consisted of six
1x1m burn plots situated in variable fuel densities of heavy moderate and light
in which 180 obsidian artifacts (with established hydration bands) were equally
distributed across the plots and at variable soil depths (soil surface, -5cm,
-10cm). Temperature data were recorded
using temperature sensitive pellets of various melting thresholds ranging
between 149-843°C. In addition a digital
thermometer paired with two thermocouples was used to measure temperature
gradient in Plot 1 during the burn.
The
prescribed burn was conducted in late fall under cool and moist conditions,
which diminished fire intensity and the potential for the fire to carry itself
across the burn area. The results of the
experiment show that in Plot 1 (light fuel) surface temperature peaked at only
85.2°C and subsurface (-5cm) temperature reached a maximum of only 6.2°C. In plots with heavier fuels, peak surfaces
temperatures are reported to have reached approximately 300°C (temperature
pellets). The impact of the burn on
obsidian specimens was minimal, and generally limited to surface specimens from
one heavy and one moderate fuel plot.
For the heavy fuel plot, 40% of specimens are reported as exhibiting
diffuse hydration bands, and in moderate fuel plot 10% of specimens were
similarly affected. Due to the
significant degree of inter and intra-plot temperature/burn intensity variability
observed during the experiment, the authors conclude that, in sagebrush
environments fuel density and the proximity of artifacts to fuels are important
variables that potentially affect the thermal alteration of obsidian hydration
bands.
Green
et al. (1997) conducted a similar experiment during a prescribed burn in
sagebrush fuels. The research
methodology consisted of placing obsidian artifacts with established hydration
bands in burn plots of variable fuel density (heavy, moderate, light). Temperature data were recorded by placing
temperatures sensitive tablet beneath each artifact. The results of the experiments demonstrated
that alteration of obsidian hydration bands is strongly associated with fuel
load and burn temperature. The authors
conclude that surface temperatures below 200°C are unlikely to produce diffuse
or destroyed obsidian hydration bands.
Additional experimentation of this nature conducted in
sagebrush fuels was
performed
by Benson (2002). In addition to
assessing the impact of prescribed burning on obsidian hydration bands, the
experiment included a second component that was focused on the potential for
thermal alteration of chert artifacts.
The experimental design consisted of subdividing a burn plot into heavy,
moderate, and light subplots (three each) in which 90 obsidian artifacts and 90
chert artifacts were equally distributed at the soil surface. Soil surface temperature data were collected
using an unspecified data logger and thermocouple system. The results of the experiment showed that
surface temperatures in the heavy fuel subplots ranged between approximately
73-725°C (specific time / temperature curves are not provided) and that the
hydration bands from all obsidian specimens were either diffuse or completely
obliterated. Surface temperatures in the
moderate fuel subplots ranged between 80-550°C and the hydration bands on 24
specimens were negatively affected. In
the light fuel plots surface temperatures ranged between 37-440°C, negative
affecting the hydration bands of 16 specimens.
The
portion of the experiment focused on the potential for thermal alteration of
chert artifacts is only vaguely discussed in the report. Specifically, the source of the chert
material is not specified; no time / temperature data are provided for chert
artifacts; and the extent of thermal alteration associated with chert artifacts
is reported as “severely damaged” with little reference to specific
observations. The only information provided
is contained within the following statement; “All of the large and many of the
medium size flakes shattered into tiny fragments. Many of the smaller flakes
were structurally unchanged, but altered in other ways (p.100)”. No further elaboration is provided. Based on the temperature data provided for
the subplots, it is likely that the chert specimens were subjected to peak
temperatures ranging between 440-725°C.
These temperatures are well within the range necessary to induce
significant thermal fracturing of chert (see dissertation chapters 3 and 4).
The author
concludes that during prescribed fires where surface temperatures exceed 300°C,
thermal alteration of obsidian hydration bands is likely to occur. However, the
author suggests that duration of heating is also an important variable operating
in tandem with temperature range. Again,
the dynamic between fuel load, burn intensity, and potential thermal alteration
of selected archaeological materials is reiterated.
Discussion
In
sum, each of the research projects discussed above offers important
contributions in assessing the potential thermal alteration of archaeological
resources during prescribed fires.
Nonetheless, several of the experimental designs share a common
weakness, which is directly related to method of temperature recording. With the exception of Deal and McLemore
(2002) and Benson (2002) temperature data were acquired using temperature
sensitive products, which only provide a broad range estimate of maximum
temperature. Excessive confidence was
placed on the reliability of these products in actually establishing the peak
temperatures at which individual artifacts were heated. Moreover, some researchers placed these
products beneath artifacts during experiments, effectively estimating the
maximum temperature beneath specimens, not on upper surface where temperature
may be 50-60% greater (see dissertation chapter 4). In addition, most researchers failed to
consider the potential for variable burn intensity and temperature with the spatially
defined boundaries of individual burn plots.
Where data loggers and thermocouples were used to establish time and
temperature gradients within burn plots, the data are often coarsely measured
over gross time intervals measured in 30-60 minute units. Capturing data points at shorter intervals
such as 1-5 seconds would provide more detailed information regarding heating
rate, duration, and peak temperature.
Clearly, more reliable and detailed methods of temperature measurement
are needed to accurately assess the impact of a given burn intensity/temperature
on archaeological materials. Additional
research that incorporates a wide range of artifact classes and a variety of
prescribed burn fuel models is also necessary to further the base of knowledge
acquired on the subject thus far. In order
to address these issues, a series of prescribed burn field experiments were
conducted. Information regarding the
experimental design, fuel environment, and results are provided in the
following section.
PRESCRIBED
FIRE EXPERIMENTS
Field experiments performed in conjunction
with prescribed burns were designed and implemented during the 2001 and 2002
field seasons. Experiments were
conducted in a variety of fuel types during planned prescribed burns at
Badlands National Park, Grand Teton National Park, Wind Cave National Park,
Pike National Forest (Colorado Front Range), and Bureau of Land Management
lands (northwestern Colorado). In addition, one experiment was conducted in
the
1.
Observe surface and subsurface time/temperature gradients generated during
prescribed fires in a variety of fuel models ranging from light (grassland) to
heavy (mixed conifer).
2. For a specific fuel model, observe and
document the effects of heat energy released during flaming combustion on a
variety of archaeological material types common to the archaeological record.
3.
Based on these data, provide broad guidelines for land managers regarding the
potential impact of prescribed fire on archaeological resources given a
specific fuel model and archaeological material type.
Research
Design
The
research design utilized during the prescribed fire experiments was influenced
by that developed by Sayler et al. (1989), but adapted and refined to meet the
specific goals of this project. The
basic method used during the experiments consisted of establishing 2x2m or 1x1m
burn units, divided into four 1m2 or 50cm2 quadrants in
which clusters of experimental artifact were positioned at the mineral soil
surface. The artifacts consisted of
modern replicates or analogs of common prehistoric and historic archaeological
materials. The artifact classes
represented included mammal bone (various deer and elk elements), freshwater
mussel shell (various species), lithics (chert and obsidian flakes), pottery
(prehistoric replicates and unprovenienced black-on-white sherds), metals
(copper, brass, lead), firearm cartridges (rifle/handgun), glass fragments
(beverage containers), ceramics (white ware), beads (wood, glass), and wood
(2x4inch pine scraps). Each quadrant
within the burn plot contained a cluster of experimental artifacts representing
the same range of material types.
Thermal alteration assessments of artifacts were based on recording
the range of thermal alteration attributes observed during post-burn
analysis. Thermal alteration attributes
included various degrees of alteration ranging from combustive residue deposits
to thermal fracturing (definitions provided in Appendix 1, Data CD). Supplemental information regarding weather
conditions, fuel load, and fire behavior were also recorded during each
experiment. Experimental artifacts
(N=832) were measured, weighed and assigned Munsell (2000) color values pre-
and post-fire. Descriptive information
and thermal alteration analysis of each artifact included in the study is
provided in Appendix 1, located on the Data CD included with the dissertation.
In
addition to the basic 2x2m burn plot experimental setup, four modifications of
the design were also conducted over the course of the prescribed burn
project. First, some prescribed burn
experiments were performed where only surface temperature data was collected
without the inclusion of experimental artifacts. These experiments were performed with the
purpose of gathering additional time/temperature data in order to validate the
data generated during previous experiments.
The second modified design consisted of trials in which upper and lower
surface temperatures of experimental artifacts were recorded during a
prescribed burn in grassland fuels and during a low-intensity wildland fire in
a mixed conifer environment. The third
manipulation of the basic experimental design included a series of log burning
experiments performed during a prescribed burn in mixed conifer fuels. These experiments were conducted to observe
the range of soil surface temperatures generated beneath heavy fuels, and the
effects of the heat energy release on lithic artifacts. The log burning
experiments consisted of a 1x1m burn unit placed arbitrarily over downed, dead
logs ranging in size from 10-30cm in diameter.
Modern replicated lithic flaking debris was placed at the soil surface
beneath the logs and subsequently burned, then later analyzed for thermal
alteration attributes.
Time and
temperature data during each experiment were recorded using two systems. The primary system, used for the majority of
the project, consisted of an Omega OM-3000 portable data logger and six Type K
hi-temperature inconel overbraided ceramic fiber insulated thermocouples
(XCIB-K-1-2-25, 25ft length, Type OST male connector, probe style 1
termination) (Omega Engineering 2000).
During burning, the data logger was housed in a fire/heat resistant case
(Sentry security chest), which in turn was over-wrapped with heat reflective
carbon cloth. The data logger was programmed to record temperature (°C) data
points every 1second, 5 seconds, or 10 seconds depending on the fuel type and
experimental situation. Thermocouples
leads were placed at the mineral soil surface within the center of each
quadrant, roughly at the center of the artifact cluster. Subsurface temperatures were also recorded by
placing thermocouples at various soil depths ranging between 1-10cm depending
on the fuel type. This data recording
system allows for the generation of detailed time/temperature curves showing
rate of heating, peak temperature, time at peak, and cooling rate via computer
download from the data logger to a PC.
The secondary
time/temperature recording system consisted of a portable data logger and
thermocouple system developed and constructed by Jim Reardon at the USDA
Intermountain Fire Sciences Laboratory (
MIXED GRASS
PRAIRIE EXPERIMENTS
Prescribed
burning experiments in grassland fuels were conducted at
2001
The
2001 experiment at Badlands NP included four burn plots, each of which
contained approximately 50 experimental artifacts. Thermocouples 1-4 were placed on the soil
surface within each 1m2 quadrants delineated within the 2X2m burn
plots. Thermocouple 6 was placed 2cm
below the soil surface at the approximate center of each burn plot to determine
subsurface temperatures during the burn.
The prescribed burn occurred in moist grassland fuels, predominantly
smooth brome (Bromus inermis), with an estimated fuel load of
approximately 4 tons per acre (1472 kg/ha).
Results
The
time/temperature data for Plot 1 are summarized in Figure 2.1 (All figures are
provided at the end of Chapter 2). These
data show maximum surface temperatures ranging widely from 418.8-82.0°C. The 418.8°C value is clearly an anomaly
compared to the other values for which peak temperatures were considerably
lower. This irregular value may be due
to a displaced thermocouple and/or the occurrence of flames coming into direct
contact with the thermocouple sensor. Overall, surface temperatures peaked and
fell to near baseline values within 7 minutes, with the greatest portion of
heating occurring within the first 1 minute of combustion. Peak temperatures reached apex levels within
30-60 seconds, and were sustained at near maximum values briefly for only 10-15
seconds. Subsurface temperature (at
–2cm) reached a maximum of only 25.8°C, an increase of only 15°C over the
baseline value. The time/temperature
curve for the subsurface thermocouple is characterized by low apex and is
considerably more protracted compared to the curves generated by surface
thermocouples. This suggests that
subsurface heating at –2cm is minimal under the conditions observed during
burning. Field notes on observed fire
behavior are indicative of low fire severity.
The flame front (backing fire) required approximately 3.5 minutes to
pass across the 2x2m unit (40-60m per hr), and flame length was estimated at
between 20-40cm.
The
time/temperature data for Plot 2 are summarized in Figure 2.2. Surface temperatures within Plot 2 peaked and
fell to near baseline temperatures within 10 minutes, with the greatest range
of heating occurring within the first minute of the time and temperature
curve. Maximum surface temperatures
recorded for each quadrant of the burn plot varied moderately ranging between
235.0-106.6˚C. Peak temperatures
were attained within 40-60 seconds, and remained elevated for a short duration
(10-15 seconds). Time/temperature curves
for each of the surface thermocouples are uniform in contour, suggesting
consistent burning across the entire plot. The maximum subsurface temperature
recorded was within the plot was 21.5˚C, which is only a slight increase
over the 12.3˚C baseline reading. Fire behavior observations show that the
flame front (backing fire) spread across the unit within approximately
2min38sec (40-60m per hr), and flame length was estimated at 20-40cm.
The
time/temperature data for Plot 3 are summarized in Figure 2.3. These data show very uniform curves with peak
surface temperatures ranging between 150.2-281.0˚C. Surface temperatures reached maximum levels
within approximately 30-50 seconds and were sustained at elevated levels only
briefly for approximately 10-15 seconds.
The entire duration of heating from baseline, to peak, to return near
baseline was approximately 11 minutes.
The maximum subsurface temperature at –2cm was recorded at 22.7˚C,
which represents a minimal 5.8°C increase over the baseline value. Observed fire behavior notation recorded the
flame front (head fire) passing over the plot within approximately 2min7sec
(300-1200m per hr), and estimated flame length ranging between 30-60cm.
The time/temperature data for Plot 4 are
summarized in Figure 2.4. Maximum
surface temperatures recorded within each quadrant varied widely from
321.2-61.6˚C.
Thermocouple
time/temperature curves for quadrant 1 and 3 are uniform in contour and similar
with regard to peak temperature (250-300°C).
Data from quadrant 2 and 4 contrast sharply compared to quadrants 1 and
3, showing low peak temperatures (60-70°C) and protracted curves. These data suggest that combustion within the
burn plot was not consistent across all quadrants. Overall, peak surface temperatures recorded
in Plot 4 peaked and fell to near baseline within 8 minutes with the largest
proportion of heating occurred during the first 1 minute of combustion. The maximum subsurface temperature recorded
within the plot was 34.6˚C, only a 13.3 °C increase over the baseline
value. Fire behavior observations show
the flame front (head fire) traversing the plot within approximately 2min
(300-1200m per hr) with estimated flame length of 30-60cm.
In general,
maximum surface temperatures between the quadrants delineated within each of
the four burn plots varied considerably ranging between 418.8-61.6˚C. These temperatures were characterized by
brief residence times in which peak and near peak values were sustained for
only 10-15 seconds. The average maximum
surface temperature recorded within each quadrant across all four burn plots
was 195.8°C. Overall, time curves for
each plot show that temperatures peaked precipitously, then fell rapidly to
near baseline within 6-9 minutes. The
steepest portions of the curves show that the majority of heating above 50 °C
occurred within the first minute of combustion.
In sum, the results of the grassland prescribed burn experiment indicated
that surface heating during combustion was rapid but brief in duration.
Subsurface heating at –2cm during combustion
of grassland fuels was negligible.
Maximum subsurface temperatures for burn plots 1-4 ranged between
21.5-34.6°C. The average peak subsurface
temperature across the four burn plots was 26.2°C, which was only an average
increase of 10.9°C over baseline temperatures recorded prior to burning. These data suggest that 2cm of soil is
sufficient to mitigate subsurface heating during cool-season prescribed burning
in grassland fuels.
Post-burn
analysis of over 200 experimental artifacts subjected to burning during the
experiment show that the limited amount of heat energy produced by combusting
grassland fuels did not generate significant thermal alteration of any artifact
classes. No potentially detrimental
thermal damage in the form of thermal fracturing, cracking, spalling, or
deformation was observed (with the exception of melted plastic on the shotgun
shells). The most significant type of
thermal alteration observed occurred in the form of partial combustion/charring
of organic specimens such as wooden beads, 2x4 inch pine scraps, and the
organic component of some bone specimens (generally limited to upper edges of
specimens where bone density was thin).
Interestingly, the pine scraps, and wooden beads did not fully combust;
only minor evidence of incomplete combustion on the upper surfaces and edges of
these specimens was present.
Over the
entire artifact sample, the only immediately discernable effect of burning was
the discoloration of the upper surfaces of specimens in the form of an adhesive
light brown combustive residue deposit, and minor blackening/charring along the
upper edges of organic specimens. The
combustive residue deposit is a highly nitrogenous condensate tar that forms on
cool surfaces (i.e., artifacts) during a fire (Yokelson et al. 1997). This deposit ranged in color from golden
brown to black depending on the extent of combustion of the tar deposit. The charred portions of organic specimens
represent the byproduct of the pyrolysis and partial combustion of those
materials, particularly wood specimens.
DeBano et al. 1998:23, refer to this as “char”, a substance that is
neither an intact organic compound nor pure carbon. In the instance of condensate tar deposition
on artifact surfaces, it is likely that under natural conditions these deposits
will weather from the surfaces of artifacts over time. In the laboratory, these deposits can be
removed via vigorous scrubbing with water and a pumice soap solution. The charred portions of organic specimens;
however, are permanent thermal alterations of the artifact structure that may,
overtime lead to enhanced degeneration of these artifact classes.
In general,
the impact of prescribed burning in grassland fuels on the experimental
artifacts is generally consistent with similar experimental results reported by
Sayler et al. (1989) and Brunswig et al. (1995). Sayler et al. (1989) report pervasive
“scorching” (combustive residue deposits) of all experimental artifacts as well
as charring of organic specimens during prescribed burning in a prairie
environment. However, the researchers
did report potlid fracturing, lead deformation, and shell damage, of which no
instances were observed during the present experiment. This is likely due to heavier fuel loading
(buck brush) in two of the experimental burn plots set up by Sayler et al.
(1989). The results of the present
experiment show that although grassland fuels are homogeneous, there was a wide
range of variability in the maximum surface temperature recordings between and
within each of the burn plots. This is
likely due to variable rates of combustion of fuels within each plot, and the
possible occurrence of flames coming in contact with a thermocouple
sensor. In addition, grassland fuels
were not heavy enough to sustain high temperatures and long residence
times. This is reflected in the recorded
subsurface temperatures that were elevated only 10-15˚ over baseline in
each of the 4 burn plots. Based on these
observations, it is suggested that prescribed burning in mixed-grass fuels
presents a minimal risk to surface artifacts, and little or no risk to
subsurface artifacts.
2002
Field
experimentation performed in conjunction with prescribed burning at
During
the Pinnacles burn project, the same methodology was used for three burn plots
with the exception that artifacts in Plot 1 were sprayed with fire retardant
immediately after combustion had ceased within the unit to assess the potential
for thermal shock during the application of fire suppressant foam. In addition, an experiment designed to assess
the temperature differential on the upper (side facing atmosphere) and lower
(side facing soil surface) surfaces of an artifact during prescribed grassland
fire conditions was also conducted. The
experiment consisted of attaching thermocouples (via binder clips) to the upper
and lower surfaces of 3 artifacts (pronghorn [Antilocapra Americana]
mandible, black-on-white pottery sherd, and Hartville Uplift chert flake). Ten trials were performed using the same
three artifacts in each trial to observe the cumulative impact of repeated
burning in grass fuels on the artifacts as well as the temperature differential
between upper and lower surfaces of each artifact during the experiment. Digital photos were taken of each artifact
beginning and following each trial. Fire behavior and fuel information were
also recorded for each trial.
Results:
Roadside Burn Project
Smooth
brome (Bromus inermis) was the dominant fuel type during the roadside
burn project. Time/temperature data for
Plot1 are provided in Figure 2.5. These
data show that maximum surface temperatures within the burn plot varied
considerably from 67.0-256.6°C during combustion. Temperatures reached apex levels rapidly
within approximately 30 seconds and were sustained at elevated levels only
briefly for 10-15 seconds. Surface heating
within the plot was relatively consistent with the exception of quadrant 1 in
which peak temperatures were limited to the 60°C range. Overall, surface temperatures within the plot
peaked and diminished to below 50 °C within approximately 7 minutes. Subsurface temperatures at –1cm increased
from 6.3-30.5°C, and temperatures at –2cm increased from 6.2-12.4°C. Fire behavior observations documented during the burn show the flame front
(backing fire) passing the 2x2m burn plot within approximately 2.5 minutes, and
flame lengths ranging between 15-50cm.
Time and temperature data indicate that the maximum amount of surface
heating occurred within the 2.5-minute window.
Post-fire
analysis of over 50 experimental artifacts from Plot 1 indicated that the
impact of the burn on the artifacts was minimal. Overall, a light brown
condensate tar deposit was pervasive across the upper surfaces of all
specimens. Some bone and wooden bead
specimens also exhibited minor charring on upper surfaces as well. However, no significant forms of thermal
alteration such as thermal fracturing, spalling, or deformation was
observed. These observations are
consistent with those reported for the experiments conducted at
Experimental
artifacts were not included in Plots 2 and 3; only temperature data were
collected during burning.
Time/temperature data for Plots 2-3 are summarized in Figure 2.6. Data for Plot 2 show maximum surface
temperatures varying considerably between 86.7-281.3°C. Heating within quadrants 1 and 3 was
characterized by rapid ascent to peak temperatures and brief residence
times. Surface heating within quadrants
2 and 4 was less severe and more protracted indicating inconsistent combustion
within the plot. Subsurface temperatures
at –1cm increased from 12.7-27.2°C, and temperatures at –2cm increased from
9.0-13.2°C. Fire behavior observations
show that the plot was burned within 1.5 minutes via a flanking fire that
generated flame lengths of 50-75cm.
Consequently, the maximum duration of heat penetration at the soil
surface was limited to this short window.
Over the entire burn window, temperatures peaked and fell to below 50°C
within approximately 4.5 minutes.
Peak
surface temperature within Plot 3 varied between 72.3-196.9 °C. The time/temperature data for the plot show
that surface heating was consistent with the exception of quadrant 2 where peak
temperatures were limited to 72.3°C.
Surface heating within the plot was precipitous and brief with the
greatest proportion of heating occurring within the first minute of combustion,
and the entire duration of heating in excess of 50°C lasting approximately 4.5
minutes. Subsurface temperatures at –1cm
increased from 11.3-19.3 °C, and temperatures at –2cm rose from 12.8-20.9 °C,
indicating minimal subsurface heat penetration.
Fire behavior observations show the flame front (head fire) crossing the
unit within minute with flames lengths ranging between 50-150cm.
Results:
Pinnacles Burn Project
An
additional experiment consisting of three burn plots was conducted at
Time/temperature
data for Plot 1 show that maximum surface temperature within the plot varied
considerably, ranging from 48.5-289.0 °C.
Temperatures at –1cm subsurface increased from 8.6 –35.3 °C, and
temperatures at –2cm increased from 8.9-12.2 °C. Fire behavior observation recorded the flame
front (flanking fire) crossing the plot within 2.5 minutes. Fire suppressant foam was applied to the unit
immediately after flaming combustion has ceased. This is reflected in the time-temperature
curves for each thermocouple, which show sharp and erratic declines in
temperature within 2 minutes versus the typical 4-8 minute range observed in
previous experiments. The abrupt change
in temperature, however, did not have a significant impact on the experimental
artifacts. None of the artifacts
exhibited fracturing or spalling that is characteristic of thermal stress
induced by irregular heating and rapid change in temperature. This is most likely do the fact that maximum
surface temperatures were low and of minimal duration during combustion within
the plot. Although temperatures did drop
sharply, it was not sufficient to create tensile stresses that could initiate
thermal fracturing within the artifacts.
Neither
experimental artifacts nor fire suppressant foam were applied during burning
within Plots 2 and 3; only temperature data were recorded. Time/temperature data for plots 2 and 3 are
summarized in Figure 2.7. Maximum
surface temperatures within Plot 2 were rather low, ranging from between only
70.9-94.0 °C. Subsurface temperatures at
–1cm increased from 3.3-35.6 °C, and subsurface temperatures at –2cm increased
from 6.5-13.5 °C. Overall, surface
temperatures within the plot peaked and fell to below 50 °C rapidly within 2.5
minutes. Fire behavior observations recorded the flame front (head fire)
crossing the burn plot within less than 1 minute. The low peak surface temperatures and short
residence times can be attributed to the flame front flashing through the fuels
quickly as the result of wind and a slight 5% slope.
The
experimental design was altered for Plot 3.
Here a juniper branch (approx 2m long x 4-8 cm diameter) was added to
the unit by the experimenter.
Thermocouples 1-4 were placed at the surface within each respective
quadrant, and thermocouples 5-6 were placed directly beneath the juniper
branch. The burn plot was burned via a
head fire that crossed the unit in less than 1 minute with some residual
flaming combustion occurring for 2.5 minutes.
The Juniper branch did not combust, and was only slightly charred. Maximum surface temperatures ranged between
60.0-417.5 °C, and maximum temperatures recorded beneath the juniper ranged
only between 27.5-87.5 °C. The peak
temperature value for thermocouple 3 of 417.5 °C is anomalous, and may be the
result of the thermocouple coming out of position at the soil surface and
coming into direct contact with a flame.
Artifact
Surface Temperature Experiment
The
final component of the Pinnacles prescribed burn project consisted of an
experiment designed to assess the temperature differential between the upper
and lower surfaces of artifacts during burning in grassland fuels. Thermocouples were attached to the upper and
lower surfaces of 3 artifacts (a pronghorn mandible, a black-on-white pottery
sherd, and a Hartville Uplift chert flake).
In total, ten trials were performed using the same three artifacts in
each trial.
The
results of the ten trials show maximum temperatures on the upper surfaces of
experimental artifacts varying broadly between 37.4-268.9°C; however, the
average maximum upper surface temperature for the overall sample was 189.3°C. Maximum temperatures recorded on the lower
surfaces of the same artifacts also varied considerably ranging between
31.4-191.3°C. The overall average
maximum lower surface temperature was 92.1°C.
The temperature differential between peak upper and lower artifact
surface temperatures ranged between 6.0-115.8°C. The average temperature difference for the
entire sample was 97.2°C resulting a 49% average temperature differential
between peak upper and lower artifact surface temperatures. Figure 2.8 illustrates a typical
representation of time/temperature curves associated with the upper and lower
surfaces of artifacts during the combustion of fine fuels. In general, heating on the upper surfaces of
artifacts was precipitous in which temperatures peaked and declined
rapidly. Lower surface temperature
curves show that heating on the underside of artifacts was more protracted and
less severe compared to that recorded on representative upper surfaces. Overall, however, this differential in
heating between artifact surfaces was not sufficient to cause catastrophic
failure in any of the artifacts even after the tenth trial. The heat energy generated by combusting fine
grassland fuels lacked the intensity and duration to create tensile stresses of
the magnitude necessary to induce thermal fracturing of the experimental
artifacts The only observable impact of
the experiment on the artifacts was the deposition of a condensate tar deposit
that initially consisted of a thin light-brown coating after trial 1, and
increased in density thereafter as each trial was completed. At the end of 10 trials the artifacts were
entirely covered with a thick, dark-brown/black coating of combustive
residue. Significant forms of thermal
alteration such as thermal spalling, fracturing, or surface cracking were not
observed. This experiment further
illustrates the limited potential for cool season prescribed fires in grassland
fuels to significantly impact common archaeological materials.
Summary
The
results of the 2002 experiments at
Although
grassland fuels are homogeneous, there was a wide range of variability in the
maximum surface temperature recordings between and within each of the burn
plots. Combustion of these fine fuels
did not generate sufficient energy release capable of sustaining high
temperatures and long residence times at the soil surface. In general, residence times of flaming
combustion within 2x2m plots are relatively brief, typically ranging between
1-3 minutes depending on fire behavior.
Backing fires will produce longer residence times and the greatest window
of heating at the soil surface, generally between 2.5 - 3 minutes. Flanking fires will produce a maximum soil
surface-heating window of approximately 1.5 minutes. Head fires produce short window of soil
surface heating, generally less than 1 minute. Based on time temperature
curves, this generally equates to window of heating above 50°C for
approximately 3-6 minutes depending on fire behavior. In general, analysis of the time temperature
curves generated during the experiments show that initial heating on the
ascending portion of the curve is precipitous in which peak temperatures are
achieved within 30-60 seconds. The
residence time of peak and near peak temperatures was brief, typically
occurring for only 10-20 seconds. The
descending portions of the curves were generally more protracted as residual
heating is sustained after flaming combustion has ceased. Peak subsurface temperatures, even at –1cm,
are negligible generally producing maximum increases of only 13.4-7.7°C at –1cm
and –2cm respectively over baseline values.
These
data are consistent with the results reported by various by biological science
researchers. Archibold et al. (1998)
report average maximum surface temperatures of 189°C and 209°C during
prescribed burning in northern mixed prairie grasses. Similarly, Bailey and Anderson (1980)
reported an average maximum soil surface temperature of 186°C during burning in
northern grassland fuels. During burning
in annual grasses, Bently and Fenner (1958) recorded maximum soil surface temperatures
of 93-120°C. In
Concerning
the potential negative impact of prescribed burin in mixed grass prairie on
archaeological resources, the results of several experiments show that the
relatively low surface temperatures and short residence times associated with
the combustion of grassland fuels generate minimal thermal alteration of
experimental artifacts deposited at the soil surface. The most pervasive form of thermal alteration
affecting artifacts was the presence of condensate tar deposit formed during
the pyrolysis and combustion of organic fuels.
This deposit was generally light brown and color and was limited to the
upper surfaces of artifacts. It is
likely that over time, these deposits will weather from the surfaces of
artifacts since the residue can be removed using water and a pumice soap
solution. Some artifact classes with
organic components such as bone, and those that are completely organic such as
wood did exhibit minimal partial combustion along edges and upper surfaces. No catastrophic forms of thermal alteration
such as thermal fracturing, spalling, deformation, or surface cracking were
observed during the series of experiments performed at the park during 2001 and
2002. The results of the experiment
demonstrate that early-season prescribed burning in mixed grass prairie
environments will have a minimal impact on archaeological material located at
the soil surface. Archaeological
resources located greater than 1cm subsurface will be largely unaffected under
the same conditions. These results are
consistent with that reported by Brunswig et al. 1995, and that reported by
Sayler et al. 1989 for low fuel load burn plots.
MIXED GRASS PRAIRIE / PONDEROSA PINE EXPERIMENTS
Prescribed burning experiments in a mixed
grass prairie (multiple species) / Ponderosa pine (Pinus ponderosa)
environment were conducted at
2001
Experimental methods employed in 2001 at
The
burn plots consisted of 4 contiguous 2X2m units placed in a location with
variable fuels at the transition between grassland and Ponderosa Pine
stands. Fuel loads ranged between
roughly 4-6 tons per acre (1472-2208 kg/ha) depending on fuel type. Plot 1 contained grassland fuels only. Plot 2 was bisected by a dead/decayed
Ponderosa log (15cm dia.), associated branches (<3cm dia.), and
grasses. Plot 3 was placed adjacent to
the Ponderosa log such that some branches (<3cm dia.) but not the log itself
were present, grasses and 2 live Ponderosa sapling (<60cm tall) were also
present. Plot 4 contained grasses, 1
live Ponderosa sapling (<60cm tall), and few branches (<3cm dia.).
Results
For
safety reason, fire behavior observations were made from a distance making it
difficult to gather precise information.
However, the flame front developed into a head fire propelled by a 10mph
wind and appeared to be moving rapidly (500-1200m per hr) across the burn
plots. Flame lengths were estimated at
between 1-3m.
The
effect of the fire on the experimental artifacts was varied. A general trend can be identified that is
directly related to fuel load composition.
Artifacts from Plots 1 and 4, placed in light fuels were minimally
impacted. Observable effects included
slight or no combustive residue deposition and minor charring on the upper
surfaces of some bone/shell specimens.
Artifacts from Plots 2 and 3, placed near the Ponderosa log and litter,
exhibited significant thermal alteration.
Bone and shell artifacts were partially combusted and thermally
fractured/fissured as well as highly friable during handling post-fire. One pottery sherd exhibited thermal spalling,
two lead sinkers were melted, and wooden beads were 75-100% combusted. Overall, each specimen exhibited deeply
charred combustive residue deposits that heavily blackened the exposed surfaces
of affected artifacts. However, thermal
damage of lithic flakes (chert and obsidian) was not observed with the
exception of one obsidian flake that exhibited the enhancement of preexisting
radial fracture lines. Overall, the
results of the artifact analysis show that the heat energy generated by
combusting grass fuels (Plots 1-4) was not sufficient to significantly impact
most experimental artifacts. This
observation is consistent with the results of multiple grassland prescribed
experiments conducted at
Unfortunately,
temperature data were not recorded for this experiment due to an overloaded
memory in the data recorder (experimenter error). To obtain comparable temperature parameters,
the data logger was setup in a separate area that approximated the fuel
conditions under which the previous experiment was conducted. No experimental artifacts were included in
this experiment, and only temperature data was recorded. Thermocouples 1-2 were placed at the soil
surface in grass fuels only.
Thermocouple 3 was placed at the soil surface under a dead/decayed
Ponderosa log (15cm). Thermocouples 4-5
were placed at the soil surface within the branches (<3cm) and litter
associated with the log.
Temperature
data for this experiment is summarized in Figure 2.9. As expected, the temperature data were highly
variable between fuel load compositions.
Maximum temperature in grass fuels reached only 92.0 ˚C, peaking
rapidly within 1-1.5 minutes with elevated temperatures being sustained briefly
for only 10-20 seconds. Overall, surface
heating in grass fuels was sustained above 50°C for approximately 7 minutes. Maximum surface temperatures in branch/grass
fuels reached apex levels over a more protracted duration of approximately 10
minutes in which maximum values for the two thermocouples were recorded at
261.5°C and 471.0˚C. Here surface
temperatures in excess of 200°C were sustained for approximately 8 minutes and
20 minutes for each respective thermocouple.
Maximum surface temperatures recorded beneath the log were the greatest,
reaching the 615.1˚C mark. Here
temperatures greater than 500°C were sustained for approximately 22 minutes,
followed by a gradual reduction in temperature in which values remained in the
200˚C range for over 2 hours. These
data illustrate the significant temperature differential generated by fuels of
various composition within a small spatial unit, and further support the
assertion that fuel load and residence time are critical variable affecting the
potential for significant thermal alteration of archaeological resources. While these temperature data are not directly
correlated with observed thermal alteration of experimental artifacts, they can
be used as an analog by which to assess the thermal alteration of experimental
artifacts observed in Plot 1-4 during the initial experiment.
2002
During
the 2002 prescribed burn project at
Results
Fire
behavior was observed at a distance as the flames backed down into the drainage
from three sides. Flame lengths were
observed to be between 20-30cm, and burned across the burn blot within approximately
5 minutes. Glowing and flaming
combustion continued for approximately another 10 minutes, and smoldered for an
additional hour.
Graphical
summarizations of the time/temperature data for the experiment are provided in
Figure 2.10. Maximum surface temperatures
ranged between 286.2-513.5°C and reached apex levels within approximately 10
minutes within each quadrant. However,
the temperature gradient within each quadrant was varied. Quadrant 2 sustained heating >400°C for
approximately 16 minutes, quadrant 3 experienced temperatures >300°C for
approximately 9 minutes, quadrant 4 sustained heating >200°C for 22 minutes,
and temperatures >200°C within quadrant 1 were only sustained for
approximately 9 minutes. This
variability is likely due to differential fuel combustion within the burn
plot. Subsurface temperatures at –1cm
increased from 4.8-74.7 °C, and temperatures at –2cm increased from 8.0-64.2
°C; both over a period of approximately 30 minutes. Overall, the greatest
proportion of surface and subsurface heating occurred over a period of 25
minutes as temperatures peaked and fell gradually to below 50 °C after
approximately 45 minutes. These data
suggest that duff and litter fuels can generate sustained surface heating in
the 200-400°C range for approximately 10-20 minutes.
Post-fire
analysis of the experimental artifacts shows that all specimens exhibited
heavily blackened combustive residue deposits on exposed surfaces. Significant
thermal alteration was pervasive for bone, antler, and shell specimens, which
exhibited intensive thermal fracturing/fissuring, heavy charring/partial
combustion, and a considerably increase in friability. Wooden beads from each quadrant sustained
combustion rates of 75-100%. In
addition, one pottery sherd from quadrant 3 exhibited thermal spalling and
fracturing, and a lead sinker form the same sample sustained complete
deformation due to melting. One glass
fragment (bottle glass) from the quadrant 4 sample exhibited thermal
spalling. None of the lithic specimens
from the overall sample exhibited thermal fracturing. Overall, the impact of the burn on the
experimental artifact was most pronounced and consistent for organic
specimens. This experiment illustrates
the potential for combusting Ponderosa pine litter and duff fuels to generate
sufficient heat energy to significantly alter some types of archaeological
resources deposited at the mineral soil surface. Thermal alteration of subsurface
archaeological deposits > -2cm is not probable. The results of this experiment are similar to
those observed during the previous experiment conducted during the Bison Flats
prescribed burn at
Summary
The
results of the two prescribed burn experiments conducted at
MIXED
CONIFER
Fire
effects experimentation was conducted in a mixed conifer environment within the
2001
Experimental
methods implemented during this project were similar to those discussed for
previous experiments. One 2X2m burn plot
was setup with approximately 50 experimental artifacts (similar to artifacts
used in previous experiments). The plot
was orientated such that a dead/decayed conifer log (30cm dia.) and associated
branches bisected the unit. Other fuels
included a 5-10cm thick duff/litter accumulation, minimal grasses, and
forbs. Thermocouples 1-4 were placed
within in each respective 1m2 quadrant at the mineral soil surface beneath
the duff/litter accumulation.
Thermocouple 5 was placed –5cm subsurface beneath the log, and
thermocouple 6 was placed –10cm subsurface underneath the log. Artifacts positioned in quadrants 1 and 3
were placed at the mineral soil surface (beneath the duff) in close association
(~5-10cm) with the log. Artifacts
associated with quadrant 2 were placed at the soil surface beneath a 5cm duff
accumulation not in association with the log (~70cm NW of log). Quadrant 4 artifacts were positioned at the
soil surface beneath an 8cm accumulation of duff, and approximately 30 cm N of
the log. The variable fuel composition
within each quadrant was structured to assess the potential for differential
soil surface heating and artifact thermal alteration.
Results
Fire
behavior observations show that a head fire, pushed by 5-8 mph wind velocity,
generated 1-3m flame lengths, and passed over the burn plot within
approximately 5 minutes. Flaming and glowing combustion continued in lighter
fuels for approximately 15 minutes; however, the log and other larger fuels
sustained flaming and glowing combustion for over 3 hours.
Maximum
surface temperatures within quadrants 1 and 3 (close proximity to log) reached
peak values of 350.9˚C and 326.0˚C gradually over a prolonged period
of approximately 4-6 hours. Surface temperatures within these quadrants also
declined slowly, whereby temperatures >100°C were sustained for 6.6-7.5
hours. Peak surface temperatures within quadrant 2 (light fuels) reached
397˚C rapidly within 8 minutes, then began to decline quickly with
temperatures >100°C being sustained for only 45 minutes. Surface heating within quadrant 4 (light
fuel) reached a maximum value of 188.7˚C over a period of approximately 45
minutes. Surface temperatures here were
sustained at >100°C for approximately 1 hour. Duff combustion within quadrant 2 was more
extensive than within quadrant 4, possibly accounting for the differential in
maximum temperature and duration of heating observed between the two
quadrants. This suggests that duff may
mitigate the soil surface heating in instances where duff is not heavily
combusted. The maximum subsurface
temperature at –5cm beneath the log was 139.0˚C, which was recorded over 4
hours after flaming combustion was initiated within the plot. Similarly, the maximum temperature at –10cm
of 107.5˚C, was recorded over 5 hours post ignition as the log continued
to combust over an extended period.
Time/temperature data for the experiment are presented graphically in
Figure 2.11. These data illustrate the
variability in surface heating recorded within the burn plot. The time/temperature curves associated with
heavy fuel (log) are smooth and protracted, whereas those associated with
lighter fuels (duff) show precipitous temperature gradients. Subsurface curves are smooth and protracted
suggesting uniform heating below the surface of the log.
The impact of the fire on the
experimental artifacts was proportional to their proximity to the heavier fuel
load of the log. Artifacts in quadrants
2 and 4 (light fuels) exhibited heavy combustive residue deposits on the upper surfaces
of all specimens. Bone and shell
specimens from these quadrants also exhibited moderate charring and partial
combustion on upper surfaces. No thermal
fracturing, spalling, or deformation was observed. Within quadrants 1 and 3, bone and shell
exhibited heavy charring and combustion, as well as thermal
fracturing/fissuring and increased friability.
In addition, lead was melted, wooden beads were partially combusted, and
obsidian artifacts exhibited enhanced radial fracture lines.
The
results of the post-fire artifact analysis illustrate the direct relationship
between the potential impact of prescribed fire on surface archaeological
materials and fuel load composition.
Artifacts positioned near the log were subjected to temperatures in excess
of 300˚C for over 1 hour in quadrant 3 and three hours in quadrant 1. Subsequently these artifacts were more
heavily impacted by the release of radiant heat energy from the heavier fuel
compared to artifacts burned in duff where temperatures peaked and fell more
rapidly. Subsurface temperatures –5cm
and –10cm beneath the log were elevated over 100˚C for 1-2 hours; however,
it is unlikely that archaeological materials deposited at these depths would
have been significantly impacted since peak temperatures were low and heating
was uniform and prolonged over a period of several hours. Overall, the results of the experiment
further support the assertion that heavier fuels equate to greater potential
heat energy output during combustion, which in turn, increases the potential
for significant thermal alteration of archaeological resources.
Log Burning Experiment
In addition
to the burn plot experiment conducted during the 2001 prescribed burn, a log
burning experiment consisting of four trials was also performed. Each of the four trials consisted of a 1x1m
burn unit positioned over a downed/dead conifer log and associated surface
litter and duff. Temperature and time
data were recorded using the data logger and 6 thermocouple leads placed in a
linear orientation beneath logs and associated litter/duff at the contact with
the mineral soil. Lithic artifacts were
also placed at the soil surface in association with the thermocouples. The lithics were modernly replicated flaking
debris representing four selected lithic material types common in the
archaeological record (obsidian n=10, porcelanite n=8, phosphoria n=6, pink
bioclastic chert n=8, N=32). Prior to
burning each specimen was measured, weighed, and assigned a Munsell color
value. Post-burn, each specimen was
analyzed for evidence of thermal alteration, weighed and assigned a Munsell
color value. The purpose of this
experiment was to collect additional temperature data associated with the
combustion of conifer log fuels, and to assess the impact of the heat energy
released during combustion on a range of lithic material types.
Results
Fuels in
Trial 1 consisted of a 15cm diameter conifer log that extended across the unit
and a thin duff accumulation, measuring approximately 1cm in depth. Fuel combustion during burning was uniform
and complete. Maximum surface
temperatures beneath the log ranged widely between 269.0-878.9°C. Temperatures peaked within 10-35 minutes and
tapered off to <100°C over a period of 4 hours. Thermal alteration of lithic specimens
consisted of a combustive residue deposit, resulting in a blackened
discoloration on the upper surfaces of all specimens. In addition to the combustive residue
deposits, the chert and phosphoria flakes also exhibited an overall color
change to a darker Munsell value suggesting that heating had altered the
mineralogy of each specimen. More
importantly, one of the chert flakes was thermally fractured into 3 pieces
during this experiment. The peak
temperature associated with this specimen was 348.9°C, which was recorded
approximately 30 minutes after initiation of flaming combustion. The obsidian flakes exhibited a metallic
sheen and the enhancement of radial fracture lines on upper surfaces. The peak temperature associated with these
specimens was 276.6°C, recorded within 30 minutes of initial combustion. Porcelinite flakes exhibited no structural
damage during the trial.
Trial
2 fuels consisted of one 10cm diameter log extending the across the unit, and a
3cm deep duff accumulation. Fuel combustion
was uniform and complete during the trial.
Maximum surface temperatures ranged between 366.4-602.0˚C. Temperatures peaked within 20-40 minutes and
declined slowly to <100°C over an extended period of approximately 4
hours. Thermal alteration of the
experimental lithics was similar to those discussed for trial 1. One of the chert flakes was thermally
fractured into 2 pieces, and exhibited the same shift in overall color as seen
in trial 1. The maximum temperature associated with this specimen was 424.1°C. In addition, one of the phosphoria flakes
exhibited a classic potlid fracture (28.33mm in dia.) on its upper
surface. The maximum temperature
associated with this specimen was 490.6°C.
The obsidian specimens exhibited a metallic sheen and enhanced radial
fracture lines on upper surfaces. The
maximum temperature associated with these specimens was 432.5°C. All specimens were discolored by combustive
residue deposition on upper surfaces, and the chert and phosphoria specimens
exhibited a shift to a darker hue suggestive of mineral alteration.
Fuels in
Trial 3 consisted of a conifer log (10cm in diameter) that extended across the
burn unit, and a duff accumulation measuring approximately 3cm in
thickness. Fuel combustion during the
trial was uniform and complete. Maximum
surface temperatures ranged between 319.7-540.8˚C. Temperatures peaked rapidly within 15-30
minutes followed by a gradual reduction in temperature in which temperatures
>100°C were sustained for over 4 hours.
Thermal alteration of the experimental lithics was consistent with that
reported for the previous trials.
Combustive residue deposits were observed on the upper surfaces of all
specimens, and chert and phosphoria specimens exhibited color alterations
suggestive of mineral oxidation. The
obsidian specimens exhibited enhanced radial fracture lines as well as a
metallic sheen on upper surfaces. The
peak temperature associated with these specimens was 450.7°C. In addition, one of the phosphoria flakes
exhibited a small (3.49mm) potlid fracture.
The other phosphoria flake exhibited fine crazing on the upper
surface. The peak temperature associated
with these specimens was 362.8°C.
The burn
unit in trial 4 included two logs extended across the 1x1m unit, each of which
measured approximately 30cm in diameter.
The duff accumulation within the unit was approximately 7 cm thick. Compared to the previous experiments, the
fuels in this experiment were the heaviest.
Fuel combustion during the trial was uniform and complete. Maximum surface temperatures ranged from
403.4˚C to 831.4˚C, peaking within a variable period of 15-60
minutes. Heating within the unit was
erratic due to natural movement of one of the logs during combustion. Thermal alteration of the experimental lithics
was similar to those reported previously.
All specimens exhibited combustive residue deposition on upper
surfaces. Obsidian specimens exhibited
enhanced radial fracture lines as well as an alteration of the upper surfaces
in which the original color was altered to a metallic sheen. The maximum temperature associated with these
specimens was 567.7°C. Interestingly,
although surface heating was considerable during the trial, no chert of
phosphoria specimens sustained thermal fractures. There was, however, one exception in which
one porcelinite flake was thermally fractured into 2 pieces. The maximum temperature associated with this
specimen was 831.4°C.
Combined
time and temperature data for all 4 experiments are summarized in Figure
2.12. The uppermost peak surface
temperature recorded for trials 1-4 ranged between 878.9-503.2˚C. During each trial, surface temperatures in
excess of 200˚C were maintained for up to 3 hours. In general, temperatures reached apex levels
within a broad period between 10-60 minutes; however, the decline in
temperature from peak levels was typically more gradual in which temperatures
>100°C were sustained for approximately 4 hours. Overall, a wide range of maximum temperatures,
and variable rates of temperature increase and decrease were recorded during
the experiment. This variability is
likely the result of differential rates of glowing and flaming combustion and
oxygen availability that occurred while the log was being consumed. These data suggest that surface heating
beneath combusting logs is characterized by relatively rapid temperature ascent
to peak levels, prolonged periods of sustained high temperature, and protracted
declines in temperature on the descending side of maximum values.
Thermal
alteration of the experimental lithics from each experiment can be summarized
as follows:
Obsidian: Blackening/carbonization, chrome-like gloss,
and linear crazing occurring on the side-up surfaces, observed in 100% of the
sample (N=10). Maximum temperatures
associated with these form of thermal alteration ranged between 276.6-567.7°C
for an average temperature of 431.8°C.
Chert: Blackening/carbonization as well as an
overall color change from 10R 7/3 to 10R 6/3, a darker value for the 10R hue,
observed in 100% of the sample (N=8).
Extensive fracturing resulting in the flake breaking into multiple
pieces, observed in 25% of the sample.
Maximum temperatures associated with fractured specimens ranged between
348.9-424.1°C.
Phosphoria:
Blackening/carbonization, and an overall color change from 10R 3/4 to 10R 3/2,
a darker chroma value, observed in 100% of the sample (N=6). Potlid fracturing was observed in 30% of the
sample, and crazing was observed in 16% of the sample. Maximum temperatures associated with thermal
fracturing and crazing ranged between 362.8-490.6°C.
Porcelanite:
Blackening/carbonization with no overall color shift, observed in 100% of the
sample. Fracturing was observed in 12%
of the sample (N=8). The maximum
temperature associated with the thermally fractured specimen was 831.4°C.
These
data suggest the levels of heat energy released during the combustion of
dead/decayed conifer logs during prescribed burns is sufficient to induce
significant thermal alteration of selected lithic material types included in
the experiment, given that specimens are deposited directly beneath logs. Significant forms of thermal alteration
observed included thermal fracturing and mineral oxidation of pink bioclastic
chert, potlid fracturing and crazing of phosphoria, enhanced radial fracture
line propagation and surface alteration (metallic sheen) of obsidian, and
thermal fracture of porcelinite under extreme temperature gradient.
2002 Pike National
Forest
Fire
impact experimentation was resumed within the
The
experiment consisted of a 1x1m burn plot placed over a 20cm diameter conifer
log that was associated with a 2-3cm duff accumulation (mostly loose pine
needle). Experimental artifacts
consisted of 2 deer (Odocoileus sp.) metatarsals, 2 black-on-white
pottery sherds, 2 Hartville Uplift chert nodules, 2 obsidian secondary flakes,
and 2
Results
Fire
behavior observations recorded during combustion show that the plot was burned
during a slow moving low-intensity backing fire that produced flame lengths of
approximately 20-30cm. Time/temperature
data for soil surface measurements are summarized graphically in Figure 2.13. The figure clearly illustrates the
considerable differences in soil surface heating associated with the log, duff
only, and subsurface beneath the log.
Temperatures directly beneath the log reached peak values of 632.6°C and
739.8°C. The temperature gradient was
characterized by a rapid ascent to apex levels within 5-7 minutes with
temperatures >500°C being sustained for 7-12 minutes and temperatures
>300°C being sustained for 20-40 minutes.
Thermocouples associated with duff fuels only reached peak levels of
223.9-259.3°C rapidly within approximately 3 minutes. Temperatures then declined rapidly to
<100°C within approximately 5-8 minutes.
Subsurface temperatures beneath the log at –2cm reached a peak value of
226.0°C, and 105.7°C at the –5m level. Here
the temperature gradient was smooth and protracted in which peak values were
recorded after approximately 1 hour, and heating >50°C was sustained for
approximately 3 hours. These data
illustrated the variability in soil surface heating on the floor of a mixed
conifer forest during a low-intensity fire.
Heating is extreme where large fuels are combusted and minimal where
only duff is consumed.
The
result of the artifact heating component of the experiment show that peak
temperatures on the upper surfaces of selected artifacts ranged between
753.5-583.4°C and maximum values on the lower surfaces of artifacts were
recorded in the 515.1-436.8°C range.
Time/temperature data are provided in graphical form in Figure 2.14. Upper surface peak temperatures were achieved
rapidly within 2-10 minutes and lower surface maxima were recorded within
approximately 10-20 minutes. Upper
surface heating was erratic and precipitous, while heating on the lower
surfaces of artifacts was more uniform and protracted. Overall, artifacts experienced temperatures
>300°C for a prolonged period of 30-60 minutes.
Thermal
alteration of experimental artifacts was significant. All specimens exhibited a heavily charred
combustive residue deposit on exposed surfaces.
Bone specimens were heavily charred, combusted, fractured/fissured, and
very friable. Peak upper and lower
surface temperatures associated with the bone specimen with attached
thermocouples were 735.5°C and 463.4°C respectively. Peak temperatures associated with the chert
nodule were 682.5°C for the upper surface and 436.8°C for the lower
surface. This specimen exhibited extreme
thermal fracturing that reduced the specimen to many small fragments (>50),
and sustained mineral oxidation (probably limonite to hematite) that altered
the original yellow-brown color to red.
The chert primary flake also exhibited thermal fractures, but to a
lesser extent than the nodule, as well as a shift in color to dark red
suggestive of mineral oxidation. The
upper surface maximum temperature recorded for the pottery sherd was 583.4°C,
and the peak lower surface value was 515.1°C.
No thermal fracturing or spalling was observed for either of the pottery
sherds, only a highly blackened combustive residue deposit on the upper surface
of the sherds was noted. This deposit
did, however, obscure the design characteristics of the sherds. Obsidian artifacts exhibited enhanced radial
fracture lines and a metallic sheen on upper surfaces, but evidence of thermal
fracturing was not observed. No
temperature data were recorded for these specimens.
Overall,
the results of this experiment illustrate the variability in surface heating
during low-intensity wildland fire in a mixed conifer environment. Soil surface temperatures were highly
variable depending on fuel load composition.
Light duff fuels generated low peak temperatures (223.9-259.3°C)
characterized by brief residence time in which temperature peaked within 3
minutes and fell off sharply within 5-8 minutes. Conversely, peak soil surface temperature
beneath a combusting log ranged between 623.56-739.8°C and were sustained at
>300°C for 20-40 minutes. Most
experimental artifacts associated with the log experienced significant degrees
of thermal alteration, particularly bone and chert specimens. However, pottery sherds were not affected
structurally. Upper surface temperatures
recorded on selected artifact were consistent with soil surface temperatures
recorded beneath the log. In sum, the
combined data illustrate the important relationship between fuel load as it
affects the potential for significant thermal alteration of archaeological
materials during wildland and prescribed fire.
Summary
In
sum, the experiments performed in conjunction with prescribed and low-intensity
wildland fires in mixed conifer fuels suggest that the impact of heat energy
released during combustion on archaeological materials is variable and largely
dependent on artifact class and the association of artifacts relative to large
fuels. Decayed logs on the forest floor
can generate soil surface temperatures in the 400-800°C. Artifacts associated with these fuels
generally exhibit significant thermal alteration such as thermal fracturing
among lithics and heavy charring, combustion, and fracturing of bone and shell
specimens. The results of several trials
in which combusting logs were present show that thermal fracturing of chert and
phosphoria artifacts is initiated when soil surface temperatures reach the
350-490°C range, and sustain pronounced fracturing when the upper surfaces of
artifacts reach the 500-680°C range.
Thermal fracturing of chert artifacts is most prominent for larger
specimens such as nodules (cores, etc.) where the potential for
disproportionate heating within the material is greater due to larger body
mass. Mineral oxidation within
RIPARIAN ZONE AND SAGEBRUSH COMMUNITY EXPERIMENTS
Prescribed
burn experiments were conducted at
Jackson Lake
Fuels in
the burn area consisted predominantly of various willow species and associated
grasses in a low-lying riparian type environment. Consequently, the soil surface contained high
levels of moisture content. The
experiment included two 2x2m burn plots in which experimental artifacts were
burned, two 1x1m burn plots in which upper and lower surface temperatures of
experimental artifacts were recorded, and two additional trials where only
surface temperatures beneath a large willow and small sagebrush were
recorded.
The
first component of the experiment consisted of one 2x2m burn plot (Plot 1) that
encompassed a small booth willow (Salix boothii) (1.5 x 2m) and
associated grasses, largely beaked sedge (Carex rostrata). Experimental artifacts (N=14), representing
the range of artifact classes (i.e., bone, shell, chert, obsidian, glass, and
metals) included in previous experiments were placed in quadrant 1 only. Temperature data were recorded using the
primary
Fire
behavior observations near each of the 3 burn plots (encompassed within a 10m2
area) show that burning within the units was accomplished by a backing
fire, which generated flame lengths of approximately 30-50cm and a rate of
spread of approximately 1m per 2.5 minutes.
The fire consumed the upper organics on the soil surface only, leaving
the willows partially scorched to approximately 45cm from the base. The duff layer was not combusted and the O
horizon remained cool and moist.
Time/temperature
data for Plot 1 are summarized in Figure 2.15.
The results from plot 1 show maximum surface temperatures ranging only
between 95.9-140.4 °C. Temperature
reached apex levels rapidly within 1.5 minutes, and only sustained heating
>50°C for approximately 9 minutes.
Peak temperatures were maintained over a period of approximately 30
seconds. Subsurface temperatures at –1cm
rose only slightly from 4.3-8.0 °C, and temperatures at –2cm were also elevated
only slightly from 4.0-7.0 °C.
Time/temperature curves for the subsurface thermocouples were very
protracted, indicating very slow heating.
Time/temperature
data for upper and lower artifacts surfaces from Plot 2 are summarized in
Figure 2.16. These data show that
heating on the upper surfaces of artifacts was precipitous and erratic, while
heating on the lower surfaces was minimal and protracted. Peak temperatures recorded on upper artifacts
surfaces reached highly variable peak temperatures of 265.7-28.0 °C. Heating on the upper surfaces of artifacts
was rapid, reaching peak levels within approximately 30 seconds. Elevated temperature were sustained only
briefly for a few seconds, then temperature fell off to <50°C within 4-10
minutes. These data are consistent with
soil surface heating observed in Plot 1.
Peak temperatures recorded on lower artifact surfaces varied between
minimally between 28.0-59.3 °C.
Time/temperature
data for artifact surfaces from Plot 3 are provided in Figure 2.17. Within this burn plot, peak temperatures on
the upper surfaces of artifacts ranged moderately between 244.5-355.0 °C. Peak values were reached within approximately
30 seconds and sustained very briefly.
However, overall heating on the upper surfaces of artifacts was more
significant within Plots 3 with temperatures of >100°C being sustained for
5-15 minutes. Heating recorded from
lower artifact surfaces attained maximum temperatures ranging from 47.8-215.2
°C. The 215.2°C is anomalous, and is
likely the result of a thermocouple becoming dislodged from the under-side of
the artifact during burning. Overall, lower surface heating was slow and
uniform, and upper surface heating was rapid and irregular.
The impact
of the fire on all of the experimental artifacts from each of the three burn
plots was minimal. Post-burn analysis of
the artifacts showed that the only observable form of thermal alteration
present was a light deposit of golden-brown combustive residue on the upper
surfaces of artifacts, similar to that observed during the mixed grass prairie
experiments. Bone and shell specimens
were not charred or partial combusted, only slightly discolored. No significant forms of thermal alteration of
experimental artifacts such as thermal spalling, cracking, or fracturing were
observed. The results of the artifact
analysis are consistent with the temperature data generated during the
experiment. The heat energy generated
during combustion was characterized by rapid heating in which peak values were
sustained for approximately 30 seconds, followed by a rapid decline in
temperature within a brief period of 5-15 minutes. The fuels within each plot did not generate
the temperature gradient necessary to initiate thermal fracturing, charring, or
other types of thermal damage within the artifact sample.
During
the experiment it became apparent that combustion of small willow and fine
grasses during the prescribed burn would not generate the heat energy necessary
to significantly impact experimental artifacts.
As a result, an additional 1x1m burn plot was established in heavier
fuels. Burn Plot 4 was placed at the
base of a large willow (3.5 x 3.5m, Salix geyeri ?) that was also
associated with an accumulation of small dead fuels. Fourteen experimental artifacts (various
classes, bone, lithic, metal, glass, etc.) were placed at the soil surface next
to the base of the willow. Thermocouple
6 was placed at the soil surface in association with the artifacts. The remaining thermocouples were placed at
25cm intervals in a linear orientation radiating outward from the base of the
willow (as in experiment 2). The purpose
of this trial was to establish the temperature ranges generated at the base of
a large willow, and at intervals radiating outwards beneath its understory.
Fire
behavior observation show that the willow ignited and torched into the crown
rapidly, and flaming combustion was observed for approximately 17 minutes. Glowing and smoldering combustion was
observed for an additional 18 minutes, and the willow was observed to be
approximately 80-85% combusted at the end of the combustion phase. Time/temperature data associated with Plot 4
are summarized in Figure 2.18. The
maximum surface temperature recorded by the thermocouple associated with the
artifacts reached 497.8 °C. Here
temperatures peaked within 9 minutes and were sustained at near peak levels for
approximately 30-60 seconds.
Temperatures in the range of >300°C were sustained for ~10 minutes,
>200°C were sustained for ~20 minutes and >100°C were maintained for ~30
minutes. The remaining 5 thermocouples recorded maximum surface temperatures
ranging between 52.5-320.2 °C. Peak
temperatures diminished in magnitude with each 25cm interval radiating from the
base of the willow. However, heating was
uniform across each thermocouple with each time/temperature curve following a
similar contour. Overall, surface
temperatures peaked and fell gradually to below 50 °C over a period of
approximately 50 minutes.
Post-burn
analysis of experimental artifacts showed the presence of a moderate to heavy
combustive residue deposit present on the exposed surfaces of all specimens.
The bottle glass specimen exhibited a thermal fracture that split the specimen
into halves. Bone specimens exhibited
charring and partial combustion on upper surfaces as well as the propagation of
surface cracks. Thermal fracturing of
lithic materials was not observed. This
trial illustrates that the residence time and maximum temperatures associated
with a large, heavily combusted willow are sufficient to significantly impact
some types of archaeological materials, particularly organics and glass. However, these materials were placed directly
at the base of the willow. Materials
deposited subsurface or at distances greater than 25cm from the base are
unlikely to be significantly impacted.
Two
additional trials were conducted during the
The willow
was burned via a backing fire with flames reaching into is crown, however,
burning was sporadic and combustion of the willow was less than 50%. The
maximum surface temperatures for thermocouples positioned near the base of the
willow ranged between 257.0-289.3 °C.
The maximum surface temperatures for thermocouples positioned 25cm from
the base ranged between 126.9-84.4 °C.
The thermocouple placed 50cm away from the base recorded a maximum
surface temperature of only 26.5 °C.
Maximum temperatures were maintained for approximately 2.5 minutes with
temperatures tapering out to below 50 °C within approximately 8.5 minutes. Maximum surface temperatures next to the
willow base were significantly higher than those 25cm away from the base. Higher concentrations of live and dead fuels
were observed near the base of the willow prior to ignition. These fuels may be heavy enough to sustain
critical temperatures in the range that can significantly impact archaeological
materials deposited at the soil surface.
The
following trial was similar to the previous with the exception that the source
of fuel was a small sagebrush (Artemisia tridentada, 1.5 x 1.5m). Thermocouples 1-2 were placed at the soil
surface adjacent to the trunk of the sagebrush.
Thermocouple 3 was placed at the soil surface 15cm away from the base beneath
the radiating branches of the sagebrush.
Thermocouples 4-5 were placed 25cm from the base, and 6 was positioned
50cm from the base. During the
experiment thermocouple 1 popped out of position at the soil surface and was
suspended at approximately 5cm above the soil surface. The maximum temperature recorded for this
thermocouple was an anomalous 701.2 °C.
The maximum surface temperature for thermocouple 2 was 212.8 °C. The
maximum surface temperatures recorded by thermocouple 3-6 ranged between
336.8-163.0 °C. Temperatures peaked and
fell gradually to below 50 °C over a period of 20 minutes. Maximum temperatures for thermocouples 2-6
were fairly consistent regardless of their position relative the trunk of the
sagebrush.
Summary
The
result of the Jackson Lake experiment demonstrate that soil surface
temperatures generated by a prescribed fire in a riparian zone dominated by
willow and sedges can vary significantly depending on fire behavior, the size
of fuels, and extent of combustion of fuels.
The fuels ignited in Plot 1 were characterized by small willows and
grasses which, when ignited, only generated maximum surface temperatures
between 95.9-140.4 °C. These
temperatures are not sufficient to generate radiant heat energy capable of
significantly affecting most archaeological materials (with the exception of
wood and other organics). Similarly,
heating of artifact surfaces within the same fuel composition produced peak
temperatures of up to 355.0°C on the upper surfaces of artifacts, and nearly
60°C on lower surfaces (excluding anomalous value). This is a significant temperature
differential; however the severity and duration the heat energy generated by
fine fuels was not sufficient to initiate stress within artifacts capable of
producing thermal fracture. However,
the results from Plot 4 illustrate that temperatures generated at the soil
surface by a large willow produce maximum temperatures of 497.8 °C (at the
base), and have a more sustained residence time compared to lighter fuels. Enough radiant heat energy is transmitted to
the soil surface by these larger fuels to significantly impact some
archaeological materials, if the materials are deposited near the base of large
willows. Overall, the impact of spring prescribed burning in a riparian zone on
archaeological materials is mitigated by high soil and fuel moisture
content. Archaeological materials will
be significantly impacted only if sufficient combustion of large fuels occurs
in tandem with closely associated surface archaeological materials. However, it is highly unlikely that
subsurface archaeological deposits will be adversely affected during prescribed
burns in riparian environments.
Kelly Prescribed Burn
Additional
fieldwork was conducted at
Plot 1
consisted of one 2x2m burn unit situated within a group of 9 small to medium
sized (1m x .75m, 7cm dia. trunk) Artemisia tridentata and associated
grasses. Sagebrush canopies were
relatively thin, and dead under-story accumulations were sparse. The 2x2m plot was divided into four 1x1m
quadrants, each containing one or two sagebrush and associated grasses. Thermocouples 1-4 were placed at the soil
surface approximately 10cm from the base of a sagebrush in each of the four
quadrants. Experimental artifacts were
place in each quadrant within a 20cm radius of each respective thermocouple
lead. Experimental artifacts were the
same for each quadrant and consisted of analogs of common prehistoric and
historic artifacts (bone, shell, chert, obsidian, pottery, beads [glass and
wood], glass, lead, copper, and brass).
Fire
behavior observations show that burn plot ignition was achieved via a head
fire, driven by a 3-5mph wind, which generated flame lengths reaching
1-2m. Flaming combustion within Plot 1
was observed for approximately 2min40sec, and fuels were 70-90% consumed as
combustion ceased. Time temperature data
for Plot 1 are summarized in Figure 2.19.
These data show that maximum surface temperatures within quadrants 1-4
ranged between 166.8-310.8 °C. Surface
temperatures reached apex levels within approximately 2-3 minutes, and elevated
temperatures were sustained for approximately 1 minute. The time / temperature
curves for thermocouples 2-4 were very similar, however, the data for
thermocouple 1 was somewhat anomalous.
Thermocouple 1 recorded a maximum temperature of 310.8 °C and maintained
temperature above 200 °C for over 6 minutes before gradually tapering off to
below 50 °C after 26 minutes. Fuels in
quadrant 1 consisted of 2 medium-sized Artemisia tridentada, one of
which was partially dead and likely had lower moisture content. Greater potential live and dead fuel mass
within quadrant 1 is likely the reason for higher temperatures and broader
temperature curve observed for thermocouple 1.
Overall, surface temperatures peaked rapidly and diminished to <100°C
with varying durations of between 4.5-13.5 minutes. Maximum subsurface temperatures at –1cm and
–2cm were 35.1 °C and 26.0 °C respectively, and were only elevated 15-25 °C
over baseline values. The results for
Plot 1 are consistent with those observed for one individual sagebrush burned
during the Jackson Lake Lodge prescribed burn in May of 2002 where maximum
surface temperatures ranged between 336.8-163.0 °C, and where temperatures fell
to below 50 °C over a period of 20 minutes.
Post-burn
analysis of over 50 experimental artifacts showed that all specimens exhibited
moderate combustive residue deposits on exposed surfaces. Bone specimens were most prone to the effects
of thermal alteration across all quadrants.
All specimens exhibited charring and partially combustion on upper
surfaces, and existing fissures and cracks were exacerbated by thermal
stress. One chert secondary flake from
quadrant 3 was thermally fractured, as was a chert nodule from quadrant 1. Peak
surface temperatures within these quadrants reached 166.8°C and 310.8°C respectively;
however, it is probable that the surfaces of these specimens experienced higher
temperatures. In addition, a glass
bottle fragment located in quadrant 2, where the peak surface temperature was
193.3°C also exhibited thermally fractures.
The results of the artifact analysis combined with recorded temperature
data indicate the radiant heat energy emitted by combusting sagebrush is
sufficient to elicit thermal fractures in chert materials, glass, and
bone. Fracturing of chert and glass was,
however, limited only affecting less than 20% of the sample. Charring and enhanced cracking and bone was
universal across the entire sample. The
deposition of combustive residue on the upper surfaces of experimental artifacts
was also pervasive. This compound is a
condensate tar produced during pyrolysis and combustion of organic plant
material, and is unlikely to significantly impact most archaeological material
types.
Plot 2
consisted of a 2x2m burn plot placed within 6 medium-large (1m x 1m, 7cm dia
trunk.) Artemisia tridentada and associated fine grasses. Fuels in this
plot were denser and had greater accumulations of dead fuels (dead under story)
compared to the fuels within Plot 1. No
experimental artifacts were included in this experiment. Thermocouples 1-6 were placed at the base of
one large sagebrush within quadrant 2.
The thermocouples were placed in a linear orientation at 3cm intervals
beginning with thermocouple 1, which was placed at the base of the sagebrush
trunk. The purpose of this experiment
was to establish time and temperature data for areas at the soil surface
beneath the canopy of large sagebrush.
The
time/temperature data for Plot 2 are summarized in Figure 2.20. Results of this experiment show maximum
temperature ranging between 238.7-522.2 °C.
Flaming combustion was observed in the plot for approximately 7 minutes
with flame lengths reaching the 1-2m range.
Post-fire combustion of fuels within the plot was estimated at 90%. Time and temperature curves for thermocouples
2-6 were very similar with temperatures peaking rapidly to over 450 °C, and
falling off gradually to 50 °C over a period of 43 minutes. Temperature for thermocouples 2-6 were
maintained above 400 °C for 6 minutes, above 300 °C for 9 minutes, and above
200 °C for up to 18 minutes.
Thermocouple 1 produced the only anomalous recording with a maximum
temperature of 238.7 °C and gradual curve that dropped to 50 °C within 30
minutes. This thermocouple was
positioned next to the sagebrush trunk, which did not combust as consistently
as its dead under story. The dead under
story associated with thermocouples 2-6 did combust uniformly and completely
resulting in higher maximum temperature peaks and exposure to longer periods of
temperatures above 200 °C under the time temperature curve.
This
experiment demonstrates that dense accumulations of large sagebrush with
significant accumulations of dead under story can produce maximum soil surface
temperatures above 500 °C accompanied by protracted temperature curves. The amount of radiant heat energy emitted
within Plot 2 would have been sufficient to produce significant thermal stress
in many classes of archaeological material, particularly bone, shell, and
cherts.
The results
of the overall experiment show that sagebrush fuels are variable, however, even
the lower end of the spectrum archaeological materials positioned at the soil
surface beneath the under story of sagebrush are subject to thermal
damage. The degree of thermal alteration
is variable as well and depends largely on artifact class, fuel load/composition,
position of artifact relevant to fuels, and fuel combustion
characteristics.
PIÑON-JUNIPER EXPERIMENT
Field
based experimentation focused on assessing the impact of prescribed burning on
archaeological resources was also conducted in a piñon-juniper environment
during the
Results
Fire
behavior observations showed that the fire in the vicinity of the burn plot was
severe. The plot was burned via a head
fire pushed by a 6-10 mph wind, which generated 20-30ft flame lengths as large
dead ground fuels were ignited and live piñon-juniper fuels torched into the
crowns. Large flames and torching
continued for several minutes.
Post-fire, log fuels and duff within the burn plot were 100% combusted
and live fuels were completely torched.
Patches of soil within and near the burn plot were oxidized to a strong
orange color. The fire was sufficiently
intense to destroy the protective fire box that contained the data logger and
rendered the LCD screen on the data logger unreadable for several hours. This burn was considerably more intense than
that encountered during any of the previous prescribed fire experiments.
Time/temperature
data for the experiment are summarized in graphical form in Figure 2.21. These data show that peak surface
temperatures within the burn plot ranged between 722.3-853.0°C. Peak surface temperatures were achieved very
rapidly within 1 minute within each of the four quadrants. Interestingly, temperatures then fell rapidly
to the 400°C range within 1-2 minutes, and were sustained within the 200-400°C
range for at least one hour. Data were
not recorded beyond that point due to an inadvertent cessation of the data
logger caused by the blacked-out LCD screen on the unit. Based on analysis of the time/temperature
curves, it is highly probable that elevated temperatures could have been
sustained for a protracted time period.
The peak surface temperature directly beneath one of the logs reached
814.3°C, and interestingly, the –5cm reading beneath the log peaked at 527.3°C. These values also peaked rapidly within 1
minute and then declined precipitously to <100°C within 1-2 minutes. However, heating directly beneath the log
began to rise again after approximately 50 minutes, reaching a
The
results of the artifact analysis show that significant thermal alteration was
pervasive across the majority of the sample.
All bone specimens were heavily charred, combusted, fractured, and
extremely friable. Each of the four
Hartville Uplift chert nodules exhibited significant degrees of thermal
fracturing and mineral oxidation (yellow-brown to red, limonite to hematite). Fragments of some nodules were located up to
50-60cm away from the location of the main body. All four
These
data suggest that combustion of piñon-juniper fuels is volatile, flashy, and
capable of generating extreme temperature gradients at the soil surface and
within the first 5cm of soil where logs are present. Archaeological materials located at the soil
surface are likely to exhibit significant thermal damage when burned under
similar conditions described for this experiment. Materials most at risk include bone, shell,
glass, chert (particularly large specimens), and lead. Unfortunately, only one trial was conducted
during the burn due to situational restrictions. Additional data regarding the characteristics
of prescribed burning in piñon-juniper environments is needed to assess the
results presented here.
SUMMARY AND
CONCLUSION
This research project was focused on assessing the impact of prescribed fire on archaeological resources through field-based experimentation conducted in conjunction with prescribed burning projects on federally managed lands. Prescribed fire is a common and effective resource management tool utilized to reduce hazardous fuels and promote healthy ecosystems. This project was focused on assessing the potential impact of prescribed burning on archaeological resources. Research was conducted under several different fuel models to include; mixed grass prairie, mixed grass prairie / ponderosa, mixed conifer, riparian, sagebrush, and piñon-juniper. The results of the experiments show that the impact of prescribed fire on archaeological resources is variable and largely dependent on fuel model, fire behavior, peak soil surface temperature, duration of heating, and artifact class. Moreover, the results suggest the prescribed burning may be performed with limited risk to the archaeological materials in some instances; however, it is critical that large fuels be reduced in the vicinity of important resources in order to mitigate significant thermal alteration of multiple artifact classes. Table 2.1 provides a generalized data summary for each of the prescribed fire experiments.
Table 2.1: Prescribed burn experiment summary data (generalized).
|
Fuel
Type |
Peak
Temp (Surface) |
Residence Time |
Fire Effect (Experimental Artifacts)
|
Mixed Grass
|
100-300°C |
10-20
sec |
Light
CB, limited bone PC only |
|
Grass/Mixed Conifer (Grass) (Grass/Litter) (Log) |
100-300°C 250-500°C 450-600°C |
10-20
sec 5-15
min 5-20
min |
Grass:
light CB all, limited bone PC Grass/Litter:
moderate CB all, bone PC, FR Log:
extensive bone FR, PC, sherd SP, glass FR, Lead
MLT, shell DL, heavy CB all |
Riparian
(Grass) ( ( |
100-200°C 100-300°C 300-500°C |
10-20
sec 1-2
min 2-8
min |
Grass:
light CB all, limited bone PC Sm.
Willow: light CB all, limited bone PC Lg.
Willow: moderate CB, bone PC, glass FR |
Sagebrush
(Small-Med) (Large) |
150-300°C 250-500°C |
1-3
min 2-4
min |
Sm-Med
Sagebrush: moderate CB all, bone PC, limited FR, chert nodule limited FR,
lead MLT Lg
Sagebrush: No artifact data |
Mixed
Conifer (Duff/Litter) (Log) |
200-400°C 400-800°C |
1-2min 5-20
min |
Duff/light
litter: heavy CB all, bone PC and FR Log:
chert FR, PL, FR, phosphoria PL, SP, obsidian CRH, lead MLT, glass FR, bone
severe PC, FR |
|
Piñon-Juniper (Large
Litter) |
700-800°C |
2-4
min |
Large
litter: severe chert FR, OX, severe bone PC, FR lead MLT, glass FR, shell DL,
PC |
Thermal Alteration Codes Definitions: CB = Combustive Residue, PC = Partial Combustion/Charring, FR = Thermal Fracture, SP = Thermal Spalling, OX = Mineral Oxidation, MLT = Melting, DL = Delamination
Research
at
Overall,
the relatively low surface temperatures and short residence times associated
with the combustion of grassland fuels generated minimal thermal alteration of
experimental artifacts deposited at the soil surface. The most pervasive form of thermal alteration
affecting artifacts was the presence of condensate tar deposit formed during
the pyrolysis and combustion of organic fuels.
It is likely that over time, these tar deposits will weather from the
surfaces of artifacts since the residue was removed relatively easily in the
laboratory using water and a pumice soap solution. Artifact classes with organic components such
as bone, and those that are completely organic such as wood did exhibit minimal
partial combustion along edges and upper surfaces. No catastrophic forms of thermal alteration
such as thermal fracturing, spalling, deformation, or surface cracking were
observed during the series of experiments performed at the park during 2001 and
2002. The results of the experiment
demonstrate that early-season prescribed burning in mixed grass prairie
environments will have a minimal impact on archaeological materials located at
the soil surface. Archaeological resources
located greater than 1cm subsurface will be largely unaffected under the same
conditions. These results are consistent
with that reported by Brunswig et al. 1995, and that reported by Sayler et al.
1989 for low fuel load burn plots. In
sum, cool-season prescribed burning in grassland fuels will produce low peak
surface temperatures characterized by brief residence times, which in turn,
will generate limited potential for significant thermal alteration of surface
artifacts and no thermal alteration of subsurface archaeological deposits.
Experiments
conducted at
The results
of several experimental trials conducted in a mixed conifer environment within
the
Prescribed
burning in a riparian environment at
The impact
of prescribed burning in sagebrush communities on archaeological resources is
variable and largely dependent on the size and density of sagebrush, the
proximity of artifacts to sagebrush understory, and artifact class. The results of the experiment conducted at
Grand Teton National Park show that peak surface heating generated by
combusting sagebrush can vary from 450-520°C for larger and densely sagebrush;
and 160-310°C for smaller more dispersed accumulation of sagebrush. Although experimental artifacts were not burned
in association with large/dense sagebrush fuels, it is likely that significant
thermal alteration of bone, glass and chert would occur if these materials were
located within the understory of the vegetation. During the experiment conducted within a burn
plot containing smaller and more dispersed accumulations of sagebrush these
artifacts types were impacted by charring, thermal fracturing, and thermal
spalling. However, significant thermal
alteration if likely to affects only surface artifacts located directly beneath
the understory of sagebrush. Thermal
alteration of subsurface archaeological deposits within sagebrush communities
is improbable. Mitigating the impact of
prescribed burning in this fuel type on surface archaeological materials could
be accomplished by fuel thinning in the vicinity of know sites.
The
prescribed burn experiment conducted in piñon-juniper fuels in northwestern
In
sum, the proportion of the radiant heat energy generated by combusting fuels
that is transmitted downward to the soil surface and surface artifacts is very
important in assessing the potential for thermal alteration of archaeological
materials. Heavy fuels combust at higher temperatures and have longer residence
times compared to light fuels such as grasses.
The physical composition and thermal properties of an artifact also
condition the potential impact of radiant heat energy. Some artifacts, due to their physical
structure, are more resistant to thermal alteration. However, the results of
the study do indicate that heat significantly affects the structural integrity
of bone, and some lithic types, particularly chert. Heavily burned bone was appreciably more
friable post-heating, which can have significant implications for the long-term
preservation of thermally altered archaeological bone. In addition, severe thermal fracturing of
chert flakes and nodules was also observed under high fire severity. In some instances, fragments of chert nodules
were recovered 50-60cm away from the body of the original specimen. In addition, Hartville and
In
brief, the important variables to consider when assessing the potential impact
of prescribed fire and wildland fire on archaeological resources are: 1) fuel
load; 2) fire behavior; 3) peak temperature and duration of heating; 4)
proximity of artifacts to fuels; and 5) class of artifact. The research presented here has been limited
to the immediately discernable effect of heat energy on artifacts. The long-term impact of thermal alteration of
artifacts such as the potential for increased weathering and decreased
preservation potential has not been addressed.
Figure 2.1
Figure 2.2
Figure 2.3
Figure 2.4
Figure 2.5
Figure 2.6
Figure 2.7
Figure 2.8
Figure 2.9
Figure 2.10
Figure 2.11
Figure 2.12
Figure 2.13
Figure 2.14
Figure 2.15
Figure 2.16
Figure 2.17
Figure2.18
Figure 2.19
Figure 2.20
Figure 2.21
CHAPTER 3
LABORATORY FURNACE HEATING OF
SELECTED MATERIALS TYPES
Introduction
Previous research
incorporating laboratory experimentation to explicitly address the potential
for thermal alteration of archaeological materials during wildland and
prescribed fires is very limited.
Steffen (2002) utilized laboratory-heating experiments to replicate the
range of thermal alteration observed on obsidian artifacts burned during the
Dome Fire in the
Kelly and Mayberry (1980) briefly outlined
a research design involving the heating of archaeological materials
representing a variety of artifact classes using an electric laboratory
furnace. Kelly and Mayberry (1980:605)
state that the “tests have not yet begun”, and it is unclear if these
experiments have been conducted since no further reference has been identified
from the literature. Bennett and
Kunzmann (1985) conducted the only published research project involving the
laboratory heating of a wide range of archaeological materials. In their experimental design,
cryptocrystalline quartz (cherts, flint, and chalcedony), obsidian, Southwestern
pottery sherds (slipped/decorated, undecorated, and corrugated), stoneware,
china, modern glass (window, and bottle), enameled tin ware, and bone (“fresh”
and “old”) were heated in an electric muffle furnace to various temperature
ranging between 200-800°C over a period of several hours. Heated specimens were assessed for
macroscopic thermal alteration as well as weight loss by calculating the
difference between the total weight loss of a specimen (after heating trials),
and the loss of chemically bound water (pre and post-trial via infrared
spectrophotometer). In addition, thermal
shock testing was also conducted whereby heated specimens were submerged in
water while specimen temperature and evidence of stress-induced failure
(cracking, spalling, checking) were recorded.
The results of Bennett
and Kunzmann’s experiment showed that chert specimens experienced post-heating
weight loss, largely attributed to the release of chemically bound water and
capillary water. Weight loss was reported
to be gradual between 200-600°C, and precipitous at temperatures of
600-800°C. The only other observable
thermal alteration of cherts documented in the experiment was surface dulling
of specimens heated to 600-800°C. No
catastrophic forms of thermal alteration such as thermal fracturing, potlid
fracturing, or surface crazing were observed by the researchers. This observation is inconsistent with the
literature surrounding the heat treatment of chert where several researchers
have reported heat-induced mineral alteration, and in some instances, thermal
fracturing and crazing among various types of chert at temperatures between
300-500°C (Ahler 1983; Griffiths et al. 1987; Schindler et al. 1982; Purdy
1974; see also Luedtke 1992 for a summary).
Bennett and Kunzmann suggest that weight
loss in cherts and flint is a function of time and temperature, and derived a
hyperbolic function to predict weight loss given a specific maximum temperature
and time (degree minutes) interval. They
further purport that cryptocrystalline quartz specimens did not exhibit
significant negative effects at temperatures below 500°C. The same was also suggested for obsidian
specimens, although they state that at temperatures above 500°C obsidian
experiences an increase in weight loss and surface alteration. Based on this observation, Bennett and
Kunzmann (1985:8) suggest that, “Obsidian is apparently more subject to heat
damage than flints.” However, they do
not elaborate, nor do they provide an adequate description of the surface thermal
alteration observed on the specimens, only comments in tabular form such as
“dulled” and “chalky.” Apparently, no
catastrophic forms of thermal alteration were observed, which does not
necessarily support their assertion that obsidian is more prone to thermal
alteration than cherts and flint.
A recently published combined volume
focused exclusively on the thermal alteration of obsidian edited by Loyd et al.
(2002) provides a wealth of information on the effects of heating on obsidian
as well as the implications for obsidian hydration of thermally altered
obsidian. Many researchers have
demonstrated that the hydration rims of obsidian are significantly affected
beginning at approximately 400°C, and become completely diminished at
temperatures above 700°C (Benson 2002; Halford and Halford 2002; Origer 1996;
Solomon 2002; Steffen 2002; Trembour 1979.
Nakazawa (2002) has experimentally demonstrated that surface cracks and
crazing begin to form on obsidian beginning at 550°C. In addition, at temperatures beyond 800°C,
Steffen (2002) has recorded vesiculation, the formation of interconnected
bubbles on the interior surface of obsidian, which eventually may elicited a
morphological change in obsidian whereby it takes on a foam-like appearance. Bennett and Kunzmann (1985) do report that
the surfaces of the obsidian specimens heated to between 500-800°C exhibited a
“dulled” and “chalky” appearance. This
is probably analogous to what Steffen refers to as “matte finish”, and “surface
sheen” (see Steffen 2002:163 for discussion).
For the various
Southwestern pottery sherds that were heated during the experiment, Bennett and
Kunzmann report that ceramic material are largely unaffected by exposure to
temperatures below 400°C. The authors
demonstrated in the experiment that heat-induced color alteration of
prehistoric ceramics occurs at temperatures above 500°C in which redware
varieties can change to a darker hue, and black-on-white wares will change to a
“slightly buff” color due to the oxidation of iron minerals. Furthermore, they suggest that the pigmented
design on black-on-white sherds is stable under high temperature if the paint
is mineral based, but may combust and fade significantly if the paint is
organic based. In addition, the
researchers state that at temperatures in excess of 600°C black-on-white sherds
can oxidize to resemble redware, and although this was not demonstrated in
their experiment and they do not provide references, other researchers have
documented the process in which the color change occurs (Burgh 1950; Colten
1953; Shepard 1956). Aside from the
heat-induce color alterations, the authors contend that prehistoric ceramics
are structurally stable up to temperatures of 600°C since most were originally
fired at temperatures lower than 600°C.
The authors did not provide any references to support this assertion;
however, several works exist which address firing temperatures of prehistoric
ceramics (e.g., Cogswell et al. 1997; Colten 1951; Feathers et al. 1998;
Goodyear 1971; Heimann and Franklin 1979; Kaiser and Lucius 1989; Rice 1987;
The historic materials
tested by tested by the researchers included modern china, modern stoneware,
enameled tinware (modern and historic), window glass and bottle glass (modern
and historic). These materials were only
heated to a maximum temperature of 500°C during the experiment, and minimal to
no thermal alteration of was recorded by the researchers. This is not surprising since most of these
materials are formed at temperatures well beyond 500°C. For example, porcelain and china ceramics are
fired at temperatures of 1280-1400°C and beyond (Rice 1987). In addition, these materials are manufactured
to withstand significant thermal stress since most are created for everyday use
in cooking. Thermal stress in modern
ceramics has received attention from several researchers in the ceramic
industry (Amberg and Hartsook 1946; Buessem 1955; Chandler 1981; Coble and
Kingery 1955; Crandall and Ging 1955; Davidge and Tapppin 1967; Grimshaw 1971;
Kingery 1955). Glass has a melting point
of approximately 700-800°C, although heating at lower temperatures may cause
glass to expand and develop thermal fractures (DeHann 1997:172). For enameled tinware, the authors state that
the volatilization of enamel coating occurs at temperatures above 500°C, and is
thus unlikely to be significantly affected by wildland or prescribed
fires.
Bennett and Kunzmann also incorporated bone into their
experiment, although information regarding species and element were not
provided. In addition, the researchers
used “fresh” and “old” specimens in the heating trials; however, no additional
information is provided regarding their categorization of fresh and old. For fresh bone specimens the authors observed
charring and water loss at temperatures between 100-300°C. At temperature beyond 300°C the researchers
observed that the “chemical structure of the bone is greatly altered”, and
becomes “chalky” in appearance (Bennett and Kunzmann 1985:12). The authors conclude that, regardless of age,
the chemical structure of bone is significantly altered at temperatures above
400°C. It is unclear what the authors
regard as chemical structure alteration.
Bone consists of two major components, an organic phase and an inorganic
phase. The organic phase (35% of dry
bone mass) consists largely of protein, mostly in the form of collagen (Posner
and Belts 1975). The inorganic phase is
composed of mineral, mostly hydroxyapatite in microcrystalline form (Ortner et
al. 1972). Several researchers have documented
the processes and involved in the thermal alteration of bone (e.g., Bennett
1999; Bonucci and Graziani 1975; Bradtmiller and Buikstra 1984; Brain 1993;
Buikstra ad Swegle 1989; Herrmann 1977; Kizzely 1973; McCutcheon 1992;
Nicholson 1993; Shipman et al. 1984; Sillen and Hoering 1993; Stiner et al.
1995; Von Endt and Ortner 1984). Shipman
et al. (1984) provide an excellent summary of the process of bone thermal
alteration, which can be summarized as: 1) change in bone color; 2) change in
the microscopic morphology of bone surfaces; 3) changes in the cystalline
structure of bone; and 4) bone shrinkage.
The degree of thermal alteration observed in bone is dependent on the
temperature at which bone is exposed, the duration of exposure, position of bone
in relation to heat source, bone composition, and bone size.
In addition to the electric furnace heating trials,
Bennett and Kunzmann (1985) also conducted thermal shock testing of artifacts
in which heated artifacts were submerged in water and assessed for damage
induced by thermal shock. The authors
report that cooling of submerged artifacts was precipitous, dropping from 400°C
to ambient temperature of rates greater than 500°C per minute. They also concede that theses cooling rates
are much greater than would be expected under field conditions during a
prescribed fire. This is probably an
accurate statement, with the possible exception of a scenario in which water or
fire retardant is sprayed on artifacts during wildland fire suppression
activities. Oppelt and Oliverius (1993)
have induced thermal shock in Mesa Verde pottery sherds by applying fire
retardant to sherds burned in experimental plots under controlled prescribed
fire conditions. Bennett and Kunzmann
report a 30% failure rate for decorated sherds and a 10% failure rated for
undecorated and corrugated specimens. In
addition, they also observed a 30-40% failure rate for china. The researchers
did not observed thermal damage for flint/chert/chalcedony specimens, but did
report a 20% failure rate for obsidian specimens. Glass specimens, window and bottle, exhibited
failure rates of 70-80% and 30% respectively.
This experiment suggests that inverse thermal shock can significantly
damage a wide variety of archaeological material type; however, the precipitous
decline in temperature achieved during the experiment is unlikely to be seen in
prescribed and wildland fire scenarios, with the possible exception of
instances in which fire suppression activities involving water or fire
retardant and heated surface artifacts are involved. Under prescribed and wildland fire
conditions, thermal stress and potential thermal shock among various artifacts
types will occur due to rapid heating of artifact surfaces from ambient
temperature to the maximum amount of heat energy that is conducted by the
artifact from the radiant heat energy emitted by the resident flames
Bennett and Kunzmann’s experiment represents the
principle body of research on laboratory heating of a range of artifact classes
to address the potential effects of natural fire on archaeological
resources. Their research design and
conceptualization of time and temperature as critical variables are innovative
and remain important contributions to the fire effects literature. The major weakness of their experiment is
embedded in the method used to heat artifacts.
The researchers used an electric laboratory furnace to heat, which can
require in excess of 30 minutes to reach a temperature of 500°C. The rate of heating in a muffle furnace is
characterized by gradual and uniform temperature gradients. The specifics of Bennett and Kunzmann’s
heating trials are not provided in great detail, and rate of heating data are
provided in terms of degree-minutes. It
is apparent, however, that the researchers heated artifacts gradually over a
period of several hours given the method of heating and their degree-minute
calculations. The rate of heating
attainable in a muffle furnace is not analogous to wildland and prescribed fire
conditions where temperatures peak rapidly (within a few minutes) as the flame
from passes the area in which archaeological materials are located and diminish
more slowly as fuels are combusted (Chapters 2 and 4). The maximum temperatures at the soil surface,
and the duration of heating generated by a natural fire are largely dependent
on fuel load fire intensity and extent of combustion of fuels (Chapter 2). These conditions cannot be replicated in a
laboratory setting using an electric muffle furnace. In order to address this issue, two
laboratory experiments were designed:
1) Experimental heating of
various artifact classes in a laboratory muffle furnace with
the purpose of substantiating or rejecting
the findings presented by Bennett and
Kunzmann (1985). (This Chapter)
2) Laboratory wildland fire
simulations in a controlled wind tunnel setting whereby
various artifact classes were subjected to
burning more representative of natural
fire.
(Chapter 4).
The details of each
experimental design and results are provided in this chapter and Chapter 4.
Research
Design
The laboratory heating
experiment was conducted using an electric muffle furnace manufactured by
Thermolyne. The experimental design
consisted of ten trials beginning with a l hour 100°C trial and incrementally
increasing the maximum temperature of each 1-hour trial an additional 100°C
until the final trial, which achieved a maximum temperature of 1000°C. Temperature was measured using the internal
thermocouple and analog display that was integral to the muffle furnace
itself. Heating was controlled by
setting the furnace thermostat to the desired heating trial temperature and
recording the amount of time the furnace required to achieve the predetermined
temperature. The muffle furnace did not
accommodate an external thermocouple and data logger system; therefore,
specific time and temperature curves were not generated for the experiment.
In each of the ten heating trials, 21 experimental
artifacts representing a variety of artifact classes were placed in the muffle
furnace for heating. In total, 210
experimental artifacts were subjected to controlled laboratory heating and
analyzed for subsequent thermal alteration.
Heating was initiated after the artifacts were placed in the muffle
furnace, and heat up time, time at temperature, and cool down time to <100°C
were recorded. Heat up and cool down
time varied given the magnitude of the desired heating time temperature;
however, once the predetermined temperature was established, it was maintained
for 1 hour before the cool down was initiated.
Prior to heating, each artifact was numbered to correspond to a specific
heating trial and general descriptions of the artifacts were recorded on a data
form and documented through digital photography. In addition, each artifact was measured for
maximum length, width, and thickness, weighed to the nearest 1 gram using an
Ohaus CS-2000 digital scale, and artifact color(s) was recorded using Munsell
soil color charts (Munsell 2000). After
a specific heating trial was terminated, each artifact was weighed to the
nearest 1 gram to assess potential post-heating weight loss, and potential
heat-induced color change was assessed using the Munsell system. In addition, artifacts were macroscopically
analyzed for different forms of thermal alteration such as thermal fracturing,
thermal spalling, crazing, partial combustion, and deformation. Post-heating thermal alteration of artifacts
was documented on the data form and through digital photography. The descriptive artifact data in tabular form
and digital photographs of artifacts pre-heating and post-heating are provided
on the data CD that accompanies the dissertation (Appendix 2, Photos, Artifact
Data).
Descriptions of the experimental artifacts heated during
each of the ten trials of the muffle furnace experiment are provided below:
Mammal Bone
Samples:
Sample 1 included
sawed and sectioned appendicular elements (humerus, radius/ulna, femur,
proximal and distal epiphyses and diaphyses) from domestic cattle (Bos sp.). These specimens had been weathering at the
soil surface in an arid environment for a period of 3 years. The cortical surfaces of these specimens were
dry, cracked and flakey. These specimens
were given a weathering classification of 3 on an ordinal scale of 1-3.
Sample 2 included
sawed and sectioned axial elements (thoracic vertebrae spines, and rib blades)
from domestic cattle (Bos sp.).
These specimens were form a single carcass that had been weathering in
the same arid environment for approximately one year. These specimens were in good condition, and
were given a weathering classification of 1 on an ordinal scale of 1-3.
Sample 3 was
comprised of complete domestic cattle (Bos sp.) teeth (mandibular and
maxillary premolars and molars). These specimens were had been weathering at
the soil surface in an arid environment for a period of 3 years. The enamel surfaces and root structures of
these specimens were dry and cracked.
The weathering classification for these specimens was recorded as 3 on an
ordinal scale of 1-3.
Sample 4 included
sawed and sectioned appendicular element (meta tarsal, proximal and distal
epiphyses and diaphyses) from an elk (Cervus elaphus), which had been
weathering in a mountain environment for an unknown duration, but probably over
a period of less than one year. These
specimens were in good condition, and given a weathering classification of 1 on
an ordinal scale of 1-3.
Sample 5 included
sawed and sectioned elk (Cervus elaphus) antler of undetermined age from
a high plains environment. These
specimens exhibited significant weathering, and were given a weathering
classification of 3 on an ordinal scale of 1-3.
Freshwater
Mussel Shell:
Large bivalve shell halves
of unspecified species from Midwestern riverine environments. Each shell half
was sawed into two equally sized sections. These specimens were in fair
condition, and were given a weathering classification of 2 on an ordinal scale
of 1-3.
Lithic
Materials:
Moderate to large sized
secondary flakes representing a variety of raw material types that were derived
from modern flint knapping activities.
Each material type is listed below:
Porcelinite (unspecified
source)
Obsidian (black, red, and
translucent, unspecified source)
Obsidian (black with fine
gray banding, unspecified source)
Hartville Uplift chert (
Pink Bioclastic chert
(unspecified source)
Phosphoria (Big
Novaculite (unspecified
source)
Silicified Wood (
Cliff House Formation
Sandstone (
Pottery and
Prehistoric: Moderately
sized individual black-on-white sherds from an unprovenienced collection of
Southwestern specimens. The black paint
on the specimens is mineral based.
Historic: Moderately sized
individual decorated whiteware fragments from an historic midden area located
on private property and dating from the 1920s-1950s. The painted decorations consisted of pink-red
flower motifs and gold banding. Specific
data regarding the manufacturer and production date were not discernable.
Glass:
Historic: Moderately sized
fragments derived from an individual amber colored screw-top bottle. The specimen was obtained from an historic
midden area located on private property and dating from the 1920s-1950s. Specific data regarding the manufacturer and
production date were not discernable.
Trial #1
(100°C)
Heating trial number 1 was conducted at maximum
temperature of 100°C for 1 hour. Heat up
time to 100°C was achieved in approximately 3 minutes, and the cool down period
continued for a duration of approximately 3 minutes. Post-heating macroscopic analysis of the
experimental artifacts revealed no significant evidence of thermal alteration
such as heat-induced color change.
However, post-heating weights of bone and antler specimens revealed a
3.3-3.5% weight loss for smaller specimens, and a 5.5-5.9% loss in mass for the
larger specimens. The elk antler section
sustained a loss in mass of 6.5%. This
suggests that internal moisture is lost, and the organic phase, primarily
collagen, of bone begins to combust at temperatures as low as 100°C. The freshwater shell specimen did not exhibit
a heat-induced loss in mass since it is composed primarily of calcium
carbonate. In sum, the 100°C heating
trial only minimally affected the organic phase of the bone and antler
specimens, and had no discernable impact on the remaining artifacts. Descriptive data and a summary of artifact
thermal alteration for this trial are summarized in Table 3.1.
Trial #2
(200°C)
The parameters for trial number 2 consisted of heating
experimental artifacts for 1 hour at a temperature of 200°C. The heat up time required for the muffle
furnace to achieve the 200°C mark was approximately 20 minutes. After the trial, the furnace required of
cooling period of approximately 1 hour before the artifacts were removed for
analysis.
Post-heating analysis of the experimental artifacts
demonstrated that bone specimens were affected most significantly during the
200°C trial. Heat-induced weight loss
associated with the combustion of the organic phase of bone ranged between
10.6-15.6% for all specimens including antler.
The color of bone and antler specimens changed from white and light gray
to strong brown and black post-heating giving the specimens a charred
appearance. The partial combustion of
the organic phase produced a light combustive residue on many of the experimental
artifacts. In addition, the more heavily
weathered specimens exhibited enhanced surface cracking of preexisting cracks
resulting from thermal stress. The tooth
specimen (mandibular third premolar) did not exhibit an observable loss in
mass, but the color of the enamel changed from white to gray brown, and the
root portion became blackened. The freshwater shell specimen exhibited a light
gray haze over the entire upper surface, but was not affected by weight loss.
Each of the lithic and obsidian specimens exhibited a
slight increase in luster. Heat-induced
color change was observed for porcelinite (greenish gray to a darker greenish
gray hue around the edges), Hartville Uplift chert (strong brown to strong
brown with mottled red), Pecos chert (light gray to light gray with mottled
weak red) pink bioclastic (pink/light gray to weak red), Fort Hood Chert (light
brownish gray to reddish gray), phosphoria (dark red to dusky red), and the
silicified wood specimen (strong brown/very dark gray to red). Heat induced color change in cherts from
their original color to pink or red is the result of the oxidation of iron
compounds to hematite, and other color changes may be attributed to the
alteration of other mineralogical impurities (Luedtke 1992; Purdy 1974; Schindler
et al. 1982). The novaculite specimen,
however, was unaffected by heating.
Flenniken and Garrison (1975) also observed that
The china and pottery specimens did not exhibit any
observable thermal alteration; however, the glass bottle fragment did exhibit
crack propagation in an area of preexisting weakness. The results of the 200°C trial indicate that
combustion of the organic phase of bone was accelerated compared to the 100°C,
and that crack propagation of existing surface cracks may be enhanced. In addition, if appears that the mineral
alteration of some lithic materials is initiated at 200°C. Pre- and post-heating Munsell values, thermal
alteration summaries, and descriptive information for each of the experimental
artifacts are provided in Table 3.2.
Trial #3
(300°C)
Heating trial number 3 was designed to heat
experimental artifacts at 300°C for 1hr to include additional heating required
during muffle furnace heat up and cool down.
The muffle furnace reached the 300°C point within approximately 20
minutes, and required a cool down period of approximately 1.5 hours before the
artifacts were removed.
The general observation surrounding the impact of the 300°C
trial on the experimental artifacts was that all specimens exhibited a light
combustive residue due to the partial combustion of the organic phase of the
bone specimens. The bone specimens
themselves where completely blackened giving them a charred appearance. Post-heating weight losses for all bone
specimens ranged between 20-27.7%. Aside
from the reduction in mass, the bone appeared to be increasingly more fragile
and exhibited crack propagation of existing surface cracks as well as the
combustion of a portion of the flakey surface on the heavily weathered
specimens. The tooth specimen
(mandibular third premolar) exhibited blackening of the enamel and the root as
well as a 22.2% loss in mass. The
freshwater shell exhibited a brown haze over the original pre-heat colors in
addition to delamination of its interior surface and a 7.7% loss in mass.
The obsidian artifacts exhibited a more lustrous
appearance post-heating, and additionally, the black and gray-banded specimen
exhibited metallic-like sheen. This is
what Steffen (2002:163) defines as “surface sheen”, which may be caused by
either organic buildup or the formation of microscopic crazing and
bubbling. The chert specimens exhibited
a slightly dulled appearance as well as heat-induced mineral alterations in
which Hartville Uplift chert changed from strong brown to dusky red, Pecos
chert was altered from light gray to reddish gray, Fort Hood chert changed from
brownish gray to brown, and pink bioclastic chert was altered from pink to dark
reddish gray. In addition, phosphoria
was altered from its original dark red to dusky red, silicified wood was
altered from strong brown/very dark gray to dusky red, and porcelinite changed
from greenish gray to gray. As was
recorded for the 200°C trial, the novaculite specimen did not exhibit any
observable color alteration. The Cliff
House Formation sandstone block section exhibited a color alteration from
brownish yellow to weak red, and a loss in mass of 1.3%.
The china fragment and black-on-white prehistoric
pottery sherd exhibited a slight haze resulting from the combustive residue
created by the combusting organic phase in bone specimens, but were otherwise
unaffected by heating to 300°C. These
materials, china in particular, were fired at temperatures well beyond 300°C,
thus it is unlikely that they would be significantly affected during this
trial. The bottle glass fragment also exhibited a slight haze as well as the
propagation of two cracks along preexisting weak points.
Overall, the results of the 300°C heating trial show
an increased combustion of the organic phase in bone resulting in mass losses
ranging between 20-27% combined with crack propagation for some specimens. Freshwater shell begins to delaminate on the
interior surface at 300°C. Chert
specimens and the silicified wood specimen exhibited clearly discernable
alterations in mineralogy resulting in prominent color changes, and one
obsidian specimen exhibited a characteristic heat-induced surface sheen. No thermal fracturing, spalling, or crazing
was observed within the lithic sample; however it is apparent that heat-induced
mineral alteration becomes prominent at 300°C.
Descriptive information, Munsell values, and observed thermal alteration
summaries for each of the experimental artifacts are provided in Table 3.3.
Trial #4
(400°C)
Heating to 400°C in the muffle furnace was achieved
within approximately 30 minutes, and cool down to less than 100°C was
accomplished over a period of approximately 1.5 hours. The experimental artifacts were heated at a
constant 400°C for 1 hour in addition to the gradual heat gradient experienced
during the heat-up and cool-down periods.
Post-heating analysis of the experimental artifacts
showed that a moderate to heavy combustive residue was deposited on the
surfaces of artifacts due to bone combustion.
Bone specimens were heavily blackened and charred in appearance and
exhibited mass losses between 29.4-34.5%.
The antler specimen exhibited an appreciable weight loss of 37.5%. The proportion of the organic phase of bone
that has been combusted has increased incrementally as heating trial
temperatures have increased from 100°C to 400°C. The organic phase represents approximately
35% of the mass of dry bone (Posner and Belts 1975). Therefore, it seems reasonable to suggest
that prolonged temperatures approaching the 400°C range are sufficient to
combust a significant portion of the bone collagen since the bone specimens in
this trial exhibited weight loss in the range of 29.4-34.5%. Other researchers have reported partial to
complete combustion of the organic phase within bone at temperatures between
300-500°C (Bonucci and Graziani 1975; Kizzely 1973; Shipman et al. 1984). In addition to bone specimens, the antler
section sustained a loss in mass of 37.5%.
The bone specimens also exhibited crack propagation as well as thermal
fractures resulting in the fragmentation of two specimens. Bone also appeared to be increasingly fragile
and exhibited an overall deeply blackened color on all surfaces giving them a
charred appearance. The tooth
(mandibular third premolar) heated during this trial sustained a 25% loss in
mass as well as an overall blackened appearance. The freshwater shell specimen exhibited a
pervasive gray-brown color and significant delamination of its interior portion
resulting in a flakey appearance as well as an 11.1% loss in mass.
Each of the obsidian specimens exhibited the metallic
sheen recorded during the 300°C trial.
The preexisting radial fracture lines, creating by the percussion of
flaking, were also enhanced and slightly propagated. In addition, the black and gray-banded
specimen exhibited “fine crazing”, defined by Steffen (2002:164) as very fine
and shallow surface fractures that form a series of polygons. Chert specimens exhibited a dulled
appearance, largely due to the combustive residue, but also experienced
significant mineral alteration resulting in color changes to darker hues for
porcelinite, Pecos Chert, pink bioclastic chert, and phosphoria. The color of the Hartville Uplift Chert was
altered from strong brown to very dusky red, and Fort Hood Chert specimen was
altered from light brownish gray to gray and dusky red. In addition the
The combustive residue deposit observed on most artifacts
also obscured the painted designs on the china fragment and the black-on-white
sherd. However, no internal color
alteration or thermal damage was observed on either specimen. The bottle glass fragment exhibited the black
residue associated with the combustion of the organic phase of the bone
specimens as well, but was also affected by thermal spalling.
The results of the 400°C indicate that significant
combustion of the organic phase of bone occurs at sustained temperatures of
400°C. In addition, most bone specimens
also exhibited thermal fracturing, crack propagation, and heavy
blackening. For lithic specimens, the
Trial #5
(500°C)
Heat-up to 500°C for trial number 5 was achieved
within approximately 45 minutes, 500°C was maintained for 1 hour, and the
cool-down period continued thereafter for 2 hours. The combustive residue that was present on
artifacts in the previous two trials (300-400°C) was not observed on any
specimens heated during the 500°C trial.
It appears that the entire organic phase of the bone
specimens was burned off during this trial.
Post-heating weight losses for all specimens ranged between
approximately 30-38%, indicating that most of the organic phase had been
combusted during heating. In addition,
the antler specimen sustained a 45.5% loss in mass after heating. Moreover, the bone and antler specimens were
grayish brown in appearance compared to the blackened appearance of bones and
antler from the 300 and 400°C trials further suggesting that collagen and other
organic had been combusted. In addition
to the weight loss, bone specimens exhibited thermal fracturing and crack
propagation, and had become increasingly fragile. The tooth specimen (mandibular third
premolar) exhibited the same grayish brown color on the root portion and a
green-gray hue on the enamel portion. In
addition, the enamel had deteriorated, showing evidence of crack propagation
and increased brittleness as well as a loss in mass of 30%. The shell specimen also sustained a sizeable
loss in mass of 25% as well as significant delamination of its interior portion
and an overall change in color to green-gray.
Significant thermal alteration of lithic materials was
recorded for silicified wood,
The china fragment and black-on-white pottery sherd
did not exhibit any observable evidence of thermal alteration. The combustive residue seen in the previous
two trials was not present here. The
bottle glass fragment exhibited thermal spalling and appeared to be slightly
more lustrous than it was prior to heating.
Although, bottle glass seems to be subject to crack propagation and
thermal spalling at temperatures ranging between 200-500°C, ceramic materials
(historic and prehistoric) remain stable under the same conditions.
The results of the 500°C trial indicate that bone;
teeth, antler and shell are significantly impacted at this temperature
resulting in combustion of the organic phase, crack propagation, and thermal
fracturing. Similarly, some lithic
material types such as
Trial #6
(600°C)
The 600°C trial was conducted over a period of 4.5
hours. Experimental artifacts were
placed in the muffle furnace and heating was initiated immediately
thereafter. The muffle furnace required
approximately 45 minutes to reach the 600°C level, which was then maintained
constant for 1 hour. The cool-down
period from 600°C to less than 100°C was achieved over a period of 2.5 hours.
As was recorded for the 500°C trial, there was no
combustive residue present on the experimental artifacts after heating was
conducted. The bone and antler specimens
took on a partially calcined appearance after the trial whereby specimens were
altered from their original white color to light brown-gray and dark
green-gray. The organic phase of the
bone specimens had been completely combusted resulting in weight loss of
approximately 32-40%, and the initiation of a calcined appearance would suggest
that the mineral phase was beginning to be affected by prolong heating at the
600°C level. These specimens also
exhibited minor thermal fractures, crack propagation, and an overall increase
in brittleness. The tooth specimen
(maxillary second molar) exhibited a green-gray color alteration of the root
portion and a dark green-gray color alteration of the enamel portion. In addition, the enamel sustained thermal
fractures and became quite brittle as a result of heating. The post-heating weight loss recorded for the
tooth was nearly 43%. The shell specimen
sustained a loss in mass of 17.6% and exhibited moderate delamination of its
interior surface in combination with an overall color change to green-gray.
For the lithic specimens, the
The historic china fragment was unaffected by heating
during this trial; however, the black-on-white prehistoric sherd exhibited a
white hue, noticeably lighter than the original light gray slip, accompanied a
faded appearance of the black mineral paint design. It is possible that the mineral paint and
gray slip may have experienced heat induced mineral oxidation as the 600°C
temperature began to exceed the original firing temperature of the ceramic. The
glass bottle fragment exhibited partial melting around the edges as well as
slight deformation as the trial approached the melting point of glass (soda
lime), which is in the 750-870°C range (DeHann 1997:446).
In sum, the results of the 600°C trial demonstrate that
the mineral phase of bone begins to be affected at this temperature and the
organic phase is completely combusted.
Bone, antler, and shell specimens also exhibited crack propagation,
thermal fractures, and delamination (shell only). Lithic specimens experienced mineral
alterations resulting in color changes ranging from complete hue change to
darkened hue values. The only exception
to this trend was the two obsidian specimens that exhibited fine crazing and a
metallic sheen, and the novaculite flake that was unaffected by heating. The two material types that are most
susceptible to heat damage are Fort Hood Chert and silicified wood, which
exhibited pervasive thermal fracturing and spalling. The 600°C trial may have exceed the original
firing temperature of the black-on-white sherd since it exhibited an overall
lightening of the slip and black design.
In addition, the glass specimen exhibited partial melting as the
temperature approached the melting point of glass. Thermal alteration summaries, Munsell values,
and descriptive information for each of the experimental artifacts tested are
provided in Table 3.6.
Trial #7
(700°)
In order to achieve the high temperature necessary to
run the trial number 7 (700°C), the muffle furnace required prolonged heat-up
and cool-down intervals. In total, the
trial was conducted over a period of approximately five hours. After the experimental artifacts were placed
in the muffle furnace, heat-up time to 700°C was achieved over a period in
excess of 1 hour. That temperature was
held constant for and additional 1 hour, after which cool-down was initiated
over a period of 3 hours.
The bone and antler specimens heated during this trial
became calcined indicating that the organic phase had been completely combusted
and that the mineral phase had been affected by heating to 700°C for a
sustained period. Macroscopically, the
bone specimens changed from their original white color to various shades of
light blue-gray, blue-gray, and dark blue-gray as well as sustaining moderate
thermal fracturing and crack propagation.
In addition, the specimens became increasingly brittle upon heating, but
also became increasingly hardened and rigid emitting a more high pitched
percussive sound when tapped.
Post-heating weight loss for bone and antlers specimens ranged between
approximately 21-40%. The enamel portion
of the tooth specimen deteriorated significantly due to thermal fractures and
crack propagation. Additionally, the
enamel was thermally altered to a blue-gray color with root section taking on a
green-gray color, and heat-induced mass loss was recorded at 27%. Thermal alteration of the shell specimen was
characterized by delamination, color change to an overall blue-gray hue, and a
loss in mass of 20%.
Lithic specimens sustained different degrees of thermal
alteration ranging from mineral alteration resulting in color changes to
pervasive thermal fracturing and deterioration.
The material types that were impacted most significantly were
The historic china fragment was macroscopically
unaffected by prolonged heating at 700°C.
However, 700°C was sufficient to effectively refire the prehistoric
black-on-white sherd whereby the gray slip was oxidized to very pale brown in
color, and the black mineral painted design became faded. The bottle glass fragment sustained partial
melting and surface deformation during the trial since 700°C is close to the
melting point of glass.
The results of the 700°C trial demonstrate that the
organic and mineral phases of bone are affected at this temperature resulting
in a calcined blue-gray appearance, thermal fractures, crack propagation, and increased
brittleness. Three lithic material types
(silicified wood,
Trial #8
(800°C)
After the experimental
artifacts were placed in the muffle furnace, it required 1.5 hours to achieve
the prescribed 800°C temperature for trial number 8. This temperature was maintained constant for
1 hour, after which the cool-down period was initiated. The muffle furnace required approximately 4
hours to cool to a temperature below 100°C.
After the cool-down period, the experimental artifacts were removed from
the furnace and macroscopically inspected for evidence of thermal
alteration.
The immediate impact of the 800°C trial on the bone and
antler specimens was a prominent color alteration from the original dull white
to predominantly bright white with an admixture of very pale brown and gray
giving the specimens a calcined appearance.
Heat induced weight loss for these specimens ranged between
approximately 32-42%, indicating that the organic phase had been completely
combusted during the trial. The specimens
also exhibited crack propagation, deep surface cracking, and some thermal
fracturing due to thermal stress. It is
likely that the mineral phase of the bone was also affected since the specimens
became quite brittle and emitted a high-pitched percussive sound when
tapped. Thermal alteration of the tooth
specimen was most significant for the enamel portion, which sustained pervasive
thermal fractures and overall deterioration.
The tooth also exhibited color alterations from white to blue-gray on
the enamel and yellow to white on the root portion. The shell exhibited a color alteration form
its original values to an overall pale red color, and the extent of
delamination of the interior portion of the specimens was not as prominent as
what had been recorded during previous trials.
Heat-induced weight losses for the tooth and shell specimens where
recorded as 27% and 25% respectively.
The impact of the 800°C trial on lithic materials
continued the trend recorded for the previous five trials where the material
types most significantly affected by heating were the
The thermal alteration of the manufactured china,
pottery, and glass specimens ranged from minor color alteration to complete
melting and deformation. The prehistoric
black-on-white sherd was essentially refired whereby the slip oxidized from
light gray to pink, likely due to mineral alterations, and the mineral paint
became oxidized and faded. The bottle
fragment melted and was completely deformed during the trial as the 800°C
maximum temperature of the trial exceeded the initial melting point of glass.
The results of the 800°C trial suggest that most material
types tested, with the exception of novaculite and china, are subject to
observable thermal alteration at this temperature. The organic and mineral phases of bone,
tooth, and antler specimens were affected during this trial, resulting in
weight loss and structural alteration.
Lithic materials exhibited color changes resulting from mineral alterations
as well as surface cracking, severe thermal fracturing, and surface crazing for
the Fort Hood chert, silicified wood, Hartville Uplift chert, pink bioclastic
chert, and obsidian flakes. Other
notable thermal alterations include the black-on-white pottery sherd, which
exhibited mineral alterations of the slip and oxidation of the black design
pigment, and the bottle glass fragment that underwent melting and subsequent
deformation. The thermal alteration
summaries, pre- and post-heating Munsell values, and descriptive information
for each of the experimental artifacts are provided in Table 3.8.
Trial #9
(900°C)
As with the previous trials,
the experimental artifacts were placed in the muffle furnace prior to the
initiation of heating, and once the prescribed temperature was achieved it was
maintained for 1 hour. The heat-up
period for the 900°C trial required a prolonged period of approximately 1.75
hours. The cool-down period was also
protracted, requiring approximately 4 hours, due to the high temperature
maintained during the trial. After
cooling, the artifacts were removed from the muffle furnace for evidence of
thermal alteration via macroscopic analysis.
Thermal alteration of bone and antler specimens during
the 900°C trial were similar to those reported for the 800°C in which specimens
exhibited a chalky calcined appearance with specimens taking on a bright white
hue intermixed with very pale brown. The
surfaces of the bone and antler specimens also exhibited deep surface fractures
and propagation of existing surface cracks as well as weight losses ranging
between 35-41% for bone and 47.5% for antler.
These specimens also became increasingly brittle and emitted a higher
pitched percussive sound when tapped.
The tooth specimen exhibited significant deterioration of the enamel,
and was also color altered to hues of white and blue-gray. The root portion of the tooth was altered
from its original yellow color to very pale brown, and overall the tooth exhibited
a 28.6% loss in mass. Thermal alteration
of the shell specimen was also similar to that reported for the previous trial
in which delamination was moderate and the overall color of the interior
portion of the shell changed to pale red.
The post-heat weight of the shell specimen was reduced by 45%, and the
specimen had become increasingly brittle.
The lithic specimens most severely impacted by heating
were the
Thermal alteration of the manufactured experimental
artifacts varied from none for the historic china fragment to significant for
the prehistoric black-on-white sherd and the bottle glass fragment. The black-on-white sherd was effectively
refired as the 900°C maximum temperature probably far exceeded the original
firing temperature of the vessel.
Mineral oxidation of the slip was apparent in which the original gray
color was altered to reddish yellow. In
addition, the black mineral paint on the sherd became faded after heating. The glass bottle fragment melted and became
significantly deformed as a result of sustained heating to 900°C, which exceeds
the melting point of glass.
In sum, thermal alteration of the experimental artifacts
heated during the 900°C was significant for the majority of material types,
with the exception of the historic china fragment and the novaculite secondary
flake. The organic and mineral phases of
bone specimens were affected during the trial resulting in weight loss,
increased brittleness, and surface fracturing.
The enamel portion of the tooth specimen was severely compromised due to
thermal fracturing. Lithic specimens
exhibited severe thermal fracturing for the
Trial #10
(1000°C)
The final heating trial of the experiment was trial
number 10 in which a temperature of 1000°C was sustained for one hour. The muffle furnace required prolonged heat-up
period of 2.5 hours to reach the prescribed 1000°C mark. Similarly, the cool-down period was
protracted, requiring over 4.5 hours to return to a temperature below
100°C. In total, the combined time
required to complete trial number 10 was approximately 8 hours.
After heating and cool-down, the experimental artifacts
were visually analyzed for discernable evidence of thermal alteration. Thermal alteration of bone, antler, and tooth
specimens was similar to that reported for the previous two trials (800°C and
900°C). These specimens exhibited a
chalky calcined white and very pale brown color alteration as well as deep
surface fractures, thermal fracturing, crack propagation, and weight losses
ranging from 29-47%. In addition, the
enamel portion of the tooth was heavily fractured and had deteriorated
significantly over its pre-heated state.
Thermal alteration of the shell specimen was also similar to that
reported for the previous two trials in which delamination of the interior
surface of the shell was moderate, and the overall color of the inner portion
of the specimen was altered to pale red.
The lithic material types most significantly impacted by
prolonged heating to 1000°C were the
The effect of sustained heating to 1000°C on the man-made
experimental artifacts was also similar to that reported for the 800°C and
900°C trials. The bottle glass fragment
experienced melting resulting in severe deformation. The prehistoric black-on-white pottery sherd
was effectively refired resulting in the oxidation of the slip from gray to
light red and the near complete oxidation or absorption of the black mineral
painted design such that is was barely visible post-heating. The historic china fragment was visually
unaffected by prolonged heating to 1000°C.
The overall impact of the 1000°C heating trial on the
experimental artifacts was significant.
The most interesting form of thermal alteration observed during the
trial was the vesiculation of the black and gray banded obsidian flake, which
was transformed into a foam-like globule.
Other notable instances of thermal alteration include thermal fracturing
of cherts ranging from severe to minor, crazing of phosphoria, and the color
alteration of all lithic material types due to mineral oxidation. The black-on-white pottery sherd was altered
from black on white to essentially red buff color absent of any design. In addition, the glass bottle fragment
experienced melting and was severely deformed. Bone, and antler, specimens exhibited
thermal fracturing, alteration of the mineral phase, and combustion of the
organic phase, which resulted in color alteration, brittleness, and weight
loss. The enamel portion of the tooth
specimen deteriorated severely due to thermal fracturing, and the shell
specimen experienced moderate delamination, increased fragility, and color
alteration of its interior surface.
Thermal alteration summaries, Munsell values, and descriptive
information for each artifact heated during the trial are provided in Table
3.10.
Summary and
Conclusion
Thermal alteration information for each trial and
artifact type is summarized in Table 3.12 at the end of the chapter. The muffle furnace experiment was conducted
for three purposes: 1) to replicate similar experiments conducted by Bennett
and Kunzmann (1985), and proposed but not performed by Kelly and Mayberry
(1980); 2) to establish the form and extent of thermal alteration affecting a
variety of common archaeological material types across differential temperature
gradients ranging from 100-1000°C; 3) to provide reference data from which to
assess the potential for thermal alteration of archaeological resources given a
particular artifact class and maximum temperature range. The temperature range defined for the
experiment is sufficient to encompass a wide variety of natural fire scenarios
ranging in severity from prescribed fire in grassland fuels to prescribed and
wildland fires under heavy fuel loads.
The results of the experiment show that the majority
of artifact classes tested were not appreciably affected during sustained
heating to 100°C. The only exception to
this general trend was the initial pyrolysis of the organic phase sustained by
bone specimens that resulted in reductions in pre-heated mass ranging between
3-7%. The same trend was continued
during the 200°C in which the organic phase of bone began to combust, but at a
greater magnitude resulting in weight losses ranging between 11-16%. In addition, the enhanced pyrolysis of
collagen during this trial resulted in bone specimens that were blackened or
charred in appearance. The combustive
residue created by this process also adhered to the surfaces of the other
experimental artifacts tested during the experiment. This residue, however, only loosely adhered
to the surfaces of artifacts and is not considered a significant form of
thermal alteration. The only additional
form of thermal alteration observed during the 200°C trial was mineral
alteration of lithic materials resulting in color changes generally
characterized by shifts to darker hues.
This was especially prominent for the
The 300°C and 400°C yielded more significant forms of
thermal alteration across most of the experimental artifacts tested. The only exceptions to this general trend
were the novaculite flake, china fragment, and pottery sherd which, upon
macroscopic analysis were unaffected by sustained heating performed during
these two trials. Bone specimens,
however, were deeply blackened and charred in appearance due to the pyrolysis of
the organic phase. This resulted in
weight loss ranging between 20-35% for bone specimens as well as antler and
tooth specimens. In addition, the
enhancement of existing surface cracks was observed for bone and antler
specimens during both trials as well as moderate thermal alteration of one bone
specimens and tooth enamel during the 400°C trial. The shell specimen sustained delamination of
its interior surface after heating to 400°C as well a weigh losses ranging
between 8-11% after both trials. Thermal
alteration of lithic materials during the 300°C was similar to that described
for the 200°C trial in which mineral alterations resulted in color changes in
most materials especially the
The 500°C and 600°C trials induced appreciable thermal
alteration for most experimental artifacts particularly organic specimens and
certain lithic material types that have been consistently affected throughout
the experiment. Both the 500°C and 600°C
trial produced crack enhancement, some thermal fracturing, and weight losses
ranging between 34-45% for bone.
However, after the 600°C bone and tooth specimens exhibited a green-gray
and brown color giving them a minor calcined appearance. Thermal degradation of tooth enamel was also
recorded after each trial. Delamination
of the interior portion of the shell specimen also continued during both trials
as did weight losses of between 18-25%.
Lithic materials exhibited heat-induced mineral alterations that
produced marked color changes, and thermal fracturing, spalling, and potlidding
were observed for the
As the temperature was increased to the 700°C and 800°C
levels during the experiment thermal alteration of most material continued to
follow a similar trend as reported for the previous two trials. Bone specimens exhibited a complete calcined
appearance in which specimens became blue-gray and green-gray in color
indicating that the hydroxyapatite is affected in this temperature range. Bone and antler specimens also exhibited
surface crack propagation and some thermal fracturing was well as weight losses
ranging between 21-45%. Delamination of
the interior portion of shell specimens and reductions in mass of 20% continued
for both trials. Tooth enamel also
continued its heat-induced degradation and crack propagation observed during
the previous two trials. The mineral
oxidation and resulting color changes were pronounced for all material types,
and the trend of significant thermal alteration of the Fort Hood chert,
Hartville Uplift chert, and silicified wood flakes continued as it had during
the 400-600°C trials. Obsidian flakes
also continued to exhibit the propagation of radial fracture lines and fine
surface crazing that had been reported for previous trials. However, the black-on-white pottery sherd
exhibited color alterations of the slip from light gray to pale brown and pink
during the 700°C and 800°C trials respectively, indicating the mineral
oxidation had and refiring had occurred.
In addition, the black mineral paint became increasingly faded after
each of the trials as well. The glass
bottle fragment exhibited limited melting during the 700°C and significant
melting during the 800°C As recorded for the previous trials, the china
fragment and novaculite flake were unaffected by sustained heating to 700°C and
800°C.
The final two heating trials generated sustained
temperatures of 900°C and 1000°C respectively.
The impact of the heating trials on the experimental artifacts was
similar for both trials, and of greater magnitude than previous trials. Bone specimens became exhibited a chalky
white and yellow calcined appearance.
Specimens were noticeably more friable and exhibited a high-pitched
percussive sound when tapped; indicating that the mineral phase had been
altered. Bone and antler specimens also
exhibited crack propagation and moderate thermal fracturing in addition to
weight reductions ranging between 27-47%.
Delamination of the interior portion of the shell specimen combined with
significant color alteration and weight reductions of 45% were recorded during
both trials. The black and gray banded
obsidian flake exhibited extreme vesiculation after the 1000°C in which the
flake was altered to a frothy foam-like green globule. The other obsidian flake only exhibited the
linear fracture propagation and surface crazing recorded during previous trials. All lithic specimens, with exception of the
novaculite flake, exhibited significant color alterations as the result of
mineral oxidation. The
Summaries of the thermal alteration recorded for each
artifact during each of the ten heating trials are provided in Table 3.1. In sum, weathered bone (lightly and heavily)
is macroscopically unaffected by heating to 100°C, but the combustion of the
organic phase is initiated at this temperature since minor weight reductions of
3-7% were recorded. At 200°C combustion
of collagen is enhanced and bone begins to exhibit a dark brown slightly
charred appearance, and existing surface crack may be enhanced under thermal
stress. Temperatures ranging between
300-500°C are sufficient to combust the majority of the organic phase of bone
resulting in weight reductions of greater than 30%. This process gives bones a blackened and
charred appearance in addition to potential crack propagation and thermal
fracture. At temperatures between
600-700°C hydroxyapatite may be subject to thermal alteration in addition to
the complete combustion of the organic phase giving bone a calcined green-gray
and blue-gray appearance. Thermal
alteration of the mineral phase of bone continues at temperatures ranging
between 800-1000°C and bone becomes increasingly friable and chalky white and
yellow in appearance. Crack propagation
and thermal fracturing is also probable at this temperature range. These results are consistent with those
reported by Brain (1993), Nicholson (1993), McCutcheon (1992), and Shipman et
al. (1984). Each of these experimenters
used variable experimental methods; however, each is roughly in agreement in
their conclusions regarding the thermal alteration of bone.
Teeth are similarly affected by thermal alteration
compared to bone. The specimens used
during this study were significantly weathered.
Previous research by Shipman et al. (1984) incorporated fresh teeth
still affixed to de-fleshed mandibles.
As a result, the specimens heated during the present experiment
exhibited a far greater degree of crack propagation and thermal fragmentation. The thermal alteration of the root portion of
teeth follow very closely the range of thermal alteration reported for bone at
each respective temperature range.
Weathered tooth enamel was shown to exhibit crack propagation and
thermal fracture at 500°C with thermal fracturing and degradation of enamel
becoming increasing severe at temperatures ranging between 600-1000°C. At high temperature the enamel becomes very
friable and extremely fragmented. Tooth enamel and root sections also exhibit a
blue-gray, green-gray, and white calcined appearance at temperatures between
600-1000°C.
No reference pertaining to the thermal alteration of
antler has been located in the literature.
However, weathered elk antler seems to follow the range of thermal
alteration described. One exception
would be that thermal fracturing was not observed for antler specimens, only
crack propagation beginning at 200°C and consistently increasing in severity up
to 1000°C. In addition, weight loss for
antler was more pronounced than was recorded for bone with losses of nearly 40%
for the 400-500°C trials and up to 47% for the 600-1000°C trials. Antler also exhibited a calcined appearance
that is roughly analogous to that described for bone and at roughly the same
temperature intervals.
Thermal alteration of shell has previously been briefly
addressed Robins and Stock (1990) using campfires to address the issue of
burned shell taphonomy. The study was
not focused on establishing different levels of observable thermal alteration
for shell at given temperatures; however, it did establish that thermally
altered shell is more friable than unaltered shell. The results of the present study have shown
that shell is not visually affected by sustained heating at 100°C and
200°C. Partial combustion of calcium
carbonate begins at 300°C and increased incrementally as temperatures reach
1000°C resulting in mass reductions initially at 8% and culminating at
45%. The delamination of the laminar
inner surface of fresh water shell and significant color alteration begins at 400°C
and continues through 1000°C. As a
result of heating to middle and higher temperature ranges, shell also becomes
increasingly friable when handled; however, no qualitative or quantitative
method was implemented to assess friability.
The two varieties of obsidian (black and gray banded, and
black, red, translucent) heated during the experiment were macroscopically
unaltered by sustained heating at 100°C, 200°C, 300°C with the exception of a
slight increase in luster. Propagation
of existing radial fracture lines and the formation of a metallic sheen were
observed for both specimens beginning at 400°C and continuing for each trial
thereafter until the termination of the experiment at the 1000°C trial. Fine surface crazing was consistently
observed for both varieties of obsidian beginning with the 600°C and for each
following trials. Vesiculation was only
observed for the black and gray banded specimen during the 1000°C trial where
the morphology of the flake was significantly altered from its preheated state
to a green-gray, frothy, foam-like globule.
Weigh losses were not recorded for either specimen during any of the
trials, including the trial in which extreme vesiculation was observed.
In brief, observable thermal alteration of obsidian
may potentially begin at temperatures of 400°C in the form of radial fracture
line propagation, and may extend to fine surface crazing at temperatures
ranging between 600-1000°C. Extreme
vesiculation may occur for some varieties of obsidian at sustained heating beyond
1000°C. Surface temperatures of 400-800°C
and above are possible during prescribed and wildland fires given the
availability of sufficient fuel load and fire intensity. Buenger (Chapter 2) has observed fracture
line propagation and fine surface crazing under field conditions during prescribed
burns in heavy fuels for the black and gray banded obsidian material types used
in the laboratory experiment, although vesiculation was not observed. However, Steffen (2002) has observed
vesiculation of obsidian under field conditions following the Jemez fire in
The chert specimens tested during the experiment included
Hartville Uplift chert,
Significant thermal alteration of cherts in the form
of thermal fracturing was first observed during the 400°C for
The other lithic material types tested during the
experiment included silicified wood, phosphoria, porcelinite, and
novaculite. Silicified wood was clearly
the most prone to thermal alteration of all the lithic materials used in the
experiment. Prominent color alteration
was observed at 200-300°C whereby the original mottled strong brown and very
dark gray color was altered to red. This
trend continued as the temperature of each trial increased accompanied by a
progressively darker red hue. Thermal
fracturing, spalling, and potlidding were initially recorded during the 400°C
trial, and were especially pronounced during the 800-1000°C in which the flake
was reduced to an accumulation of small fragments. The phosphoria specimen exhibited color
alterations from dark red to dusky red beginning at 200°C and continuing
through 1000°C. Crazing (internal
fracturing) was observed for phosphoria during the 900°C and 1000°C, and minor
potlid fracturing was recorded during the 900°C only. Other than alteration of its mineralogy,
phosphoria seems to be rather resistant to thermal alteration up to high temperatures
in the 900-1000°C. Thermal alteration of
porcelinite was limited to color alterations, which were generally not
prominent until the 900-1000°C trials where the original green-gray color of
the material was altered to dark grey and very dark gray. No catastrophic forms of thermal alteration
such as thermal fracturing or spalling were observed for porcelinite. Finally, novaculite is clearly the most
resistant to thermal alteration of all the lithic material types tested. Thermal alteration of novaculite was not
macroscopically observed during any of the trials, even sustained heating to
900-1000°C. This is consistent with research conducted by Flenniken and
Garrison (1975), which conclude that
In sum, the thermal alteration of lithic materials is
initially observable in the form of color alterations beginning at temperatures
as low as 200-300°C for some material types.
Significant thermal alteration of cherts (particularly
The thermal alteration of the Cliff House Formation
sandstone block section was limited to color alteration resulting from mineral
oxidation and slight weight loss. This
material type is very prevalent as an architectural masonry stone at
The thermal alteration of manufactured materials
(historic china, glass, and prehistoric black-on-white pottery sherds) tested
during the muffle furnace experiment varied depending on the material type. The Southwestern black-on-white sherds were
visually unaffected by sustained heating up to 500°C, which is probably close
to the original firing temperature of the vessel of this design. At 600°C, a slight color alteration of the
gray slip in the form of a white haze was observed as well as minor fading of
the black mineral paint used for the design.
After sustained heating at 700°C, oxidation of the slip was initiated
whereby its color was altered to very pale brown, and the black mineral paint
became faded. This trend continued for
the 800°C trial, but the slip altered in color to pink, and the design became
faded. Following the 900°C trial,
oxidation of the slip resulted in a reddish yellow hue as well as a faded
design; and the 1000°C produced a light red slip and the near total
obliteration of the black mineral painted design. Black-on-white sherd heated during the
experiment were essentially refired at temperatures ranging between 700-1000°C
where by the mineralogy of the slip was altered via the oxidation of iron
minerals. This compares well with data
reported by Burgh (1960) in which black-on-white specimens were thermally
altered to red-on-buff in an electric furnace.
Burgh (1960) also notes a pottery collection containing refired sherds
that were burned under natural conditions in which iron contained within sherds
was oxidized to yellow and red hues. The
results of the present experiment suggest that oxidation of iron and subsequent
color alteration occurs at temperatures ranging between 700-1000°C. This observation may potentially be used to
assess fire severity of ancestral
Thermal alteration of the historic glass bottle
fragments consisted of an increase in luster and the propagation linear surface
cracks in areas of preexisting fragility at temperatures ranging between
200-300°C. Thermal spalling along the
fractured edges of the specimen was observed at temperatures of 400 and 500°C,
as was an overall increase in luster.
Partial melting along the fracture edges of the glass fragment was
observed after sustained heating to 600°C, and melting in the form of circular
surface depressions was recorded after the 700°C trial. Significant melting and deformation of glass
fragments was observed during the 800-1000°C where the approximate 700-800°C
melting point of glass was exceeded.
Melted glass is a very consistent and reliable indicator of fire
intensity due to its established melting point.
Melted glass from surface contexts burned during wildland or prescribed
fires would be a near certain indicator of high fire severity.
Thermal alteration of the historic china fragments
used in the experiment was non-existent at all temperature thresholds
(100-1000°C). Thermal alteration of
china is improbable unless the temperature exceeds the original firing
temperature of white-ware china (1200°C), or if sufficient thermal shock is
generated by extreme fluctuations in temperature gradient. Significant thermal alteration of china
during wildland and prescribed fires is unlikely, except in instances in which
a specimen may have preexisting points of fragility.
The data
presented above can be used as a rough guide from which to assess the potential
impact of prescribed and wildland fire on archaeological resources. When assessing the impact of wildland fire on
archaeological resources, this information can be used to gauge fire severity
and potential maximum temperature thresholds when conducting post-fire
inventories. This information may also
be used to establish parameters to be incorporated into prescribed burning
plans where fuel load, potential burn intensity, and the type of archaeological
resource is known.
Regarding the muffle furnace experiment; however, it
should be noted that heating in a laboratory muffle furnace is not necessarily
analogous to the release of radiant heat energy produced by a natural
fire. In Chapter 2, it was shown that
surface heating during prescribed fires in wide range of fuel loads is
precipitous under rather short residence times.
Heating and cooling of artifacts within the muffle furnace was generally
quite steady and of significant duration.
Artifacts are much more susceptible to thermal shock when subjected to
precipitous and intense heating that produces uneven temperature gradients
within the body of the artifact. Thermal
shock resulting in fracturing and spalling of lithics, sandstone, pottery, and
other brittle materials has been shown to occur during sudden and precipitous
temperature change, which induce differential expansion and contraction
(Hettema et al. 1998; Purdy 1974; Rice 1987; Schiffer et al. 1994). Although, thermal fracturing of artifacts
during the muffle furnace experiment was observed, heating during the trials
was generally characterized by steady rises in temperature over a protracted
period. It is likely that variables
other than maximum temperature such as rate of heating are also important when
addressing the potential for thermal alteration of archaeological materials
under laboratory conditions. With this
assumption in mind, a laboratory experiment that more closely approximated
actual wildland fire conditions was developed and implemented. In addition, an experiment explicitly
designed to address the thermal alteration of Cliff House Formation Sandstone
was also conducted. The specifics of these experiments and the results are
provided in Chapter 4.
Table (3.1) Muffle Furnace Artifact Heating Experiment: Thermal Alteration Summary
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Temp °C |