Bare Bones Guide to Fire Effects on Cultural Resources
For Cultural Resource Specialists

by Kate Winthrop, BLM

I. Introduction

This document briefly synthesizes some of the technical information available on the effects of fire on cultural resources. This synthesis should assist cultural resource specialists with their contributions to fire management planning, compliance for prescribed fire projects, and participation in wildland fire use or wildfire events.

Research on fire effects is on-going. A publication on this topic will soon be released under the USFS Rocky Mountain Research Station “Rainbow” series, and much of the data here is from drafts of articles for that publication. While there is a lot we do not know, there is also a considerable amount of work accomplished on this topic. This brief guide summarizes the results of some of these technical studies.

Fire effects to cultural resources, and the appropriate ways to manage for these effects, are context dependent. Fire itself is dependent on a suite of variables which change across the landscape; fire in grassland is likely to produce different effects to cultural materials than fire through a forest with heavy duff. Different types of archaeological materials, such as varieties of toolstone or types of ceramics may react differently in similar fire-related circumstances. This guide offers technical information which cultural resource specialists can use to craft locally and regionally appropriate strategies for protecting cultural resources within the context of fire. 

References: References are cited at the end of the document. Where possible, links are provided to resources that are already available on the web. The “Rainbow” publication on “Fire Effects to Cultural Resources” will soon be available on the web and those links will be added to this document. The Western Archaeological Conservation Center, National Park Service, is currently digitizing many reports concerning fire effects on cultural resources. Those reports will also be available on the web in the next few months, and will be linked to this document.

II. Fire Basics

Fire effects to cultural resources vary depending on temperature and duration of exposure to heat. Generally, higher temperatures and/or longer duration of exposure to heat increase the potential for damage to cultural resources. Variables that affect temperature and duration include (Wiltz n.d., Hanes 2001):

•  Type of fuel
•  Fuel load/ distribution
•  Moisture content of fuels
•  Soil type, soil moisture
•  Weather, terrain

As a general rule, fire does not affect buried cultural materials. Studies show that even a few centimeters of soil cover (10 cm) are sufficient to protect cultural materials (Oster, n.d.). However, there are times when conditions do carry heat below the surface, with the potential to affect buried materials. These conditions include:

•  Stumps that smolder and burn have the potential of affecting buried materials that are in the vicinity.
•  Heavy duff, surface logs, and roots that smolder and burn also have the potential to expose subsurface materials to heat over a period of time, and hence have the potential to affect cultural materials.

Fires that burn hot and fast through a site may have less of an effect on certain types of cultural materials than fires that smolder in the duff, or than logs that burn for a period of time.

III. Cultural Resource Basics

When assessing the potential effects of fire on cultural resources there are some fundamental considerations (Hanes 2001, Duke et al. 2003):

•  Even if fire affects certain cultural materials, that effect may not be important. That is, the effect may not actually diminish characteristics that make a site eligible to the National Register. For example, high heat may destroy obsidian hydration bands on surface artifacts, but the surface component of the site may not be of particular value in the site’s overall assessment. Fire may burn the solder out of a hole-in-cap can, but this effect does not diminish the can’s ability to provide chronological information for a site.
•  Wildland fire is generally more destructive to cultural resources than prescribed fire, since it includes both uncontrolled fire effects and the effects of fire suppression. Management decisions may need to balance the potential effects of a prescribed burn with the risk of damage from an uncontrolled wildfire.
•  Fire history may be important. When assessing the potential effects of fire to cultural resources, cultural resource specialists should consider the nature of past fires compared to the potential for fire at the current time. For example, have fires routinely burned through an area? Have conditions (e.g. fuels and fuel loads) changed significantly over time? Will the effects of fire today be significantly different—and pose a greater threat to cultural materials than in the past?
•  Prescribed fire can be controlled. Cultural resource specialists can work with fire managers to determine the predicted temperature and duration of a fire through an area, and possibly to modify burn plans to minimize effects to cultural resources.
•  Protecting cultural resources during fire begins with fire management planning. This is the place to define vulnerable cultural resources, appropriate protection measures for them, and appropriate management responses with regard to cultural resources in the event of wildland fire or a wildland fire use event.
•  As always, consultation with SHPO, Tribes, and other appropriate entities should be part of the project planning process, especially when designing fire-specific protocols for identification and protection of potentially affected cultural resources.

III. Fire Effects on Lithics (Deal n.d., Buenger 2003)

Fire can affect chipped and groundstone tools, primarily through changes in morphology rather than in chemistry. Residues on artifacts are not necessarily destroyed by fire. As a general rule-of-thumb, hotter temperatures and longer exposure to fire may affect lithic materials. When these materials are important, it may be necessary to take protective measures.

Obsidian
Fire can modify or destroy obsidian hydration rinds, but does not affect obsidian source analysis (Shackley and Dillon 2002). High temperatures, such as those experienced in a catastrophic wildfire, may be sufficient to cause obsidian to bubble and crack, loosing shape as well as hydration capacity.

The exact temperature at which obsidian is affected varies, probably due to components of the field environment and/or differences in source materials. Duration of exposure increases the effect of heat on obsidian. High temperatures and smoldering fires can both affect hydration bands.

Obsidian: Approximate Temperature Guide (Deal n.d., Buenger 2003,
Loyd et al. 2002)

Temperature	Effect
300 C (572 F)	Hydration band begins to become diffuse
400 C (752 F)	Hydration band not visible
450 C – 800 C (842-1472 F)	Enhanced fracture lines
760 C (1292 F)	Obsidian may melt

Chert
Fire can also affect chert (including various silicates), through fracturing, pot-lidding, crazing, shattering, changes in color and internal luster, and other such effects which might reduce an artifact’s ability to render information about the past. Temperatures which affect chert vary, possibly dependent upon source or other variables such as prior heat-treatment for tool manufacture. Generally, longer and/or hotter fires produce more intense effects upon chert artifacts (Deal n.d., Waechter n.d.).

Chert: Approximate Temperature Guide (Deal n.d., Buenger 2003)

Temperature	Effect
350 C (662 F)	May become distorted, brittle or explosive
350 - 550 C (662 – 1022 F)	Cracking, Fracture

IV. Fire Effects on Ceramics (Rude n.d., Buenger 2003, Haecker n.d.)

Different types of clays, inclusions, and manufacturing techniques lead to different effects among distinct pottery types. Since all pottery—historic and prehistoric—has been fired to some degree, heat damage is not as significant a consideration for this artifact type as it is for others. Generally, structural damage does not occur until temperatures exceed the original firing temperature. The main type of damage noted is to the surface decoration or glaze.

Fire may affect the potential for thermoluminescence (TL) dating. However, surface potsherds are generally not used for this technique, and buried potsherds are not likely to be affected by fire. Another study also showed that TL dating was not affected at temperatures below 400 C (752 F), indicating that moderate intensity wildland and prescribed fire may not have an impact on TL dating (Rude n.d.).

Ceramic	Firing Temperature	Temperature Effects
Prehistoric	> 350 C (662 F)	Minor effects (sooting, fading, discoloration)
Prehistoric	> 600 C (1112 F)	Structural change possible
Prehistoric	> 400 C (752 F)	TL dating potential compromised
Historic: Unrefined Earthenware	500–900 C (932–1652 F)	Glazes may crackle and spall at low fire temperatures
Refined Earthenware (“whiteware”)	1100–1500 C (2012-2732 F)	Glaze may crackle at low fire temperatures
Stoneware	900–1100 C (1652-2012 F)	Temperatures above firing point may oxidize glaze or crack shards
Porcelain	1250-1450 C (2282-2642 F)	Temperatures above firing may oxidize glaze or crack shards

V. Fire Effects on Organic Materials

Material	Temperature	Effect
Bone	200 C – 400C (392-752 F)	Bone chars, becomes darkened
Bone	600 C – 800 C (1112-1472 F)	Bone becomes calcined
Shell	>300 C – 400 C (572-752 F)	May delaminate, burn

VI. Fire Effects on Historic Materials

The following chart provides melting points for materials commonly found at historic sites. Fire may produce complex interactions which affect these baseline temperatures, however. Metal alloys may react differently, and metal artifacts/ materials which do not melt may warp. The chart is derived from Haecker (n.d.).

MATERIAL	TEMP (F)	TEMP (C)
Plastics	167-509	75-265
Solder (tin-alloy)	275-350	135-177
Tin	449	232
Pot Metal (copper-lead alloy)	572-752	300-400
White pot metal	572-752	300-400
Lead	621	327
Zinc	707	375
Glass	1100-2600	593-1427
Unrefined Earthenware	1112 – 1832	600-1000
Aluminum	1220	660
Brass (yellow)	1710	932
Silver	1760	960
Stoneware	1832-2192	1000-1200
Gold	1945	1063
Copper	1981	1082
Refined Earthenware	2192-2912	1200-1600
Cast Iron	1920-2550	1350-1400
Steel (stainless)	2600	1427
Nickel	2651	1455
Steel (carbon)	2760	1516
Iron	2795	1535
Porcelain	2822	1550

•         Rapid cooling: dumps of any of these materials on hot surfaces may cause effects to archaeological materials (e.g. artifact fracture) from rapid temperature change;
•         Materials dumped onto fragile archaeological features may break/ displace them;
•         Long-term retardants contain salts which can be desiccants, which damage old, fragile wood and may cause spalling in sandstone; chemicals may cause corrosion in metals; iron oxide additives may leave a permanent red stain and corrosion inhibitors in the retardant may turn surfaces, especially metals, blue or black;
•         Foams may hasten rusting on metal surfaces by removing protective coatings and may cause wood to flake due to swelling and contracting;
•         Water enhancers are desiccants and may damage wood surfaces, strip surfaces of finishes, and damage sandstone; they are also difficult to remove from wood surfaces, especially for old or fragile wood.
•         Retardant should be washed off important structures as soon as possible. Pre-soaking, then hand-brushing with water and a mild detergent may work for sandstone or painted wood. Metals and glass may be wiped with water and a mild detergent. Power washing, sand-blasting, and acid based washes may damage historic materials.
• There are various potential effects from use of retardants, foams, and water:
•         Rapid cooling: dumps of any of these materials on hot surfaces may cause effects to archaeological materials (e.g. artifact fracture) from rapid temperature change;
•         Materials dumped onto fragile archaeological features may break/ displace them;
•         Long-term retardants contain salts which can be desiccants, which damage old, fragile wood and may cause spalling in sandstone; chemicals may cause corrosion in metals; iron oxide additives may leave a permanent red stain and corrosion inhibitors in the retardant may turn surfaces, especially metals, blue or black;
•         Foams may hasten rusting on metal surfaces by removing protective coatings and may cause wood to flake due to swelling and contracting;
•         Water enhancers are desiccants and may damage wood surfaces, strip surfaces of finishes, and damage sandstone; they are also difficult to remove from wood surfaces, especially for old or fragile wood.
•         Retardant should be washed off important structures as soon as possible. Pre-soaking, then hand-brushing with water and a mild detergent may work for sandstone or painted wood. Metals and glass may be wiped with water and a mild detergent. Power washing, sand-blasting, and acid based washes may damage historic materials.

•         Increased visibility from vegetation burn-off and consequently greater vulnerability to vandalism
•         Physical damage to sites from snags/ trees falling
•         Soil erosion and loss of archaeological data
•         Increased damage from rain, new drainage patterns, flood
•         Increased rodent and insect activity within site soil matrix

XI. Protection Protocols

•         Serve as a technical specialist during fire events; the best protection for cultural resources during a wildfire is to have knowledgeable professionals ready and able to participate in the suppression effort.
•         Prepare plans for protecting high value cultural resources before a fire occurs, and make sure that appropriate authorities know about and have access to these plans. Define ahead of time those high value cultural resources which are really worth saving. “Fire proof” vulnerable sites ahead of time when possible.
•         Work with prescribed fire project planners to accommodate cultural resource concerns in the burn prescriptions.
•         Ensure that cultural resource concerns are included in fire management plans, especially with regard to appropriate management responses to fire whenever it might occur in specific areas. For example, where there are areas of high value cultural resources and these are also areas where fires will be suppressed, ensure that plans include the necessity for “ordering up” a cultural resource specialist when a fire occurs.
•         Track fire effects on cultural materials in local contexts, and share that information regionally. When possible, do “before/after” experiments of prescribed burns, to assess the effects of fire in specific, local contexts on those archaeological materials which are typical in your area.

•         Identify and avoid vulnerable cultural resources. Note that avoidance may contribute to greater likelihood of wildfire in the future when sites have high fuel loads, or that avoidance may create “vegetation islands” that identify sites to vandals. If necessary, work with fire planners to minimize these effects.
•         Record and collect information that would be lost during a fire. For important rock art, thorough recordation and collection of samples of the surface varnish for dating may be the best protection possible.
•         Manually reduce fuels on and/or around vulnerable sites; pile debris offsite.
•         Create fire breaks near/ around sites. This may be an effective way to protect rock art panels, for example.
•         Use retardant or foam to protect structures.
•         Wrap structures in fire proof materials to protect from fire.
•         Remove logs/ heavy fuels from vulnerable sites/ features (e.g. clear snags off bedrock mortars), or cover with foam or retardant prior to burn.
•         Flush cut and cover stumps with dirt, foam, or retardant, where burnout could affect subsurface cultural resources.
•         Modify burn plans to minimize effects to cultural resources, such as burning when duff has high moisture.
•         Identify and reduce hazard trees next to structures.
•         Use low intensity backing fire in areas near historic features.
•         Saturate ground/grass adjacent to vulnerable structures with water, foam, or gel before burning.
•         Preburn site at lower intensity than planned for surrounding areas.
•         Limit fire intensity and duration over vulnerable sites.
•         Use a fast-moving, higher intensity fire over lithic scatters, where rock materials are vulnerable to longer-duration heating.
•         Wrap carved trees, dengroglyphs, and other such features in fire retardant fabric.
•         Limb carved trees to reduce ladder fuels, if possible to do so.
•         Cover rock art in fire retardant fabric.
•         Minimize fuels and smoke near rock art.
•         Cover fuels near rock art with foam, water, or retardant, avoiding the rock art.

For a good discussion of protection measures for historic structures, see Matz (2002)

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