Fire Effects on Archaeological Materials

Items from prehistoric and historic societies are fragile and nonrenewable resources (Swan and Francis 1991) that provide evidence used by archaeologists, historians, and other types of researchers to reconstruct the behaviors and worldviews of people who are no longer living. Artifacts, or manufactured items from prehistoric societies, were constructed from a variety of source materials. Different source materials have different reactions to heat. Some of the most common source materials for archaeological and historical artifacts that can be found in the southern United States are chert (Whyte 1983, 1984), stone, bone, shell, clay, metal, glass, wood, and leather (California Department of Forestry 1983). Fires have caused damage to rock art, petroglyphs, and caves (see Duncan 1990).

Numerous field experiments have tested the effects of heat treatment to archaeological materials (e.g., Whyte 1983, 1984). Some research looks at the effects of heating raw materials that were commonly used to construct artifacts. Some of the materials that have been studied and that were used by prehistoric people in the South are obsidian (e.g., Linderman 1990), chert (Patterson 1995; Whyte 1983, 1984), flaked stone (Whyte 1983), flint (Griffiths and others 1987), silica (Purdy 1974), and bone (Shipman, Foster and Schoeninger 1984).

Archaeologists, dendrochronologists, and other scientists who reconstruct fire histories have noted the effects of past fires on artifacts and historical structures as a secondary or peripheral aspect of their research. Some archaeologists (e.g., Whyte 1990) have attempted to interpret fire scars on prehistoric materials to determine whether they were burned directly by humans or after humans disposed of them; for example, fish bones found in a hearth. Numerous studies have been conducted by state and federal agencies because the effects of fire on cultural resources are of concern to public lands managers. The National Park Service has produced numerous studies and publications on fire and cultural resources (e.g., Kelly and Mayberry 1980).

Artifacts that lie on the surface of the ground are more exposed to fire and thus more vulnerable to fire damage. In the South many of the materials left by prehistoric communities are subsurface artifacts (buried beneath the ground) in contrast to the West where surface artifacts are more common (Mayer 2003). Subsurface artifacts are less vulnerable to fire. Most artifacts that are 5cm or more below the surface in forested ecosystems or 1 cm below the surface in grasslands will escape damage. However, artifacts up to 20 cm below the surface may be impacted where fuels burn long enough or roots or stumps carry heat deep enough to bake and oxidize the soil (Lentz, Gaunt, and Wilmer 1996; Pilles 1982). Burning roots and stumps can also displace soil, thus dislocating nearby artifacts (Powell 1987). Soil and fuel conditions influence the effects of fire on subsurface artifacts. For instance, in places where fuels or soils are wet or fuels are heavy the residence time (duration) of the burn may be longer and subsurface artifacts are more likely to be adversely affected. Smoldering roots can be as hot as 1500°C (DeBano, Neary, and fFolliott 1998). Low- and moderate-intensity fires tend not to alter subsurface ceramic artifacts. Likewise, stone artifacts that are buried are less likely to be altered by fire than those on the surface.

Cultural resources fall into three categories based on their vulnerability to fire (DeBano, Neary, and fFolliott 1998):

• high vulnerability: organic, surface resources
• moderate vulnerability:  wooden structures from the historic era and lithic scatters
• low vulnerability:  unexcavated, subsurface artifacts

Artifacts composed of organic materials are vulnerable to being adversely affected by fires. Whereas inorganic materials are not vulnerable to fires below 400°C, organic materials will ignite at 280°C (DeBano, Neary, and fFolliott 1998). When fires reach 285°C bone is partially or completely charred or turns reddish brown. All bones do not burn uniformly, but differentially; for example, the bones of some fish species are more damaged by fire than other species (Whyte 1990). Above 525°C bone develops colors of grey, blue, white, and black. The effects of fire on wood depend on the type of wood and the fire behavior. Many types of wood burn above 590°F. Fires above 300°C can destroy surface pollen, but subsurface pollen is not usually affected. Pollen and macrobotanical remains that are on the surface of artifacts such as grinding stones can burn away resulting in the loss of data (Lentz, Gaunt, and Wilmer 1996). Fire can damage or destroy vegetable fibers and other plant remains. Moderate-intensity fires can cause shell artifacts to burn to ashes.

When a fire reduces the vegetative cover, archaeological materials that were previously covered may become more visible (Leonard 2002; Pilles 1982). In some cases this can damage cultural resources because they become more vulnerable to non-systematic collection or to deterioration by environmental processes thus damaging their value as historical evidence. In other cases, the discovery of undocumented cultural resources can lead to scientific advances and enhancement of cultural knowledge.

Fire can cause a number of structural changes in ceramics. They can crack or break apart. Fire can deposit soot, cause smudging, and alter pigments on ceramic artifacts. Surface designs may be obscured or obliterated (Lissoway 1990). The organic paints that decorate some ceramic artifacts can become brittle or be burned away in temperatures of 350°C or higher (DeBano, Neary, and fFolliott 1998). Mineral paints may be blackened (NWCG 1995). Fire may cause ceramic materials to become brittle and develop a glossy sheen, a process known as vitrification. The burning of resinous vegetation such as pine needles can deposit adhesions, or sticky substances that bond to ceramics. Moderate-intensity fires may deposit carbon substances and adhesions on ceramic artifacts found on the surface in addition to causing pigment alteration, oxidation, cracking, fracturing, and spalling (or breaking) (DeBano, Neary, and fFolliott 1998). Some of the effects that high-intensity fires have on surface ceramics are oxidation, brittleness, and paint dissipation. Above 1,292°F, ceramic artifacts can be refired, altering their structure.

Low-intensity fires are unlikely to affect lithics – artifacts made of stone. Above 375°C, stone artifacts on the surface or subsurface may become discolored, cracked, and could possibly disintegrate (DeBano, Neary, and fFolliott 1998).  Fires above 500°C may affect lithics in the following ways:

• discoloration
• sooting
• brittleness
• cracking
• crazing
• potlids: vapors and gases build up causing pieces to slough off and leaving depressions on the lithic

Spalling occurs on flaked stone above 662°F.

Heat treatment damages artifacts made from obsidian in several ways. Crazing – asymmetrical fissuring and cracking – begins to occur on obsidian artifacts exposed to fires above 540°C. Above 750°C the obsidian becomes progressively distorted (Hatch and others 1990) and begins to melt above 760°C (1,400°F) (Trembour 1990). Severe heat causes obsidian to bubble, bloat, and vesiculate.

The effects of fire on cultural resources from the historic era are usually more apparent than they are on those from the prehistoric era because the structures are more visible and many of the materials are more flammable (Anderson 1983). Glass, for instance, will fracture above 93°F, damaging any historical artifacts made from this material. Fire can cause damage or destruction to log cabins, wooden beams, wood frames and other wood components on historic buildings (Lissoway 1990). It can cause discoloration or partial disintegration of stone masonry and sandstone.

Fire Effects on Archaeological Dating Techniques

Fire has the potential to interfere with several common techniques used in dating artifacts. Archaeologists attempt to determine the age of artifacts to reconstruct the function and meaning of cultural resources. Ash or soot can change the results of carbon-14 dating, inaccurately producing dates that make an artifact appear younger than it is. Very low-intensity prescribed burns in light fuels have minimal impacts on obsidian hydration rims (Linderman 1990, 1993). Fires above 170°C-200°C alter the results of obsidian hydration dating (Skinner 2002), a technique that calculates the time when hydration bands were formed to measure the origin of a manufactured artifact. Fires above 675°C (997°F) reorient the electrons that are measured in archaeomagnetic dating techniques used to determine the age of stone artifacts. Fire can alter the ability of thermoluminescence to diagnose the age of pottery sherds (Rowlett and Johannessen 1979).

Fires that damage or destroy wood artifacts, and live or dead trees prevent scientists from using dendrochronology to determine dates of origin, tree felling, or of construction.