Chert
“Fire can 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.).” (Winthrop 2004)

Thermal Alteration of Chert

Luedtke (1992) provides an intensive summary of the physical characteristics of chert as well as an overview on the effects of thermal alteration of cherts.

Numerous researchers have conducted research concerned with the intentional heat treatment of various types of chert (Ahler 1983; Beauchamp and Purdy 1984; Bleed and Meier 1980; Collins and Fenwick 1974; Domanski and Webb 1992; Flenniken and Garrison 1975; Gregg and Grybush 1976; Griffiths et al. 1987; Hester 1973; Joyce 1985; Mandville 1973; Melcher and Zimmerman 1977; Olausson 1983; Ozker 1976; Patterson 1984, 1995; Perkins 1985; Price et al. 1982; Purdy 1974; Purdy and Brooks 1971; Rick 1978; Rick and Chappell 1983; Robins et al. 1978; Rondeau 1995; Roberts et al. 1978; Schindler et al. 1982; Shippee 1963; Weymouth and Mandeville 1975).   

processes involved during thermal alteration (Purdy (1974)

100-150˚C (212-302 F) - free water is evaporated from pores and cracks (0.4-2.0% weight loss)

200-300˚C (302-572 F) - goethite oxidizes to hematite (Schindler et al. 1982)  

         350˚C (662 F) - May become distorted, brittle or explosive

350-500˚C (662-932 F) - chemically bounded water within the chert is driven off, and sulfur and iron compounds begin to oxidize (Purdy 1974; Griffiths et al. 1987; Shepard 1971).

         350 - 550˚C (662 – 1022 F) - Cracking, Fracture

> 500˚C (932 F) - carbon and other non-silica materials begin to oxidize, decompose, dehydrate, and potentially fuse (Shepard 1971). 

observable changes (Ahler 1983; Griffiths et al. 1987; Schindler et al. 1982; Purdy 1974)

1. color change,
   

2. increased luster,
   

3. reduced tensile strength,
   

4. fracturing (blocky/angular),
   

5. fracturing (potlid), and
   

6. crazing (internal fracturing) . 
   

From Buenger 2002 - “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 Florida, there is often a color shift from pink to red, which is the result of various iron compounds oxidizing to hematite (Purdy 1974).  In Bald Eagle jaspers (Pennsylvania), a shift from yellow to red is the result of goethite being thermally altered to hematite (Schindler et al. 1982).  In general, a color change within cherts and jaspers with heating is related to the alteration of iron minerals such as hematite, goethite, limonite, and pyrite (Luedtke 1992).

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). 

References

Melcher, C. L., and Zimmerman, D. K., 1977. Thermoluminescent determination of prehistoric heat treatment of chert artifacts. Science 197, 1359-1362.

Luedtke, B. E., 1992. An archaeologist's guide to chert and flint. Institute of Archaeology, University of California, Los Angeles. 

Mandeville, M. D., 1973. A consideration of the thermal pretreatment of chert. Plains Anthropologist 18, 177-202.

Patterson, L., 1995. Thermal damage of chert. Lithic Technology Journal 20, 72-80.

Perkins, L. R. I., 1985. Experiments in heat-treating west-central Mississippi chert. Mississippi Archaeology 20, 19-40.