Thermally Altered Siliceous Stone from Prehistoric Contexts: Intentional versus Unintentional Alteration

Abstract

Recent studies have demonstrated that the flaking characteristics of certain siliceous stone materials are enhanced by carefully controlled annealing and that intentional annealing was indeed practiced by prehistoric Native Americans. A note of caution is extended here to investigators who might extend evidence for thermally altered siliceous stone from a site to interpretations that intentional annealing was practiced at the site. The use of fire in quarrying has a long history throughout the world, and much heat-treated material from sites may have been thermally altered unintentionally, the result of quarrying practices.

Use-Alteration Analysis of Fire-Cracked Rocks

“…Heat treatment prior to knapping improves the raw materials’ workability when they are heated slowly and out of direct contact with intense heat, at relatively low temperatures (Gregg and Grybush 1976). The most obvious indicators for lithic heat treatment are changes to color and luster in some raw material types (e.g., quartzite and chert), although the latter may only become clear if a flake has been removed following heat exposure (Deal 2012:99; Gregg and Grybush 1976:190). When a heat-treated rock is knapped, it can produce a flake scar bearing differential luster (House 1975:84; Prous 2004:85).…”

Comment on “Tomato Springs: The Identification of a Jasper Trade and Production Center in Southern California”

“…Seventy-seven percent seems to be a very high percentage of "shatter" regardless of proposed functional implications (Henry et al 1976;Patterson and Sollberger 1978). Both "broken flakes" and angular pieces could be the result of core reduction, biface production, or even thermal spalling during heat treatment, or as a result of chaparral fires (Crabtree and Butler 1964;Gregg and Grybush 1976;Jeter 1980;Purdy 1974). Because both "broken flakes" and angular chunks lacking bulbs of force or platforms were included in her "shatter" category, it is difficult to evaluate her position (Cottrell 1985:844).…”

Temporal nature and recycling of Upper Paleolithic artifacts: the burned tools from the Molí del Salt site (Vimbodí i Poblet, northeastern Spain)

“…For example, the thermal preparation of flint was used in the manufacture of Solutrean points in Parpalló Cave (Tiffagom, 2006). However, determining the intentional nature of this heat treatment is not always straightforward and the accidental burning of lithics cannot be ruled out in some cases (Gregg and Grybush, 1976). In addition, thermal treatment seems superfluous for the manufacture of most Upper Paleolithic tools, which do not require high technical skill and are commonly made on unburned blanks.…”