The selection of boiling stones from a heterogeneous supply

Abstract

Heating stones are an ancient technology found across time and space in the archaeological record. Most often studied in the contexts of cooking activities and overall density at a location, the decision-making used in obtaining stones to heat for particular tasks has been overlooked. The availability of stones varies from place to place and is controlled by the local supply. On the rocky beaches of Puget Sound, part of the glacially-carved Salish Sea in Northwestern North America, the choices are vast, as the supply of cobbles and pebbles is heterogeneous and ubiquitous. Archaeologists have previously demonstrated that rocks which are heated and quenched in water, or cooled quickly, break in distinct ways compared to rocks which are heated and air-dried, or cooled slowly. The fine-grain and coarse-grain textures of different rocks influence the breakage and fracture patterns resulting from expansion and contraction during heating and quenching. If field archaeologists can differentiate slow-cooled versus fast-cooled culturally-heated rocks, researchers can begin to consider behavior related to boiling stone technology in the Salish Sea. This study tests the utility of coarse-grain and fine-grain rocks for repeated use in boiling by examining the thermal properties of different local rock types as well as the susceptibility of each rock type to breakage when heated and submerged in water.