The nature of fire-cracked rock: new insights from ethnoarchaeological and laboratory experiments

Abstract

Fire-cracked rock (FCR) is the archaeological by-product of the systemic use of hot rocks for cooking and heating purposes. As a record of various cooking and heating facilities, FCR has substantial potential for addressing research questions on past settlement and subsistence systems. To be able to address these questions requires reliable methods that can identify or infer how a given FCR was used. A series of ethnoarchaeological replication experiments are conducted in order to improve the understanding of the geothennodynamics of FCR production. FCR from two experimental data sets are cut to expose flat surfaces that are inspected for thermal-weathering characteristics under low-power magnification. Analytical results indicate that rock structure and the length of heat application have the most control over thermal weathering. The five most important rock structure characteristics are (1) strength of the bond between grains/crystals, (2) degree of porosity, (3) grain/crystal mineralogy, (4) presence of discontinuities, and (5) grain size; thin section analysis can identify these characteristics for any rock sample. The length of heat application varies dependent on the type of cooking or heating facility, for instance, a typical earth oven remains hot for a longer duration than a typical stone-boil facility; multiple uses of a rock in a facility also increases the sum total of heat application. In broad terms, experimental tests indicate igneous rock types withstand thermal weathering better than metamorphic or sedimentary rock types. Ethnoarchaeological results are applied to six archaeological FCR features, and provide new insights on the use-history of five of the features. Microscopic observations of archaeological FCR also all for the re-evaluation of the current model which explains the occurrence of various FCR shape types within features.