Thermally-Shocked and fire-cracked Rock: Defining attributes of archaeological boiling stones

This work investigates ancient heating technologies via fire-cracked rock (FCR) analysis to inform upon foodways and land-use intensification. How FCR was used in the past is quite variable globally, and depends on such factors as the organization of settlements and use of raw materials at multiple scales. This analysis identifies thermal properties that influence fracture mechanics relevant to fire-cracked rock (FCR) recovered from many types of archaeological sites around the world. This article has two goals. The first is to establish macroscopic attributes diagnostic for boiling stones, a sub-class of FCR that can inform on acquisition and use life based on thermal properties. The second is to present sampling considerations for FCR analyses.

Building on results of an earlier study (Shantry 2020), four material types—basalt, basaltic andesite, diorite, and granodiorite—are examined here microscopically to determine what makes them resistant to thermal shock and preferable for boiling tasks. The four material types were imaged on a microprobe and examined petrographically before and after thermal shock. Results show strong bonding and smooth textures influence the resistance to thermal shock in these igneous rocks. Attributes of crazing on cortex, crenulated edges, intersecting fracture lines, and peak fractures were found to be the result of thermal shock.

Sampling for FCR should include a variety of primary and secondary contexts to capture the entire use-life of the materials. Accumulation of FCR involves decisions related to acquisition and management of raw materials. Feature contexts where FCR is associated with other materials greatly inform on the nature of hot rock usage. However, FCR is commonly found in secondary contexts or on the periphery of feature deposits. Sampling methods that can accommodate FCR outside of feature contexts are likely to provide finer-grained understandings of heating technologies.