Stone Tool Analysis: Heat fracture
Rapid thermal expansion in brittle stones—such as flint and chert—can cause cracks to form internally. These initiate at flaws where the differential rates of expansion create fields of stress. If the stress overcomes the material’s elasticity, the stone will break. This is often caused by exposure to the intense heat of fires, but it can also occur in freeze-thaw cycles and on stones exposed on the surface in deserts.
Stone that is raised to extreme temperatures can crenate into cuboidal and angular pieces. Quartz undergoes phase changes from about 573º C, resulting in an increase in expansion that exacerbates internal fracturing in silicious stones. These fractures are irreversible and undesirable for stoneworking.
Deliberate heat-treatment is a process of carefully cooking stone in an earth oven or hearth to change the internal structure and make it more amenable for flaking. Archaeologists sometimes infer from heat-fractured pieces found in archaeological sites that heat-treatment was occurring, but it is more likely that the stone was accidentally exposed to heat by scuffing into cooking features or hearths, or other taphonomic processes. Successful heat-treatment will not result in heat fracture fragments. The possible practice of heat-treatment must be evaluated relative to the differences in gloss between pre-heat treatment flake scars and flake scars created after heat-treatment.
