Stone Tool Analysis: Heat fracture

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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. 


A potlid is a disc-shaped flake that is flat on one side and convex on the other.  Potlids initiate at a flaw on the stone, often microscopic in size, located at the apex of the convex side.  The stresses created by extreme temperatures, or temperature change, cause the stone to fail at the flaw and a crack to form.  The fracture front propagates outward and upwards towards the free face of the stone, popping out the potlid flake from the surface. 

Elko point, NV

Elko Point

Timor Tanged Point

Timor Tanged Point