TECHNOLOGY

Heat-Treating Stone Tools

~72,000 years ago

Palaeolithic · Middle Stone Age · Last glacial

At Pinnacle Point, in South Africa, people were heating stone before knapping it about 72,000 years ago, to make it easier to work.

Heat treatment of stone is a process consisting in heating a rock before knapping in order to improve its fracture properties. It is attested at Pinnacle Point, on the southern coast of South Africa, on silcrete, a siliceous rock formed by the induration of sediments and difficult to work in its raw state. Buried beneath a hearth and brought to temperature, the stone undergoes an internal modification not detectable by external examination: it manifests only at the moment of knapping, the fracture becoming cleaner and more predictable. The process is documented from 164,000 years ago and becomes the dominant mode of silcrete preparation around 72,000 years ago.

Why it matters

The operation constitutes a controlled transformation of matter, planned before execution and verifiable only afterwards, long predating ceramics and metallurgy. The level of competence it implies is debated: the initial study saw in it evidence of elevated cognitive capacities, while later experimental work has shown that a simple surface fire, at domestic hearth temperature, already improves the rock appreciably.

Dating & uncertainty

The bounds follow the archaeological signal: heated silcrete appears around 164,000 years ago and becomes dominant around 72,000 years ago, the date retained here. Identifying a heat-treated stone rests on a set of measured criteria, surface gloss, fracture plane morphology and roughness recorded under three-dimensional microscopy, interpreted by means of probabilistic models calibrated on experimentally heated rock.

Our species’ history as one day

18:30:515 h 29 min before midnight

if the 315,000 years of Homo sapiens were compressed into a single day