[TheForge] ancient heat treatment of stone tools

[Peter Fels & Phoebe Palmer]

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Ancient toolmakers learned that trick, too. And archaeological research 
from South Africa pushes back the date of the earliest use of heat 
treatment at least 45,000 years, to more than 70,000 years ago.

Kyle S. Brown, a doctoral student at the University of Cape Town, and 
colleagues report finding stone tools that show signs of being heated to 
about 600 degrees Fahrenheit. Heat-treating in this way, most likely by 
burying under a fire, made the stone easier to knap, or shape into a 
tool by striking with another stone.

Archaeologists were studying several sites on the South African coast, 
with artifacts dating from 72,000 to 164,000 years ago that would have 
been made by modern humans from the African Middle Stone Age. Mr. Brown, 
an archaeological knapper who tries to replicate ancient tools, said 
they noticed that blades found at the site, made from a stone called 
silcrete, did not match silcrete obtained from outcroppings in the area. 
“We realized we were missing something,” he said.

They experimented by heat-treating some of the stone themselves. “When 
we pulled it out of the fire and flaked it, it did look like the kind of 
stone we were finding at our site,” Mr. Brown said. Their findings are 
published in Science.

The researchers had to show that the tools they found were intentionally 
heated to improve workability, not accidentally through a bushfire or 
other means. They found tools in areas where there was no evidence of 
burning. And they conducted tests on some of the artifacts, including 
one that showed that flaked surfaces had a glossiness that occurs only 
when the stone has been heated, proving that the stones were heated 
first and then worked into tools.

Mr. Brown said that the consensus among archaeologists had been that 
systematic heat treatment first occurred in Europe about 25,000 years 
ago. The current work, he added, “is almost indisputable evidence” for 
heat treatment 72,000 years ago, and perhaps as early as 164,000 years 
ago, although researchers need more samples from the earlier period to 
be sure.