Heat-treating tools

14 August 2009
Posted by Chris Smith.

Human ancestors used fire to get the best out of stone tools as much as 164 thousand years ago, according to new research published in the journal Science this week.

camp fireKyle Brown, from the University of Cape Town, and an international team of colleagues, looked at excavations from multiple sites in South Africa, including Pinnacle Point on the south coast, specifically for tool fragments made from silcrete rock.  Silcrete is a finely grained rock that is traditionally thought of highly workable in its natural state, and silcrete tools are known to have been made in Australia as well as Africa.  However, Brown and colleagues tried it out, and found it to be difficult to shape in a consistent way.  Indigenous Australian knappers are known to have used to treat the silcrete with fire, which changes the crystal structure of the rock, making it much more workable.

Using a number of archaeological techniques, archaeomagnetism, thermoluminescence and gloss analysis, the researchers compared excavated tool fragments with raw or heat-treated samples.  Heating a material will change its magnetic properties, especially if heated to above the Curie point, the temperature at which a material loses its magnetic ability.  Heating to less than this temperature still leaves a predictable change in magnetic properties, sealing the thermodynamic history into the rock.  Likewise the history can be seen through the amount of light emitted on heating, as heating will allow electrons to be released from a mineral -producing thermoluminescence.  Over time, the minerals are exposed to radiation, and build up more potential luminescence.  High temperatures can effectively re-set the strength of thermoluminescence, telling you when the mineral was heated, and to what temperature.  These methods confirmed that all of their collected samples had been heat treated.  Gloss analysis, a measure of how shiny the surface of the rock is, then confirmed that the tools were shaped after heat treatment, even those found to be around 164 thousand years old, and certainly in tools formed 72 thousand years ago.

Further analysis of the sites showed no evidence for extensive fire, suggesting that these were intentionally exposed to high temperature, rather than caught in a wild fire.

The importance of heat treating is that it allows one to use local, poorer quality materials, and still get the performance of higher quality tools.  This shows a huge intellectual leap from the use of fire for heat and light, and this cognitive change is still not well understood.  From this data, it seems that our ancestors were developing the ability to use fire as an engineering tool in South Africa as much as 164 thousand years ago, and were certainly doing so around 71 thousand years ago from when there is widespread evidence of increasingly complex symbolic behaviour.  This seems to have allowed the development of advanced tools in Africa before their development in the rest of the world, and being able to make the best of local materials could well have been a behavioural advantage when migrating up through Eurasia.

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