Friday, August 14, 2009

Heat-treated Tool Stone

We've known for quite some time that it wasn't uncommon for prehistoric peoples to heat-treat their lithic raw material to improve its flaking properties. In fact, I believe W.H. Holmes recognized the practice in some of his pioneering work in lithic studies in the 1890s.

It's been observed ethnographically and assumed archaeologically that raw material is buried in a shallow pit and a fire built over it. Lots of experimental work has been done along this line over the last 30 years or so both with fires and temperature controlled kilns and it has shown to markedly improve the quality of some materials. Sometimes the material is improved by vitrifying the silicates in the stone making it more brittle and other times more complex chemical reactions produce the desired effect.

When observing artifacts in the field, you can often tell if material has been heat-treated by color changes, crazing of the stone surface, or the presence of pot-lid fractures. Most anyone who's done much lithic analysis has seen it.

All of this is a sort of long-winded introduction to an article in this week's NY Times concerning research in South Africa that has pushed back the earliest documented occurrence of heat-treating. Previously, the concensus had been that the practice first occured in Europe about 25,000 years ago, but now material recovered from sites on the South African coast documents it there at least 72,000 years ago.

I found this interesting in that with all the lithic analysis I have done over the years, I had never really thought of heat-treating in terms of chronological implications. I'm sure this comes from my background of research limited to North America.

Here, unlike in the Old World, there is no real trajectory of improving development in lithic tools through time. In fact, some of the earliest tools we find here (such as Clovis) are the most complex and reflect the most sophistication and skill in manufacture. Some of the tool assemblages from Late Prehistoric cultures are extremely primitive - we tend to use the term "expedient" which is less value-laden. For example, in the fieldwork I did last year in Imperial County, on the Late Prehistoric sites (dating around AD 1300 - 1600) there were hardly any formal tools (such as projectile points or scrapers) at all. Really 99.9% of the assemblage consisted of a simple flake someone had knocked off a cobble, used once, and thrown away. That's hardly above a Homo erectus level of technology, yet these were fairly sophisticated farming people, who lived in settled villages, had good pottery, and lots of ground stone tools. They simply kept to a level of technology sufficient for their needs.

You see examples of this all over North America. It would appear that there was a menu of lithic manufacturing techniques (heat-treating among them) that most cultures had available to them from the first entry of humans into the New World. Those that a particular culture chose to use were a function of their particular subsistence technologies, resource availability, or sometimes just interest or esthetics. Some tool forms do change with time and are used as chronological markers, but looking at a total assemblage and characterizing it as simple or complex doesn't necessarily give you a handle on how old it may be.


Matt Mullenix said...

Henry Chappell's Native people in The Callings work with metals borrowed from our culture but spend time contemplating the work of older Native cultures (which they find around their own environment) and wondering about possible advantages of it. That was an interesting and surprising thought.

ironrailsironweights said...

Technological regression of a similar sort happened in parts of the Middle East. The wheel was in use for a period of time, around 2,000 BC if I'm not mistaken, but then fell into disuse for transportation purposes for centuries (water and pottery wheels remained) because the use of camels as draft animals made it largely unnecessary.