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Heat treatment

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  • Heat treatment

    Heat-treatment of lithics has been used in North America for thousands of years. Points made from heat-treated cores are reported from the Clovis Anzick site in Montana for example.
    Until fairly recently, we believed the technology (as an intentional technique) originated in Europe around 25,000 years ago. In 2008, Kyle Brown presented a convincing set of arguments for its use in the Middle Stone Age of South Africa 60,000 years ago to improve the knappability of "silcrete"… a rock similar in appearance and properties to quartzite, but formed by different processes. Subsequently, that date has been pushed back to at least 72,000 years and possibly as far back as 164,000 years.
    It’s worth bearing in mind that rocks other than flint/chert/novaculite can potentially benefit from heat treatment (and that not all such rocks experience a benefit at all). Jasper and quartzite were also heat treated, for example. Also, not all observed heat treatment is necessarily deliberate. Lithics that have experience wildfires and rocks used for hearth-building can also become heat treated by pure accident. There are also areas (eg some parts of Texas) where cherty lithic materials have been exposed to heat from proximity to lava flows and can be found “ready-cooked” in a manner that represents a weak form of contact metamorphism (more properly called “induration”).
    It’s also not correct to think of heat treatment as a “hardening” process. It alters the structural integrity of the material in a manner that makes it more “glassy” such that it can be flaked in a more controlled manner. The fractures created by the knapping process become more conchoidal and imperfections or cleavage planes which would otherwise work against the knapper's intent can be negated.
    Exactly how that happens, what heat treatment regime is required to optimise it, and how to recognise it vary considerably across different rock types. Ultimately it developed to a sophisticated process requiring careful control of time and temperature. In general, it’s really difficult to tell whether a lithic has been heat treated unless you have a control sample of the original material as a comparison.
    At the Anzick site I mentioned, the artefact assemblage includes complete bifaces, blanks and preforms for points, bifacial flake cores and other miscellaneous artefacts. Some of the cores are heat treated, and also some of the points have been produced from heat treated cores. The assemblage was also uniquely associated with human remains (of a 1-2 year old child) and proximity to fragmentary remains (of a 6-8 year old child) which has enabled more precise dating. The initial radiocarbon dates were inconsistent but more recent dates have been provided as around 13,000 Before Present for the younger child (Morrow and Fiedel, 2006) and around 2,400 years later for the older child (Owsley and Hunt, 2001). The assemblage is most closely associated with the younger child and the earlier date.
    Heat treating at the preform stage has its roots in the early archaic and evidence exists from numerous sites. Initially, heat treatment appears to have been sub-optimal (ie not taken to full completion in a manner that maximises the improvement). In some cases, it’s not much more than a surface effect which has created surface colour changes and only mild textural improvement within the lithic.
    There’s no universal date that you could regard as an adoption date on a widespread basis… it depends on the area and the culture. Also, as I said, not every lithic material actually benefits from the treatment… and there are many areas where lithic sources were of sufficiently high quality that there’s no evidence of the technology being used at all (because it wasn’t needed).
    Information provided by painshill
    Last edited by gregszybala; 02-05-2016, 08:04 PM.
    Searching the fields of Northwest Indiana and Southwestern Michigan