Previous research suggested that the chemical composition of rock varnish, a micrometer thick Si/AI/Mn/Fe coating, changed predictably through time and so could be used as a chronometer. In particular, the cation ratio, (Ca+K)/Ti, was posited to decrease as the varnish aged. However, many earlier varnish studies were not well documented and were based on chemical analyses now shown to be inaccurate. This paper describes a test and the first direct comparison between two different analytic methods used previously to “cation-ratio date” archeologic and geologic materials. It provides methodology sufficiently detailed for replication of analytical and sampling techniques.
Our analyses of rock varnish from a prehistoric chert quarry in California show that the two principal methods used to “cation-ratio date” rock varnish generate consistently different elemental abundances and are therefore not comparable. Neither analytic method produced a significant time-dependent trend in varnish chemistry, suggesting rock varnish cation ratios are not a reproducible and thus probably not a reliable way to date lithic artifacts.