Cation-ratio dating involves the analysis of biogenic rock varnish, a micrometer-thick coating of manganese, silicon, aluminum and iron, believed to age predictably and thus serve as a chronometer. In particular, the ratio of certain cations, (calcium + potassium)/titanium, has been posited to decrease as the varnish ages. In recent decades, some scholars have challenged these assumptions and questioned the usefulness of the technique as a reliable way to date lithic artifacts.
