To compare methods of rock-varnish chemical analysis and cation-ratio determination,we prepared three glass standards, synthetic analogues of rock varnish. Analysis of these standards demonstrates that material of varnishlike composition can be accurately and precisely analyzed by means of quantitative, energy-dispersive electron microscopy (SEM-EDS).However, a blind interlaboratory comparison and reexamination of published data show that most published cation ratios of varnish are probably inaccurate because the technique used most frequently to determine rock-varnish composition, proton-induced X-ray emission (PIXE), did not accurately measure the concentration of Ti in the presence of Ba, a ubiquitous component of rock varnish. This inaccuracy suggests that the premise of cation-ratio dating and dates generated by this method should be reevaluated.

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