A technique for dating the rock varnish that develops over the petroglyphic component of rock art. The technique works by determining the ratio of calcium and potassium to titanium—(Ca + K/Ti)‐concentrations within the rock varnish. Calcium and potassium ions are more mobile than titanium and so tend to be more readily leached from the surface. Thus the older the varnish the nearer the ratio gets to 1 : 1. The ion concentrations are determined using chemical analysis, the calibration of the ratios being dependent on having comparative samples from historical graffiti or other petroglyphs of known age.