Measuring the Properties of Rock Varnish on Petroglyphs: A Comparison of Three Methods of Analysis Used on Dated Graffiti and Ancient Petroglyphs at Three Sites in Arizona

Abstract

Archaeologists, as well as petroglyph and pictograph researchers more specifically, have striven for years to develop scientific means for dating petroglyphs, through both direct and indirect methods. For years, the conventional wisdom has been that the older a petroglyph is, the darker it will appear because of the constant process of varnish formation that occurs on rocks in arid environments. This study focused on analyzing varnish formation on both dated inscriptions and undated precolumbian petroglyphs to see if a relative dating chronology could be created based on varnish darkness. I studied petroglyphs at three sites in southern Arizona because they had a number of dated inscriptions that showed potential for analysis alongside many more ancient inscriptions. Three methods of analysis were used. First, digital photographs of both dated and undated inscriptions were color corrected to the standard International Federation of Rock Art Organizations (IFRAO) color card in PhotoShop and then analyzed for red-green-blue means (RGB) on selected sites of an area of 36 pixels. Second, a Sekonic light meter was used to measure both ambient light striking the petroglyphs and reflected light from the grooves of the inscriptions and these were averaged to obtain a reflectance (R) ratio. Third, a HunterLab spectrophotometer was used to measure reflected light from an internal UV light source and the result analyzed for a light (L*) value where zero was black and 100 was white. The results from the analysis of 79 images (both ancient and modern) contributes to and validates the previous work by Brazeau (2007), Whitley et al. (1984) and Wright (2011) that showed that varnish color can be accurately measured by modern tools. Older precolumbian petroglyphs are darker on average than more recent date inscriptions, and this can be demonstrated through scientific research on varnish color. However, it was not possible to accurately discern differences in varnish color on just the historically dated inscriptions. The time frame is too short to provide reliable, consistent readings. More research is needed in the future, perhaps using different color cards, using different software programs for color analysis, and utilizing more sensitive instruments that can measure very narrrow grooves in both dated and undated inscriptions in order to analyze larger data sets. There is still much to learn about rock varnish and its formation and process of growth.