Uncovering the chemistry of color change in rock art in Leang Tedongnge (Pangkep Regency, South Sulawesi, Indonesia)

The color change process is among the various threats endangering the preservation of the Maros-Pangkep rock art. It is necessary to be concerned about this issue because it can cause rock art to lose its original hue, fade, obscure, or darken. Among the rock art sites in the Maros-Pangkep Region experiencing the color change of rock art pigments is Leang Tedongnge. The color change process is indicated by several hand stencils with bright red and yellow colors, while others show a dark, dull red hue or a greyish tone. In this work, we aim to assess the mechanism of the color change process by studying the physicochemical properties of the pigment. Here, we employed a combined multi-analytical characterization, including optical microscopy observation, elemental analyzes of X-ray fluorescence (XRF), scanning electron microscopy coupled with energy dispersive electron spectroscopy (SEM-EDS), and Raman spectroscopy. The result indicates that the color-change process of the rock art of Leang Tedongnge was due to the formation of a greyish crust layer made up predominantly of gypsum minerals. The layer caused the pigment to become dull, fade or darken and obscured the original color of the rock art. The layer is formed by the reaction between calcium ion dissolved in the seeping of karst water with sulphate ions from the minerals deposited at the rock's surface. Moreover, the reaction might also involve the sulfate ions derived from anthropogenic sulfur emissions produced by post-harvesting straw burning near the rock art site. To the best of our knowledge, this work is the first report to present a detailed-physicochemical assessment of pigment material regarding the color change for Indonesian rock art.