Was yellow lead chromate pigment used during Middle Stone Age at Sibudu rock shelter (South Africa)?

A recent study using the combination of optical microscopy and Raman spectroscopy revealed the presence of bright yellow residues of lead (II) chromate associated with grinding striations on a 58,000-year-old grindstone excavated at Sibudu rock shelter in South Africa. Lead (II) chromate (PbCrO4) exists in nature as the rare mineral crocoite, however was available as the synthetic pigment, chrome yellow, from the 19th century. At the time of the residue analysis on the grindstone, it was not possible to determine whether the yellow residues were due to modern contamination or a result of ancient use. Careful analyses of the excavation protocol followed at Sibudu rock shelter and all implements that might possibly have been in contact with the grindstone after excavation identified yellow spray paint as a possible origin of the contamination. Here, we performed experimental grinding of the mineral crocoite on a sandstone slab, sprayed yellow paint on rhyolite and compared optical microscopy and Raman spectroscopic analysis results to the archaeological sample. The optical appearance of the archaeological residues, the absence of associated minerals of crocoite, and the presence of organic matter mixed with lead chromate demonstrate that the residues on the archaeological grindstone stem from modern contamination. This paper draws attention to contamination issues originating from excavation practices.