An occurrence of pararealgar as an original or altered artists' pigment

A yellow pigment on a painting has been identified as pararealgar, an arsenic sulphide compound. The occurrence of pararealgar in European paintings has not been previously reported. It may have been misidentified as orpiment if it was characterized solely on the basis of colour and elemental analysis. Pararealgar is found in nature as a mineral and can also be formed by exposing realgar to light at wavelengths between approximately 500 and 670nm. Pararealgar can be distinguished unambiguously from other arsenic sulphide compounds, especially realgar and orpiment, by using X-ray diffraction or polarized light microscopy. The X-ray diffraction patterns of realgar (α-AsS), pararealgar and orpiment are tabulated, together with that of an intermediate phase, x. The microscopic characteristics of pararealgar are compared to those of realgar and orpiment.