Realgar

Realgar occurs as a soft, brittle mineral of a striking deep red to orange color, typically found in the same ore deposits as orpiment and frequently mentioned alongside it in mineralogical and alchemical texts from antiquity onward. Its name derives from Arabic, rahj al-ghar, powder of the mine or powder of the cave, a name that traveled into Latin and other European languages largely unchanged, testifying to the substantial transmission of practical mineralogical vocabulary from Arabic sources into medieval European alchemy. Ground to powder, realgar served as a pigment valued for a warm red-orange unmatched by most other available minerals, used in painting and, more cautiously given its toxicity, in some traditional medicines and pest control preparations, since arsenic compounds were recognized, at least empirically, as effective against vermin.

Alchemists treated realgar as a companion substance to orpiment, both arsenic sulfides differing mainly in color and in the ratio of arsenic to sulfur, and both were investigated as potential sources of arsenic for further compound preparations. Because both minerals are genuinely toxic, historical accounts of their handling, while less cautious than modern safety standards would require, do show an awareness that these were dangerous materials, not simply attractive pigments.

Realgar is identified as an arsenic sulfide close to the formula As4S4, sometimes written more simply as AsS, an identification well supported by modern mineralogical analysis of the mineral's structure and composition. A genuinely interesting complication, unknown to any historical observer, is that realgar is photosensitive: prolonged exposure to light slowly converts it into a different, paler yellow-orange mineral called pararealgar, with a distinct crystal structure though a very similar overall composition, meaning that an old sample of what was originally realgar may, without any deliberate processing, have partly transformed into a mineralogically distinct substance simply by sitting in daylight. This adds a layer of honest uncertainty to any claim about the exact identity of a specific historical realgar sample examined today, though it does not affect the basic chemical identification of realgar itself as an arsenic sulfide, closely related to and frequently associated with orpiment throughout the historical record. Modern conservators examining old paintings and manuscripts now routinely check whether a suspect red-orange pigment sample has partly converted to pararealgar before drawing conclusions about its original color, a direct practical consequence of this photochemistry for anyone studying historical artworks today.