What It Actually Is

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.