Realgar, orange pigment

Realgar is employed as a depilatory in tanning, its red colour being a desirable feature in the treatment of hides and skins. It has been used as a paint pigment under the name of arsenic orange, but it is not too permanent and is now seldom used. Its early use as a remedy for asthma, etc., has been mentioned (p. 23). Unless carefully prepared, the commercial product is liable to contain white arsenic, and the poisonous nature of the latter tends to limit the applications of realgar. 

Orpiment is As2S3 and realgar is As4S4. Orpiment is yellow and realgar is orange-red. They are both used as pigments. 

Realgar re- al- gar, -g9r [ME, fr. ML, ff. Catalan, fr. Arabic rahj al-ghar powder of the mine] (15c) n. AS2S2. Arsenic disulfide mineral, which was used as a pigment under the name, arsenic orange . Its chemical and physical properties are similar to those of orpiment. In modern times, it is not used as a pigment because of its toxicity. 

---