As atated above, the chief use of orpiment is as a pigment ; but it is also employed as a dye; in the preparation of fireworks, and in some depilatoriea. The native sulphide is preferred to the artificial variety by artists and dyers by reason of its richer colour (see Pigments).
Realgar. (FR., Realyar ; Gan., Rothes Schwefelarsena), Formula, or disulphide of arsenic, is a deep orange-red substance, soluble in water, and highly volatile and poisonous. It is found native iu some volcanic districts, especially in the neighbourhood of Naples ; but the com mercial article is made by distilling in earthenware retorts, arsenical pyrites, or a mixture of sulphur and arsenic, or of orpiment and sulphur, or of arsenious acid, aulphur, and charcoal, in the proper proportions ; it has not the brilliant colour of the native mineral, and is much more poisonous.
On a large scale, the manufacture is carried ou in the following way :—The ingredients are mixed together in such proportions that the mixture shall contain 15 per cent. of arsenic, and from 26 to 28 per cent. of sulphur, in order to make allowance for the volatilization of a portion of the latter substance. The mixture is then placed in a aeries of earthenware retorts, which are charged
every twelve hours with about 60 lb.; this quantity should fill them three parts full. These are then gradually heated to rednesa for from eight to twelve hours, during which time the realgar distils off, and is collected in earthen receivers, similar to the retorts, but perforated with small holes to permit the escape of these gases. After the operation, the receivers are emptied, and the crude product is remelted. This is performed in cast-iron pots, the contents being well agitated, and the slag carefully removed. The requisite amount of aulphur or arsenic is then added, accord ing to the colour of the mixture, or else a proper quantity of realgar containing an excess of the required conatituent, and the mass is again stirred. When, on cooling, it exhibita the correct colour and compactness, it is run off into conical moulds of sheet iron, cooled, and broken up; it is some times refined by re-sublimation. The chief use of realgar is as a pigment ; and in pyrotechny, in the preparation of white fires (see Pigments).