Realgar (arsenic di sulphide) is a deep orange-red substance, soluble in water, and highly volatile and poisonous. It is found native in some volcanic districts, especially in the neighbourhood of Naples; but the commercial 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, sulphur, and charcoal; it has not the brilliant colour of the native mineral, and is much more poisonous. On a large scale, the manufacture is carried on in the following way: - The ingredients are mixed in such proportions that the mixture shall contain 15 per cent, arsenic, and 26 to 28 per cent, sulphur, in order to make allowance for the volatilization of a portion of the latter. The mixture is then placed in earthenware retorts, which are charged every 12 hours with about 60 lb.; this quantity should fill them 3/4 full. These are gradually heated to redness for 8 to 12 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 re-melted. This is performed in cast-iron pots, the contents being well agitated, and the slag carefully removed. The requisite amount of sulphur or arsenic is added, according to the colour of the mixture, or a proper quantity of realgar containing an excess of the required constituent, and the mass is again stirred. When, on cooling, it exhibits the correct colour and compactness, it is run off into conical moulds of sheet-iron, cooled, and broken up; it is sometimes refined by re-sublimation.