A general term used to denote any substance or mixture added to assist the fusion of minerals. In the large way, limestone and fusible spar are used as fluxes. The fluxes made use of in philosophical experiments consist usually of alkalies, which render earthy mixtures fusible by converting them into glass, or else glass itself into powder. Alkaline fluxes are either the crude flux, the white flux, or the black flux. Crude flux is a mixture of nitre and tartar, which is put into the crucible along with the metal intended to be fused. White flux is formed by projecting equal parts of a mixture of nitre and tartar, by moderate proportions at a time, into a red hot crucible. In the detonation which ensues, the nitric acid is decomposed and flies off with the tartaric acid.

and the remainder consists of the potash in a state of considerable purity. Black flux consists of two parts of tartar to one of nitre, on which account the combustion is incomplete, and a considerable portion of the tartaric acid is decomposed by mere heat, and leaves behind a quantity of charcoal, on which the colour depends. It is used in the reduction of metallic ores, which it effects by combining with the oxygen of the oxide. Mowean's reducing flux is made of eight parts of pulverized glass, one of calcined borax, and half a part of charcoal. Care must be taken to use a glass which contains no lead.