FLUX, a general term made use of to denote any substance or mixture added to assist the fusion of minerals In the large way, limestone or fluor spar are used as fluxes ; but in small assays, the method of the great operations is not always follow ed, though it would be very frequently of advantage to do so. The fluxes made use of in assays, or philosophical experiments, consist usually of alkalies, which render the earthy mixturesfusible, 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 with the mineral intended to be fused. The deto nation of the nitre with the inflammable matter of the tartar is of service in some operations ; though generally it is attend ed with inconvenience, on account of the swelling of the materials, which may throw them out of the vessel, if proper care be not taken either to throw in only a little of the mixture at a time, or to pro vide a large vessel.
White flux is formed by projecting equal parts of a mixture of nitre and tar tar, by moderate portions at a time, into an ignited crucible. In the detonation which ensues, the nitric acid is decom posed, and flies off with the tartarous acid, and the remainder consists of the potash in a state of considerable purity. This has
been called fixed nitre.
Black flux differs from the preceding, in the proportion of its ingredients. In this the weight of the tartar is double that of the nitre ; on which account the com bustion is incomplete, and a considerable portion of the tartarons acid is decom posed by the mere heat, and leaves a quan tity of coal behind, on which the black colour depends. It is used where metallic ores are intended to be reduced, and ef fects this purpose by combining with the oxygen of the oxide.
There is danger of loss in the treat ment of sulphurous ores with alkaline fluxes : for, though much or the greater part of the sulphur may be dissipated by roasting, yet that which remains will form a sulphuret with the alkali, which is a very powerful solvent of metallic bodies. The advantage of M. Morveau's reducing flux seems to depend on its containing no on combined alkali. It is made of eight parts of pulverized glass, one of calcined borax, and half a part of powder of char coal. Care must be taken to use a glass which contains no lead. The white glasses contain in general a large proportion, and the green bottle glasses are not perhaps entirely free from it.