~Borax~ is a hydrated biborate of soda, containing nearly half its weight of water. When heated it swells up, loses its water, and fuses into a glass. The swelling up may become a source of loss in the assay by pushing some of the contents out of the crucible. To avoid this, _fused_ or _dried borax_ may be used, in which case a little more than half the amount of borax indicated will suffice. Borax will flux almost anything, but it is especially valuable in fluxing lime, &c., and metallic oxides; as also in those cases in which it is desired to keep certain of the latter in the slag and out of the button of metal. ~Oxide of Lead~, in the form of red lead or litharge, is a valuable flux; it easily dissolves those metallic oxides which are either infusible or difficultly fusible of themselves, such as oxides of iron or copper. The resulting slag is strongly basic and very corrosive; no crucible will long withstand the attack of a fused mixture of oxides of lead and copper. With silicates, also, it forms very fusible double silicates; but in the absence of silicates and borates it has no action upon lime or magnesia. Whether the lead be added as litharge or as red lead, it will exist in the slag as monoxide (litharge); the excess of oxygen of the red lead is thus available for oxidising purposes. If this oxidising power is prejudicial, it may be neutralised by mixing the red lead with 1 per cent. of charcoal. ~Glass~: broken beakers and flasks, cleaned, dried, and powdered will do. It should be free from lead. ~Fluor~: fluor-spar as free as possible from other minerals, powdered. It helps to flux phosphate of lime, &c., and infusible silicates. ~Lime~: should be fresh and powdered. It must not be slaked. Powdered white marble (carbonate of lime) will do; but nearly double the quantity must be taken. One part of lime produces the same effect as 1.8 parts of the carbonate of lime. ~Tartar~ and "black flux," are reducing agents as well as fluxes. The "black flux," which may be obtained by heating tartar, is a mixture of carbonate of potash and charcoal.