The usual operations of smelting require that an ore shall always be mixed with a flux. The ore is either acid or basic, and the flux must be either basic or acid, opposite to the character of the ore. At the temperature of the furnace, neither the acid nor the basic substance in an ore will melt, and in order to melt and dispose of it, there must be introduced with the ore a certain amount of flux, which combines chemically with the earthy substances of the ore, forming slag. At the temperature of the furnace the slag is molten, and when slag and metal collect in the hearth, the slag floats on the metal and protects it from oxidation.

The materials used as fluxes in smelting are (1) silica and silicate of aluminum as acid fluxes, and (2) limestone and magnesite (an impure magnesia) as basic fluxes. These fluxes are the same materials as those found with ores which determine their acid or basic character, hence it is necessary to place with an ore the flux opposite in character to that which the ore is found naturally to contain.

In making up a furnace charge, the chemical composition of the ore must be determined in the laboratory, and a flux must be chosen, in kind and quantity, which will unite completely with the earthy materials of the ore. Too little flux will not take up all the refuse parts of the ore, and too much will act in disintegrating the fire brick lining of a furnace in spots not protected by a slag coating. A practical smelter can judge from the color of a broken piece of cold slag whether or not the flux is in sufficient quantity, and an occasional analysis of the slag is a beneficial check. Sulphur and phosphorus must be avoided in fluxes, and are not desirable in ores.