The leading products of the blast-furnace are argentiferous lead (base bullion), matte, slag and flue-dust (fine particles of charge and volatilized metal carried out of the furnace by the ascending gas current).
The resulting lead button hammered P into shape and carefully cleansed from slag is ready for the cupel.
The slag and metal produced are then run off and the latter is cast into bars; these are in general contaminated with iron, arsenic, copper and other impurities.
In a natural state it is obtained from bones, guano and wood ashes; and in an artificial condition from basic slag or Thomas's phosphate, coprolites and superphosphate of lime.
It differs from ordinary slag cement (see above) in that it is not a pozzuolanic cement depending on the interaction of granulated slag and lime.
The duty of the limestone (CaCO 3) is to furnish enough lime to form with the gangue of the ore and the ash of the fuel a lime silicate or slag of such a composition (1) that it will melt at the temperature which it reaches at about level A, of fig.
The reason why at this level the walls must form an upright instead of an inverted cone, why the furnace must widen downward instead of narrowing, is, according to some metallurgists, that this shape is needed in order that, in spite of the pastiness of the slag in this formative period of incipient fusion, this layer may descend freely as the lower part of the column is gradually eaten away.
The slag and matte formed float upon the lead in the crucible and are tapped, usually together, at intervals into slag-pots, where the heavy matter settles on the bottom and the light slag on the top. When cold they are readily separated by a blow from a hammer.
Such parts as may be subjected to extreme heat and the fretting action of molten material, as the tuyere and slag breasts of blast furnaces, and the fire bridges and bed plates of reverberatory furnaces, are often made in cast iron with double walls, a current of water or air being kept circulating through the intermediate space.
A year or two later field trials were begun in England, with the final result that basic slag has become recognized as a valuable source of phosphorus for growing crops, and is now in constant demand for application to the soil as a fertilizer.