As quarried or mined free sulphur is always contaminated with limestone, gypsum, clay, &c.; the principle underlying its extraction from these impurities is one of simple liquation, i.e.
The purification by liquation is assisted by poling the lead when it is below redness.
Liquation, if not followed by poling, is carried on as a rule in a reverberatory furnace with an oblong, slightly trough-shaped inclined hearth; if the lead is to be poled it is usually melted down in a cast-iron kettle.
In skimming the crust from the surface of the lead some unalloyed lead is also drawn off, and has to be separated by an additional operation (liquation), as, running lower in silver than the crust, it would otherwise reduce its silver content and increase the amount of lead to be cupelled.
All tin, except a small quantity produced by the shaft furnace process from exceptionally pure stream tin ore, requires refining by liquation and "boiling" before it is ready for the market.
The extraction from ores in which the bismuth is present in the metallic condition may be accomplished by a simple liquation, or melting, in which the temperature is just sufficient to melt the bismuth, or by a complete fusion of the ore.
In the liquation process the ore is heated in inclined cylindrical retorts, and the molten metal is tapped at the lower end; the residues being removed from the upper end.
Casting also is complicated by the shrinkage which occurs in cooling down from the molten state, and in some alloys by the formation of eutectics, and the liquation of some constituents.
A dry method of purification consists in a liquation on a hearth of peculiar construction, which occasions the separation of the unreduced bismuth sulphide and the bulk of the other impurities.