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.