Refining

Refining (also perhaps called by the mathematical term affining) is the process of purification of a (1) substance or a (2) form. The term is usually used of a natural resource that is almost in a usable form, but which is more useful in its pure form. For instance, most types of natural petroleum will burn straight from the ground, but it will burn poorly and quickly clog an engine with residues and by-products. The term is broad, and may include more drastic transformations, such as the reduction of ore to metal (for which see Refining (metallurgy)).

The refining of liquids is often accomplished by distillation or fractionation. Gases can be refined in this way as well, by being cooled and/or compressed until they liquefy. Gases and liquids can also be refined by extraction with a selective solvent that dissolves away either the substance of interest, or the unwanted impurities.

Many solids can be refined by growing crystals in a solution of the impure material; the regular structure of the crystal tends to favor the desired material and exclude other kinds of particles.

Refining (metallurgy)

Refining (as in non-metallurgical uses) consists of purifying an impure material, in this case a metal. It is to be distinguished from other processes such as smelting and calcining in that those two involve a chemical change to the raw material, whereas in refining, the final material is usually identical chemically to the original one, only it is purer. The processes used are of many types, including pyrometallurgical and hydrometallurgical techniques.

Lead

Cupellation

One ancient process for extracting the silver from lead was cupellation. Lead was melted in a bone ash 'test' or 'cupel' and air blown across the surface. This oxidised the lead to litharge, leaving a button of silver. In ancient times, the litharge was discarded, but more usually it was re-smelted to lead. "Pigs" of Roman lead have been found marked EX ARG (argentum is Latin for Silver). This presumably indicated that the lead had already been de-silvered. This process was viable economically if the lead contained at least 8 troy ounces of silver per ton of lead (178 ppm).