refining, any of various processes for separating impurities from crude or semifinished materials. It includes the finer processes of metallurgy, the fractional distillation of petroleum into its commercial products, and the purifying of cane, beet, and maple sugar and many other substances. The nature of the refining process depends on such factors as the type of material involved, the value of the end product, and the degree of purity necessary. The purification of a metal is based upon physical or chemical differences between the metal and its accompanying impurities, including density, melting point, magnetic properties, and reaction to certain chemicals. It may be accomplished by a continuation of a process used in separating the metal from its ore. Electrolysis is much used in the refining of various metals (e.g., copper); the open hearth process (see steel) and the basic oxygen process are used in the refining of iron; the amalgamation process and the cyanide process are chemical means of metal refining.