Physical and electrochemical properties of a metal are greatly influenced by the level of impurities. Thus ductility, electrical conductivity, and corrosion resistance improve remarkably with increased purity. Zinc, for example, cannot be rolled unless its tin content is less than 20 ppm. The castability of metals and alloys, particularly their ability to fill a mold, is influenced by the presence of a trace amount of oxygen in the molten material. Zinc of ultra high purity (99.999%) retains its bright surface for many years in ordinary room air, but even a few parts per million of a metallic impurity reduce the corrosion resistance of a Zn-Al alloy sufficiently to make it useless. Refining is, therefore, an important metallurgical operation. Refining processes vary considerably from one metal to the other and the purity called for depends on the use for which the metal is intended.