A brief description on slags was mentioned briefly in the previous item. Slags are a very important part of the process because they may tell us if the smelting was carried out in the right way. As was mentioned, in the making of Dore bars (Gold Bullion) oxides and other impurities are eliminated by means of high temperature reactions with fluxes. Most of the impurities and fluxing agents combine to form a liquid silicate called slag that float above the liquid Dore metal and is removed from the crucible. Normally, crucibles are fed with a retorting product and reprocessed slags from previous operations due they can contain precious metals in variable proportion. Slags are glassy, stony, hard and compact; and their properties mainly on how the material was cooled. The cooling procedure has influence on how a particular slag can be reprocessed.
In general, slag from smelting process arises from extraneous materials such as rust and oxides; oxidation of elements in the charge (e.g. iron, copper, zinc); residues from fuels; fluxes employed in the process (silica, borax, sodium nitrate, sodium carbonate, fluorspar); crucible erosion. Then, slag can vary in appearance and chemical composition.
Slags can be categorized in three groups. The first one is formed by allowing the molten slag to cool relatively slow under ambient conditions and the final cooling can be accelerated with water spray. The second one is cooled through a water jacket that leads to rapid steam generation and formation of several cavities within the slag. These cavities modify the slag density. The third group is formed by granulated slag that is formed by quenching molten slag in water. The very rapid cooling causes solidification of the slag as sand sized particles of glass.