Second in importance is the carbonate, calamine (q.v.) or zinc spar, which at one time was the principal ore; it almost invariably contains the carbonates of cadmium, iron, manganese, magnesium and calcium, and may be contaminated with clay, oxides of iron, galena and calcite; "white calamine" owes its colour to much clay; "red calamine" to admixed iron and manganese oxides.
In these processes the electric current is used solely to generate heat, either to induce chemical reactions between admixed substances, or to produce a physical (allotropic) modification of a given substance.
In 1885 the brothers Cowles patented a process for the electrothermal reduction of oxidized ores by exposure to an intense current of electricity when admixed with carbon in a retort.
When sold for lighting and heating purposes, it is further admixed with 0.5% of heavy benzene boiling at 150°-200° C. Provisions are also made for special denaturizing processes as in Germany.
The former is found, generally mixed with iron, copper and arsenic oxides, in Bohemia, Siberia, Cornwall, France (Meymac) and other localities; it also occurs admixed with bismuth carbonate and hydrate.