Page "Smelting" Paragraph 12

Of the seven metals known in antiquity only gold occurred regularly in native form in the natural environment.

The others – copper, lead, silver, tin, iron and mercury – occur primarily as minerals, though copper is occasionally found in its native state in commercially significant quantities.

These minerals are primarily carbonates, sulfides, or oxides of the metal, mixed with other components such as silica and alumina.

Roasting the carbonate and sulfide minerals in air converts them to oxides.

The oxides, in turn, are smelted into the metal.

Carbon monoxide was ( and is ) the reducing agent of choice for smelting.

It is easily produced during the heating process, and as a gas comes into intimate contact with the ore.