A vitreous Substance (usually mixed with coolants) that is fused under heat to a metal surface for decorative purposes. It is normally applied ground up as a paste. The colours in enamels are obtained from metal oxides; for example, copper (green), iron or silver (yellow), cobalt (blue), gold (red), manganese (purple). The technique was used by the Romans, Celts and in medieval times. In the 18th century a method of painting in enamels (mixed in powdered form with oil) was widely employed for portrait miniatures and for the decoration of objects such as snuff boxes. Enamel colours were also employed for decorating of porcelain.