Encaustic colours are the various metallic oxides given in the following list. When more than one substance is used for colouring, the proportions must be varied to suit the tint required. For violet, use iron or manganese with soda: for purple, chloride of gold with tin and chloride of silver; for black, iron, manganese, uranium, and iridium; for blue, cobalt, carbonate of cobalt, smalt, or silicate of cobalt. Zinc will brighten blue colours. For indigo, use the materials employed for both violet and blue; for turquoise, use copper with soda, or cobalt with zinc and soda phosphate; green may he obtained with copper, either with or without antimony, or by chrome with cobalt; for bronze green, use nickel. Zinc, or carbonate of zinc, will brighten this colour. For olive green, use nickel with cobalt; for yellow, use antimony with potash, titanium, chromate of lead, and chromate of barium. Zinc will brighten this yellow. For buff, use the materials for yellow, with iron, sepia, sienna, ochre, and umber; for orangey use uranium, or the sulphide of antimony with iron; for red, use iron, chromate of iron, sulphate of copper, and ochre; for carmine, chloride of silver; for pink, iron and chrome, with potash; for brown, use iron, chromate of iron, manganese, with or without cobalt, ochre, and hammer cinder; for grey, use ii on, cobalt, iridium, platinum, titanium; for white, white clay and 5 per cent, tin oxide.
The colouring oxides are used in quantities of 5 to 10 per cent., sometimes 15 to 20 per cent. No definite rule can be given for quantity. Some of these oxides act as fluxes, and great skill must be used in their employment, in order to prevent the melting of the tile.