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alumina would require nitre, and excess of alumina over potash, clay ; while excess of alumina over both potash and silica would have to be introduced as bauxite. B 2 3 would go in as borax, excess of B 2 O 3 over Na 2 as boric acid, and excess of Na 2 over B 2 O 3 as soda ash, or soda spar if alumina be sufficient. B 2 3 might also be introduced with CaO as colemanite, but CaO generally goes in as 58 CERAMIC CHEMISTRY. whiting or fluorspar, unless the P 2 5 is sufficient to hold it as bone ash. Magnesia, zinc, and barium may be intro- duced as silicates or as oxides or carbonates. Any excess of silica over all the bases is added as flint. Fluoride may pair with CaO as fluorspar or with sodium and aluminium in cryolite. CERAMIC CHEMISTRY. CHAPTER VIII. THE PRODUCTION OF COLOUR. Owing to the high and protracted firing to which pottery is subjected, it is impossible to employ organic stains for the production of colour. We are, therefore, confined to the few metals which form coloured salts and to complex inorganic stains similar to ultramarine or the masses formed when zinc or aluminium salts are heated on charcoal with a cobalt solution. But though we lose in palette by the temperature of firing and the elimination of organic dyes, we gain considerably by the permanence of the ware thus produced. The process