The metals produced in covered pots are usually softer and contain more lead and fluxes than those produced in open pots. In using open pots the heat of the furnace has direct access to the surface of the metal therein. In the case of covered pots, the heat has to be conducted through the cover of the pot, which retards the heat to a certain extent. On this account, softer mixtures are used in covered pots. ------------------------------------------------------------------------ CHAPTER V COLOURED GLASS AND ARTIFICIAL GEMS In colouring glass, either or several of the following colorific oxides may be used. They are added to the batch before fusion. Varying proportions are added, according to the depth of the colour desired. Occasionally the colour is influenced by the nature and composition of the rest of the batch. In some instances several colouring oxides are used. In this way many delicate tints are obtained; in fact, there are but few colours that cannot be produced in glass. =For Green Glasses= the following oxides may be used: Chromium oxide, 2 to 6 per cent. of the batch; black oxide of copper, ·5 to 3 per cent.; red iron oxide, ·5 to 1 per cent.; or a mixture of two or three of the above oxides in less proportions. Salts of chromium, copper, or iron may be used as the carbonates, sulphates, and chromates. =For Blue Glasses=, cobalt oxide, ·1 to 1 per cent. of the batch; zaffre blue or smalts, 1 to 3 per cent.; nickel oxide, 2 to 4 per cent.; iron oxide, 1 to 2 per cent.; black oxide of copper, 2 per cent. =For Violet and Purple=, manganese oxide, 2 to 4 per cent. of the batch. =For Rubies=, red oxide of copper, gold chloride, purple of cassius, antimony oxysulphide, selenium metal in small proportions. =For Yellows=, uranium yellow, 4 to 6 per cent. of the batch; potassium antimoniate, 10 per cent.; carbon, 6 per cent.; sulphur, 5 per cent.; ferric oxide, 2 to 4 per cent. Silver nitrate and cadmium sulphide are also used. =Black Glass= is obtained from mixtures of cobalt oxide, nickel oxide, iron oxide, platinum and iridium. Many very dark or black bottle glasses are obtained by using basalt, iron ores, or greenstone in a powdered form, added to the batch ingredients. =White Glasses= or =Opal= are obtained by using phosphate of lime, talc, cryolite, alumina, zinc oxide, calcium fluoride, either singly or in double replacements of the bases present in the glass batches. Many of the colouring oxides give distinctive colours to glass of different compositions; also the resulting colours may vary with the same colouring ingredient, according to reducing or oxidising meltings. Thus, in a batch of reducing composition, red copper oxide gives ruby glass, but in oxidising compositions the colour given is green or bluish-green. Iron oxide in an oxidising batch gives a yellow. In the reducing batch it gives bluish or green results. Manganese is similarly affected.