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, T 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. 28
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.
Many colouring oxides give more brilliant tints with glasses made from the silicates of potash and lime than if used in glasses composed from silicates of lead and soda. For many colours the lead glasses are preferred. In colouring the batches, the colouring oxides must be intimately mixed with the batch materials before fusion, more especially in the preparation of the pale tints, where only small quantities of colouring are necessary. It is a well-known fact that careful mixings give good meltings, for then the materials are more evenly distributed and uniformly attacked during the melting. Careful and exact weighings are necessary when using colorific oxides, and a pot is kept for each respective colour melted, so that the different colours and crystal glasses do not get contaminated with each other. When open pots are used for colours, the colour pots should be kept together in one section of the furnace, so that whilst melting, especially during the boiling up of the batches, the colours do not splash over into the other pots containing crystal metal.
As a rule, smaller pots are used for coloured glass; generally they are only a third of the size of crystal melting pots. When this is so, they are set together under one arch of the furnace, and the workman informed which pots contain the respective colours. All colour cuttings and scraps should be kept separate from other cullet for re-use. Coloured glasses are expensive, and no waste of glass should be permitted.