It is very generally supposed that any one of certain metals, if its condition of oxidation, or its proportion be varied, will, in combination with glass, produce the several effects of colour into which white light can be decomposed. Thus copper, when suitably treated, will produce the effects of blue, green, and red. Metals enter into combination with glass in various ways. The effect of avanturino-glass is due to the suspension in the body of the glass of minute particles of metallic copper. When oxide of gold is used as a colouring agent, it often happens that some oxide is reduced to the metallic state, and the result is a glass, which, when viewed by reflected light, appears to be of a dull, opaque, red colour, but, by transmitted light, yields a beautiful opaline blue. Opacity is probably due to an insoluble excess of metallic oxide held in suspension in the glass. White opacity is obtained by tho use of arsenic trioxide, tin dioxide, lime phosphate, powdered talc or cryolite. The effect of blackness is obtained by the oxides of iridium, manganese, cobalt, copper, or iron in excess.
Gold to be used in colouring glass is first dissolved in acina-regia ; the solution, together with oxides of antimony and tin, is added to the ordinary ingredients of flint-glass. The ruby colour is in a great measure due to the reducing action exercised upon the gold salt by the stannous oxide. Ruby-glass is usually gathered from the crucible in the form of lumps, weighing lb. As it is gathered from the crucible, it is perfectly colourless, and only acquires its colour after it has been chilled and reheated in the annealing-kiln. The ruby lumps, after having been annealed, are reheated, as they are required, and used for casing the flint-glass. Articles are never made of solid ruby-glass, partly on account of its cost, but chiefly because the colour is so powerful that an almost invisible film imparts a rich colour to the article upon which it is spread.
The red colour of copper ruby-glass is due to cuprous oxide, and all substances liable to part with oxygen, and to convert the cuprous into cupric oxide, must be avoided in its preparation. In addition to avoiding oxidizing agents, such as red-lead, and oxide of manganese, it is necessary to add reducing agents, to counteract such effects of oxidation as are unavoidable. Stannous oxide and iron scales or filings, are for this purpose mixed with the raw materials. The ruby colour produced is intense, and can only be used as a casing for colourless glass. The ruby-glass, when gathered from the crucible, is of a pale greenish-blue colour, and, like the gold ruby, requires to be partially cooled and again heated before the red colour appears. If reheating is carried too far, the red is replaced by a dull-brown tint. If copper and iron scales be added in great excess, an opaque red mass is obtained.
Cuprio and ouprous oxides, when used without reducing agents, produce peacock-blue or green ; the result apparently depends rather on the quantity than on the state of oxidation of the copper. A very minute proportion of cupric oxide will give a distinctly blue tint. Ferric oxide in the presence of manganese dioxide, which parts with its oxygen, and thereby tends to maintain the oxidation of the iron, produces a rich yellow. Ferrous oxide (FeO) gives a dull-green ; it is
obtained either by the oxidation of metallic iron in the crucible, or by the reduction of ferric oxide.
Manganese dioxide by itself and in large quantity gives violet. If the mixture be heated too long, the oxygen ie driven off, and the glass is rendered colourless. A red is obtained by a mixture of manganese dioxide and ferric oxide. A minute trace of cobalt oxide imparts a deep purple-blue. Nickel oxide produces a deep red-brown. The oxides of chromium are very slightly soluble in glass ; a minute quantity gives an emerald-green or yellow colour ; any excess will remain in the form of glistening crystals in the body of the glass, and tends to its disintegration. Antimony trioxide imparts a faint-yellow tint ; excess tends to produce opacity. Oxide of cadmium gives a pale-yellow. Uranic eeequioxide produces a bright-yellow, but its peculiar property of fluorescence, already referred to, gives to the glass, when viewed by transmitted light, a bluish-green effect. Oxide of silver, in common with cuprous oxide, possesses the power of staining glass, when applied as a pigment to its surface, and heated. This is a more convenient way of obtaining the yellow colour which silver oxide gives to glass, as, when mixed with the raw materials of glass, and placed in a crucible, it is only with the greatest difficulty that the oxide can be prevented from becoming reduced. If reduced, metallic silver sinks to the bottom of the crucible, and the glass remains colourless.
Glass Mosaic for Windows.—The heading of this section is used advisedly. The common expressions " stained " and "painted " glass are misleading, because, in the production of decorative windows, stain, enamel, or paint ought to play a very subordinate part, and because excellent effects can be obtained without them. It is true that there are coloured windows in which the effect of colour is obtained solely by the use of ordinary pigments, cemented to the surface of white transparent glass by means of gum or varnish, or by the employment of enamel colours fused to the surface of white glass by means of heat. These methods do not produce the essential conditions of a good ornamental window, namely, transparency or translucency combined with durability. It is unnecessary to point out the instability of colour depending for its existence upon the strength of a gum or varnish. It has been proved by long experience that enamel colours cannot resist lengthened exposure to air and moisture, however effective they may be for internal domestic decoration. It is obvious that opaque powders fastened upon glass must destroy its transparency and translucency. Permanent and transparent effects of colour can alone be obtained by the mosaic treatment of fragments of coloured glasses. For this reason, the mosaic method will alone be con sidered. By mosaic treatment, is meant the representation of the different colours of a design by separate pieces of coloured glass. The general effect may be heightened by the appropriate appli cation of the transparent yellow silver stain, and by the sparing use of an enamel brown or black for outlines and shading, both the stain and enamel being fixed by heat.