Excerpt from Decorative Glass Processes

Coloured glasses are produced by the addition of various metallic oxides to the frit before melting - red by the addition of copper or gold; yellow by silver, iron or anti mony blue by cobalt; green by copper; and so on-though it is stated that under certain conditions every colour in the spectrum can be produced by the use of iron alone, varying the temperature according to the colour required.

The ingredients are crushed, mixed well together and melted in a crucible. The molten metal, as it is then called, is either cast or blown into shape, according to the character of the glass which is being made. In some cases, as when bottles or slabs are blown into a mould, there is something like a combination Of the two.

Glass is proof against all acids except hydrofluoric, which is used for decorating it by means of etching. (a glass can be made from bones, however, which is proof against the action of fluoric.) It is capable of being ground or abraded by means of sand, emery, etc., and polished by putty powder and rouge. It is painted upon by means of pig ments-composed of highly fusible glass containing a large proportion of metallic oxide by way of colouring matter. This is fixed by firing in a kiln, when the pigment is melted and the surface of the glass sufficiently softened to admit of the colour Sinking into it and becoming incorporated with it. It is cut into shapes of considerable variety by means of the diamond or the small steel wheel which is, under some circumstances, taking its place.

Owing to the power which glass has of refracting light and the fact that refraction takes place at the point of con tact between the glass and the atmosphere - that is, the surface. Of the glass - the character of that surface is an all important factor in.the appearance of the glass, apart from any consideration of colour or impurities in the material. Polished plate glass, having a perfectly smooth surface, which allows of the angles of refraction being alike for all rays of light which pass through it, is transparent, and under some circumstances practically invisible. Sheet glass ranks next to plate in this respect, its occasionally wavy surface producing distortion of objects seen through it. A certain amount of reflection is also possible when the rays of light which fall upon the surface of the glass are stronger than those which impinge upon the same point from behind, even in the case of perfectly clear glass without any backing. Both of these facts are turned to account in' the manufacture of ornamental glasses, the surface being roughened either in casual haphazard fashion or in more or less mechanical pattern to produce a brilliant glittering effect, transparency being, of course, sacrificed. These glasses serve, in addition to the ornamental purpose primarily intended, a further useful purpose in scattering and diffusing the light which passes through them by means of the many difiering angles of refraction given by the roughened surface, and so affording a means of improv ing the lighting of rooms which do not receive a sufficient amount of direct light.

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