Making Glass

COLOURS

Unless the raw materials are very pure, glass made by mixing and heating sand, soda ash and limestone will normally be green, the depth of the colorants present in the raw materials. a sand containing as little as one-thousandth part of iron oxide will give normal soda-lime glass, used for windows and glass containers, a greenish tint.

For many products, instead of using high purity (and thus expensive) raw materials, glass manufacturers may decolourise the glass by adding minute amounts of other colorants which produce complementary colours to green so that the finished articles appear colourless. Thus selenium (which gives a pink colour) and cobalt (which gives blue) can be added to soda-lime glass to offset the effect of the green or yellow due to the iron and this is done in the manufacture of glass containers. Nickel may be used similarly in the decolourising of lead crystal glass.

Different additions may produce different coloured glasses, the range of possible colours being almost infinite. Some of the most frequently used colorants and the colours they produce are listed below. The colour depends on the state of oxidation of the colorant, the type of glass in which it is used, and thermal treatment.

The use of large amounts of several different colourants will tend to produce black glasses. Opaque or opal glasses can be produced by the addition of appropriate amounts of fluoride or phosphate compounds, which produce crystal growth, known in the glass industry as devitirification.