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Carbonate of LimeLimesparLimestoneParis WhiteWhiteningCalcium Carbonate

Dolomite

FluorsparFluoride of Lime

Phosphate of Lime

Carbonate of BariumWitherite

MagnesiaStrontia

Zinc Oxide

Cryolite

Alumina.

Oxide of Lead.

Tin OxideAntimony Oxide

ManganeseArsenicNickel Oxides

Cullet.

Basalt

Of the various silicates used in glassmaking, the silicate of alumina is the most refractory. The silicates of lime and barium are rather refractory, but under a strong heat and in the presence of other silicates they can be readily formed. The silicates of the alkalies, lead, and many of the other metals are formed at much lower temperatures. In the case of the silicate of iron, manganese, or copper, a strong affinity is shown between the metal and the silica, and a black or dark-coloured slag with a very low melting point is formed. Such slags are very active in corroding the masonry and pots of the furnace.

No single silicate is entirely free from colour. Each gives a slight distinctive coloration, the lead silicate being yellowish and the soda silicate greenish, but by the judicious mixture of different silicates and the use of decolorizers, such as manganese, nickel, etc., compound silicates are obtained, giving less perceptible colours or crystal effects. In optical glassmaking the use of the ordinary decolorizers is not permissible, and the purity of the materials used becomes the most important factor.