Zinc Oxide is a base used in the manufacture of many optical glasses. With boric acid it gives silicates of a low coefficient of expansion and special optical values. Used with cryolite, it forms a very dense opal suitable for pressed ware. It is rather more expensive than the other bases used.
Cryolite is a natural opacifying ingredient used in making opal glasses. It consists of a combination of the fluorides of aluminium and sodium, and is one of the most active fluxes known to glass and enamel makers. Its cutting chemical attack on the pots is very intensive. It is imported from Greenland. An artificially manufactured form of cryolite is known, which is a little cheaper than the natural variety and gives similar results in opacifying glass.
Alumina. This is sometimes present to a small extent in glass makers’ sands. As such it is not a dangerous impurity. It exists in combination with silica and potash to a large extent in feldspars, china clays, and granites. Alumina, when used, has a decided influence upon the viscosity and permanency of glass. In large proportions it noticeably diminishes the fusibility of glass, and makes it more or less translucent. Owing to the refractory nature of alumina it is with difficulty that it can be diffused in alkaline silicates, borates, or lead silicates; consequently any considerable proportion present in glass may cause cords or striae, which are objectionable defects in the glass.
Oxide of Lead. Red Lead, or Minium, is much used in the manufacture of enamels, table glassware, and heavy optical glass. It gives great brilliancy and density to all glasses in which it is used, but if used in excess the glass is attacked readily by mineral acids and becomes unstable. Red lead is a powerful flux, even at low temperatures, and forms the chief base in making best crystal ware and enamels. The red oxide of lead used by glass manufacturers is a mixture of the monoxide and peroxide. Glass manufacturers, in buying red lead, should realise that it is the peroxide present which is the active oxidising agent, and that at least 27 per cent. should be present. A dull, dark red oxide shows a low percentage of peroxide; a bright orange red a high percentage. Impure red oxides of lead may be adulterated with barytes, finely divided metallic lead, or added water. Such impure varieties should be avoided. The red oxide of lead is preferred to the other oxides and forms of lead for glassmaking, on account of its greater oxidising action, which is desirable in producing crystal glassware.
Tin Oxide and Antimony Oxide are used as opacifiers. When used they generally remain suspended in a finely divided form in the glass. Used in small quantities they have a favourable influence in the development of ruby-coloured glasses.
Manganese, Arsenic, and Nickel Oxides are used in glassmaking as “decolorizers,” which will be treated in a later chapter.
Cullet. In all glasses a proportion of “cullet,” or broken glass scrap, is used. This cullet is usually of the same composition as the glass mixture or “batch.” The use of cullet facilitates the melting, and assists in giving homogeneity to the resultant glass by breaking up the cords and striae which tend to develop in most glasses.
In the commoner varieties of bottle glass Basalt and other igneous rocks are crushed and used. These are naturally occurring silicates containing lime, alumina, alkalies, iron, and other elements in varying proportions. They are used more on account of their cheapness, and produce dark, dirty-coloured glasses, which in the case of common bottles are not objected to. In some instances iron, manganese or carbon is added to produce black bottle glass.
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.