Boric Acid acts as an acid in glass, as does silicic acid. It renders glass more fusible and brilliant; it has a searching action upon the colourising properties of certain metallic oxides when they are dissolved in the glass. It is an expensive ingredient, but is considerably used in optical and special chemical glassware in replacing a portion of the silicates ordinarily used and forming borates. It cannot be used in large amounts, as an excess produces glass of a less stable nature.
Borax, or Borate of Soda, consists of boric acid combined with soda. It is a very useful glassmaking material and is an active fluxing agent. If used in excess in glass mixtures it causes considerable ebullition, or boiling of the metal. In moderate proportions it is used in the manufacture of enamels for glass, as it helps to dissolve the colorific oxides and diffuse the colouring throughout the enamel mass.
Tincal, and Borate of Lime, are other forms in which borates may be introduced into glass.
Carbonate of Lime, Limespar, Limestone, Paris White, or Whitening are all forms of Calcium Carbonate. It is an earthy base and is added to the simple alkaline silicates and borates to form insoluble combinations or double silicates of soda and lime. By the use of lime, glasses are rendered more permanent and unchangeable when in use. Lime forms a very powerful flux at high temperatures. The quantity used must be carefully regulated according to the proportion of other bases present; otherwise an inferior or less stable glass may be produced. In excess it causes glass to assume a devitrified state.
Dolomite is a Magnesium Limestone, and is a natural stone which is available for use in making glass in tank furnaces.
Fluorspar, or Fluoride of Lime, is used in giving opacity and translucency to glass. It can only be used in small amounts, as the presence of any large proportion attacks the clay of the pots, causing serious damage by the sharp cutting chemical action due to the evolution of fluorine gas.
Phosphate of Lime is another material which produces opacity and translucency, but does not seriously attack the pots. Bone ash is a form of phosphate of lime, and is procured by calcining bones until all organic matter is consumed.
Carbonate of Barium, or Barytes, is a very heavy, white powder, and is a form of earthy base available for use in glassmaking. It can be used to replace lime, with similar results. By replacing other elements in the glass which are of lower density, barium can be used to increase the density of glass. Like lime it is a very powerful flux in glass at high temperatures. It gives increased brilliancy and little coloration. For this reason it is very useful in the manufacture of pressed glassware, giving a glass which leaves the moulds with better gloss than is found to be the case with lime glasses.
Magnesia and Strontia are other bases which are less used in glassmaking.
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 alumina and soda, and is one of the most active fluxes known to glass and enamel makers. Its cutting chemical attack on the fire-clay 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.
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 felspars, 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 causes cords or striae, which are objectionable defects in the glass.
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.