=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.