Sapphire. Powdered strass glass, 1,000 parts; pure cobalt oxide, 15 parts.
Topaz. Powdered strass glass, 1,000 parts; antimony oxide, 50 parts; uranium yellow, 10 parts.
Garnet. Powdered strass glass, 1,000 parts; antimony oxysulphide, 100 parts; gold chloride in solution, 1 part; pure manganese oxide, 4 parts.
Turquoise. Powdered strass glass, 1,000 parts; cobalt oxide, ·5 parts; black copper oxide, 10 parts; white opal glass, made with tin oxide, 200 parts.
After suitable pieces of glass of the requisite tints are obtained, they are cut and ground on a Lapidary’s wheel, then polished, engraved, and set as gems.
Artificial Pearls are now cleverly made in glass. A tube of the requisite size made of translucent or opal glass is cut into small sections, which are heated on a tray to softening point whilst set in a rotatory movement. As the heat increases they gradually melt in and seal at the openings, when they are removed from the tray and sorted.
CHAPTER VI
DECOLORIZERS
Decolorizers are the agents employed by the glass maker to neutralise or subdue the objectionable tints given by the colouring action of small traces of iron oxide, which exists as an impurity present in the materials used or otherwise become accidentally admixed during the process of the manufacture of glassware.
The small additions of manganese dioxide, arsenic, nitre, nickel oxide, selenium, antimony, oxide, etc., to glass batches may be considered as decolorizers. The most commonly used of these materials is manganese dioxide, so the action of this material will be explained. Every glass maker finds that one or other of the raw materials he uses may contain impurities. It is seldom that glass makers’ sand can be obtained that does not contain traces of iron oxide present as an impurity. Again, the cullet collected from the glass house often contains iron scale or rust from the blowing-irons, which firmly adheres to the glass and gets admixed with the batch for re-melting. The presence of even very small traces of iron in glass becomes evident as a pale sea-green tint when viewed through any thickness of metal. The chemical action of the glass upon the walls of the pot is continually dissolving a minute quantity of iron from the and diffusing it throughout the metal, giving it a tendency to the pale-green tint.