In the manufacture of the glasses for imitation " paste " jewels, every effort is made to procure pure materials and colorific oxides. The base for making artificial gems is a very heavy lead crystal glass termed " Strass paste," which gives great brilliancy and refraction. The composition of such a paste would be: Best white sand 100 parts, pure red oxide of lead 150 parts, dry potash carbonate 30 parts. These should be thoroughly well melted until clear and free from seed, and the molten mass ladled out of the pot and quenched in cold water, or " de-graded." This assists in making the paste homogeneous. After repeated melting and de-grading, the paste or cullet is collected, dried, and crushed for use in making the coloured pastes. Usually, this strass metal is melted in small, white porcelain crucible pots holding about 5 to 10 kilogrammes of the metal and heated in a properly regulated gas and air injector furnace. The coloured paste is kept in fusion for a whole day, after which it is slowly cooled and annealed within the pot, and the gems cut from the lumps of glass thus obtained. The following are some of the compositions used in the preparation of the respective gems.
Powdered strass paste, 1,000 parts; white calcium phosphate, 200 parts; uranium yellow, 5 parts; pure manganese oxide, 3 parts; antimony oxide, 8 parts.
Powdered strass paste, 1,000 parts; purple of cassius, 1 part; white oxide of tin, 5 parts; antimony oxide, 10 parts.
Powdered strass, 1,000 parts; antimony oxy-sulphide, 10 parts; cobalt oxide, -25 parts.
Powdered strass glass, 1,000 parts; purest manganese oxide, 8 parts; pure cobalt oxide, 2 parts.
Powdered strass glass, 1,000 parts; green chrome oxide, 1 part; black copper oxide, 8 parts.
Powdered strass glass, 1,000 parts; pure cobalt oxide, 15 parts.
Powdered strass glass, 1,000 parts; antimony oxide, 50 parts; uranium yellow, 10 parts.
Powdered strass glass, 1,000 parts; antimony oxysulphide, 100 parts; gold chloride in solution, 1 part; pure manganese oxide, 4 parts.
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.