Glue Moulds
Plaster casts of intricate objects may be obtained by making the mould of glue. Bunches of grapes, etc., for instance, are taken in their natural shape by covering them all over with glue, then cutting through the middle and extracting the grapes, after which the halves are joined together accurately and the cavity filled with plaster. A perfect cast of whatever object has been treated will be discovered after the glue has been melted off with boiling water. See ELASTIC MOULDS.
Gluten is vegetable fibrin. If wheat flour is made into a dough and kneaded on a sieve under a stream of water, the starch is carried away, leaving a gray, tough, and elastic substance having the appearance of animal skin, and which when dried has a glue-like aspect; hence the name. See STARCH. Glycerine is a colorless, inodorous fluid, of a sweet taste. The usual method of obtaining it on a small scale is from olive-oil. See OILS. Gold is a metal very widely diffused, occurring principally in grains, but sometimes in larger pieces weighing some pounds; it also occurs in the crystalline form in some instances. Its color is yellow, lustre brilliant; specific gravity 19,258. It is the most malleable of metals, is ductile to a high degree, and as soft as lead when pure. It melts at 2587 deg. F., and is not affected by air or water at any temperature.
The usual solvent for gold is aqua regia, a mixture of nitric acid 1, chlorhydric acid 3 to 4. The name of this solvent means royal water, so called from its power of dissolving the king of metals.
Gold, like silver, in a pure state is seldom used in the arts, except perhaps as a solder for vessels of platinum for laboratory uses. Dentist's and gilding gold usually contains about 6 grains of copper to one ounce of gold. Standard gold, copper 1, gold 11, has a density of 17.157, and is harder and more fusible than gold when pure. The French standard is copper 1, gold 9. Carat is a term used to designate one of the parts or units of a certain number which is taken as the standard of pure gold. In the United States the number is 24; hence pure gold is said to be 24 carats fine. If it contain 2 parts alloy, it is then 22 carats, etc.
Gold is separated from all its ores, except silver, by amalgamation with mercury. (See AMALGAMATION.) It is obtained from silver by boiling it with nitric acid, which dissolves out the silver, leaving the pure gold. See SEPRATING METALS.
Most metals combine with gold, increasing its hardness but impairing its ductility.
With silver 29.2, gold 70.8, the jeweller's composition called green gold is produced. The maximum degree of hardness with silver is obtained when the silver constitutes one third of the alloy; with copper it is one eighteenth. Twenty per cent of iron with gold produces the jeweller's gray gold, and 75 per cent makes an alloy of silvery whiteness, hard enough for cutting instruments.
Thirteen hundred miles of silver wire may be covered by one ounce of gold, the leaf is reduced to the 290,000th part of an inch, and a leaf of 56 square inches may be beaten out of one grain of this metal. See GILDING; ALLOY; TOMBAC; GOLD ALLOY.