Gun-metal
A soft gun-metal that will bear drifting is made from: copper 16, tin 1. Harder, for heavy guns: copper 9, tin 1. A small proportion of zinc aids the alloy to mix well, and increases the malleability without materially affecting its hardness. Sterling's gun-metal is copper 50, zinc 25, iron 1 to 8. Rosthorn's is copper 56.33, tin 0.49, zinc 41.29, iron 1.84. See ALLOY; BRASS; BRONZE; TIN; COPPER.
Gutta-percha is very similar in many respects to caoutchouc, being the dried juice of the Isonandra gutta tree. When mixed with about one fourth of linseed-oil it makes a good substance for obtaining moulds from undercut patterns. After obtaining the mixture and kneading it into cakes of suitable thickness, soften the surface before the fire and press firmly on the pattern, and, before it becomes cold, remove the mould and set it in cold water at once, otherwise it will shrink out of shape. The gutta-percha may be softened by heat until it is possible to press it into the most intricate recesses, but let the final touches leave the mould about equal in thickness all over, taking it off whilst warm, and plunging into water. See ELASTIC MOULDS; INDIA-RUBBER; PLASTER CASTS.
Gypsum is the sulphate of lime. This salt is found in many parts of the world, forming very extensive rocky beds. When pure and transparent it is known as selenite, and in its other varieties as gypsum, alabaster, and plaster of Paris. Powdered gypsum parts with its water of crystallization when subjected to a temperature of 300 deg. If it be then made into a liquid paste with water, it again combines with it, and at once commences to harden and resume its stony condition. It is entirely owing to this wonderful property that it can be used for obtaining impressions of objects by taking casts whilst in a liquid state. It is mixed with glue and colored for architectural purposes, the objects cast being then called stucco-work. See PLASTER CASTS; PLASTER OF PARIS; STUCCO-WORK.