Gold is the most easily weldable of all the metals. Like lead, it can be welded cold and, provided it is free from certain impurities, it can be joined at all temperatures. Gold is generally placed first in tables of malleability and ductility. These properties are destroyed by certain impurities, notably antimony, arsenic, and bismuth. One part of bismuth in 1,920 parts gold is alone sufficient to interfere with the working properties of gold.

Pure gold melts at 1062 deg. Cent, and will not oxidize at any temperature. Hence the surface will be clean and weldable. The welding property is very apparent with gold leaf, which must not be allowed to fold on itself lest the surfaces stick together. Gold fillings in dentistry are made by the cold welding of gold leaf.

Pure gold is soft and is seldom used in the arts. To render it strong and durable for coinage and jewelry, it is alloyed with copper and sometimes with silver. Thus the gold coins of Great Britain contain eleven parts gold and one part copper. Those of France and the United States nine parts gold and one part copper. For jewelry both copper and silver are used, the purity of an alloy being designated by the number of carats of gold in a total of 24 carats.

The gold of coinage and jewelry cannot be joined without a flux. The flux may be boracic acid or a solution of zinc chlorid and water. In the making and repairing of gold jewelry a mouth blowpipe is used for small and delicate work, and for larger pieces a gas blowpipe with a foot-pump air-blast or compressed air.

The flux mentioned is used in case the surfaces oxidize when heated. If the gold is so alloyed as to be hard up to the melting point, the weld will be a melt-weld. Low-carat alloys melt considerably lower than pure gold. Hence the melt-weld would be made at about the temperature that high-carat alloys would be plastic enough to weld.

Gold can also be readily welded by electricity.

For ordinary cheap and quick joining in jewelry, gold is soldered with soft solder, a mixture of two parts tin and one part lead.. The flux is a solution of zinc chlorid in water. Such a joint is condemned by the best jeweling practice: the joint is not strong; it is a different color than the gold; the solder is apt to destroy the strength of the gold at the joint.

The following table of hard solders, given by Gee,1 are yellow alloys of high melting point and make strong solders:

These alloys are rolled into ribbons and cut up into "pallions," or may be applied in dust made by filing. A description of the soldering of gold would properly belong to a treatise on the goldsmith's art.