Copper is one of the oldest, if not the very oldest, metals of history. It was used almost entirely as an alloy with tin or zinc, until recent times. The aborigines of North America, however, found the pure metal already smelted for them on the shores of Lake Superior; and the tools they used we find to-day, made of nearly pure metal, mistakenly said to be "tempered by a lost process."

Pure copper melts at 1080 deg. Cent., is fourth in ductility, and sixth in malleability.1 If free from certain impurities, such as sulphur and carbon, it becomes plastic above red heat. Under an oxidizing flame it will burn or scale, and part of the scale will be absorbed by the metal surface.

It is a curious fact that, though copper is a weldable metal, as appears from its properties, it is hardly ever welded. The common method of joining copper, brass, and bronze has always been to solder, braze, or rivet the pieces. The welding property is occasionally mentioned,2 but most metal workers are ignorant of the possibility. While the effect of impurities on the welding property appears not to have been gone into, we may presume that the same substances that cause red shortness and assist in oxidation are also detrimental to welding; and that electrolytic and Lake copper, being nearly pure, are also most weldable. "Over-poled" copper, containing carbon, and copper smelted from sulphid ores are red-short, and generally unworkable.

The fact that the welding of copper is almost an unknown art is strikingly shown by the fact that, in reply to the query of a correspondent, the editor of a leading technical publication recently replied as follows: That copper was not weldable; that it flew to pieces if hammered when hot; and that it burned rapidly at welding heat, and would not braze perfectly.

1 Prechtl.

2 American Machinist, Oct. 23, 1902; Schnabel's Metallurgy, p. 1.

The flux for copper welding usually contains borax or boracic acid and a phosphate salt. One flux recommended is two parts sodium phosphate and one of boracic acid;1 another is one part yellow potassium prussiate and twenty parts of borax.2 A pinch of rosin is sometimes added to the flux. When using a phosphate in the flux care must be taken not to bring the copper in contact with free carbon, because copper phosphate will form and will prevent sound welding.3

To weld, the metal is heated to redness, when it becomes plastic. The calcined flux is sprinkled on the surface and the pieces are then joined at a yellow heat and hammered together as in iron welding. When using a phosphate flux, do not touch the copper with coke or charcoal. A gas or oil flame is preferable, or the pieces can be heated in an electric welder or with a high-temperature torch. An ordinary hammer and anvil can be used, but on account of the rapid conduction of heat away from the joint, a piece of brick or stone can be substituted for the anvil and a wooden mallet for the iron hammer. Copper at a red or yellow heat is very plastic, if not red-short. So it is well to upset the metal considerably at the joint to allow for working with the hammer.

Copper is welded by the electric process, and a melt-weld can be made with the hydrogen or acetylene burner. But in either case it has been shown that the fibrous structure is destroyed and a crystalline joint occurs. As with wrought iron, the copper weld must be hammered or drawn to restore the fiber. Copper welding is generally considered unsatisfactory, soldering and brazing being preferred, as either can be done below the critical temperature of crystallization.

The smith welding of pure copper is considered more difficult than the smith welding of wrought iron. And because pure copper is inferior in strength to pure iron, it is unlikely that the welding property will ever be generally taken advantage of. But the knowledge that copper can be smith welded and that the welds can be made of 100 per cent, strength, may be occasionally found to be useful in the arts.

1 American Machinist, Sept. 25, 1902.

2 Ibid.

3 Ibid.