Copper is probably the most difficult of all the metals to work by the file or lathe, but pure copper may be cut like cheese with a graver, and consequently it is extensively used for plates where the number of impressions required is not very large. In filing copper the file should be well chalked, and in cutting it in the lathe use plenty of soapy water, and let the solution of soap be pretty strong. In polishing copper it will be found that owing to its softness, it burnishes easily (see article on polishing metals), but where it is polished by means of abrasive processes, that is, by the use of powders which grind it or wear it down, great care must be taken to have the powders free from particles which are larger than the average, as these would be sure to scratch the metal, owing to its softness. For polishing copper by abrasion, only the softer polishing powders should be used, such as rotten stone, prepared chalk, and soft rouge. These are used with oil at first, but the last touches are given dry.
Copper may be welded by the use of proper fluxes. The best compound for this purpose is a mixture of one part of phosphate of soda and two parts of boracic acid. This welding powder should be strewn on the surface of the copper at a red heat; the pieces should then be heated up to a full cherry red, or yellow heat, and brought immediately under the hammer, when they may be as readily welded as iron itself. For instance, it is possible to weld together a small rod of copper which has been broken; the ends should be beveled, laid on one another, seized by a pair of tongs, and placed together with the latter in the fire and heated; the welding powder should then be strewn on the ends, which, after a further heating, may be welded so soundly as to bend and stretch as if they had never been broken. It is necessary to carefully observe two things in the course of the operation. First, the greatest care must be taken that no charcoal or other solid carbon comes into contact with the points to be welded, as otherwise phosphide of copper would be formed, which would cover the surface of the copper and effectually prevent a weld. In this case it is only by careful treatment in an oxidizing fire and a plentiful application of the welding powder that the copper can again be welded. It is, therefore, advisable to heat the copper in a flame, as, for instance, a gas flame. Second, as copper is a much softer metal than iron, it is much softer at the required beat than the latter at its welding heat, and the parts welded can not offer any great resistance to the blows of the hammer. They must, therefore, be so shaped as to be enabled to resist such blows as well as may be, and it is also well to use a wooden hammer, which does not exercise so great a force on account of its lightness. Mr. Rust, the inventor of this process, states that, as long ago as 1854, he welded strips of copper plates together and drew them into a rod; he also made a chain, the links of which bad been made of pretty thick wire and welded.
The following process is said to give very good results: First make the article entirely bright by file, scratch brush, or any of the usual modes. Apply to the surface a coating of cream of tartar, then sprinkle the surface with a saturated solution of sulphate of copper, and rub with a hard brush. The coating of copper deposited on the iron is said to be very even and durable.