In the United States copper is usually roughly divided into three grades: Lake copper, that which is obtained from the mines of the Lake Superior region; electrolytic copper, that which has been refined by the electrolytic process; casting copper, that which is not entirely refined, but carries varying amounts of impurities. This last named grade of copper is disappearing from the market because of the development of scientific alloying where the composition of the metals used must be definitely known.

The specific gravity of copper is 8.82, its melting point 1,981 degrees Fahrenheit. Its tensile strength varies with its physical condition, but is as follows: in cast copper, 26,000 pounds per square inch; wire, 55,000 pounds per square inch.

It is the best conductor of both heat and electricity, slightly excelling even silver in the latter respect. Chemically pure silver was for years believed to have the highest electrical conductivity of all the metals, and accordingly the basis for the 100 per cent standard. Recently, however, copper has been produced with such a high degree of purity as to indicate an electrical conductivity of 104 per cent.

Copper is very malleable and ductile and may be drawn into very fine wire or rolled into thin foil one two-hundredth of an inch thick. It becomes harder as it is worked, but by heating to 608 degrees Fahrenheit it regains its malleability. It may be thrown into water while red hot and cooled quickly or it may be allowed to cool slowly in the air and it will be equally soft in either case.

Copper or brass in sheets, bars, rods, or in the form of wire, tube, rivets, etc., can readily be obtained from any of the manufacturers whose addresses are given in Chapter VII (Sources Of Materials And Equipment).

For many years there has been a popular belief, current even at the present time, that the ancient Egyptians, the North American Indians, the mound-builders, and the cliff dwellers possessed the secret of hardening and tempering copper, and many a poor deluded enthusiast has spent time and money in an endeavor to rediscover the supposed secret, and thereby win for himself the fabulous reward that was supposedly awaiting the discoverer. While it is perfectly true that many of the copper articles that have been found in various parts of the world are harder than is usual in copper, it has been proven that the hardness was caused by the accidental addition of a small quantity of some other metal such as nickel, tin, etc. In many cases the ore itself contained enough of the foreign metal to give the hardness noticed. With the modern methods of scientific alloying it is now possible to produce a copper alloy that is harder by far than any piece made by the ancients.

The commonest forms in which copper is placed upon the market are as follows:

Ingots, of about 60 pounds in weight, with two deep depressions so that they can the more readily be cut into three parts for convenient handling in melting and casting.

"Wire bars," about three and one-half inches square and five feet long, averaging in weight about 350 pounds. These are used for drawing down into wire of various shapes and sizes.

Copper for rolling into sheets is placed on the market in the form of square cakes weighing from 100 to 1,000 pounds.

Anodes of impure copper to be refined by the electrolytic process. These usually weigh about 200 pounds each.

Cathodes of pure copper, weighing the same as the anodes. These are the product of the electrolytic process, and are bought by manufacturers to roll into sheets.

Sheet copper of varying grades and thickness. These sheets are generally 30 inches wide and 60 inches long. It is cheaper to buy an entire sheet, as the dealers charge considerable for cutting into a sheet. The sheets are sometimes designated and sold by weight. A 30 by 60 sheet, which is 18-gage, Brown-Sharpe gage, weighs about 20 pounds, and is called a 20 lb. sheet.

There are two methods of rolling copper into sheets, the straight rolling system and the "Welsh" cross rolling system. The straight rolling system consists in rolling a plate of copper between hardened steel rolls to about 0.25 or 0.75 inch thick, cutting it to the proper weight for the desired gage of sheet, then heating and rolling in packs on finishing rolls to the desired length.

Hot rolled copper is not as smooth or tough as cold rolled copper, so if possible order direct from the wholesalers and specify cold rolled and annealed copper as it is smoother, tougher, and more elastic.

The Welsh cross rolling system is much slower and more expensive and is being superseded by the straight rolling. Cross rolling is done by rolling cake copper to about 25 inches wide and about one-quarter inch thick, cutting to the desired weight and rolling crosswise to the desired width.