Copper is a light, hard metal, very liable to rust; all salts, unctuous bodies, and many other natural substances are solvents of it. It is remarkably sonorous, being the basis of all alloys in which that quality is sought for; and its divisibility is great. After hammering, its appearance is silky, and its lustre seems to be increased. Heated to fusion, it absorbs oxygen, oxidizes at the surface, and becomes covered with a black crust; and it may be converted into suboxide altogether by a strong heat in the muffle. At a high white heat it burns with a greenish coloured flame. In dry air, copper is unchangeable; in moist air, and in presence of carbonic acid, sulphuretted hydrogen, or other acids, it becomes dark, and assumes a bronze hue. It is a red-coloured metal, with a specific gravity of 8*78, which may be increased to 9.00 by hammering. The tenacity of cast copper is sufficient to support a weight of 19,000 lb. to the sq. in., or rather more than half as much as good cast iron. When heated, it rapidly loses strength, and at a dull-red heat is little more than half as strong as at ordinary temperatures. Pure copper in mass is hard, but capable of being cut with a knife. Its hardness is increased by hammering, rolling into sheet, or drawing into wire.
If either of these processes be repeated sufficiently long, the metal is rendered extremely brittle; but its tenacity and malleability may be restored by a process of annealing. A fracture of the metal in a soft state is red, shining, granular, and, if very soft metal, it is crystalline. When hard, its fracture presents a fibrous, lightish-red, silky appearance. The increase of specific gravity from 8.78 to 9.0 by hammering was supposed to arise from the condensation of the particles of the mass; but when copper is melted in contact with the atmosphere, it absorbs oxygen, and becomes slightly porous. This absorption is prevented by fusion under common salt. The density of the metal so fused has been found to be 8.921; after being subjected to a pressure of 300,000 lb., it has been increased only to 8.930. The difference is so slight that it is probably owing to a diminution of the spaces still remaining, rather than to an approximation of the particles of the mass to each other. It has a disagreeable taste and smell. It is an excellent conductor of heat and electricity.
Its power of conducting heat is 898, taking gold at 1000, iron being 374, and lead 179. A bar of copper heated from 32o to 212° F. (0° to 100° C.) is lengthened about 1/380 part, while iron is extended only 1/310. Copper in a finely-divided state, if pressed together and made red-hot, as in Wollas-ton's process of "preparing platinum, may be welded together into a solid mass, provided it has been soaked in oil previous to ignition, in order to prevent oxidation. It expands in solidifying. The addition of 0.1 per cent. of potassium, zinc, or lead, will prevent expansion in cooling. It boils at an intense white heat, but is not volatilized. Exposed to intense heat in a close vessel, it incurs no sensible loss of weight. In a finely-divided state, on ignited charcoal, it burns like tinder; if, on being lighted, it be put into an atmosphere of oxygen, the combustion becomes very vivid, the metal being converted into protoxide. Lengthened exposure of the bright metal to a humid atmosphere causes its oxidation, and it becomes covered with a green carbonate of copper, commonly, but incorrectly, termed verdigris. Heated to redness in contact with the air, it oxidizes, and the scales of oxide fall off.
With care, at a low-red heat, bars of it may be worked by the smith in the same manner as iron. It is soluble in acids, but mostly so in the oxygenating acids.
Copper occurs chiefly in 3 forms:-native, as sulphide (copper pyrites), and as carbonates. The 2 latter forms undergo a series of operations for the extraction of the metal. These often commence by a rough process of classification in the mine; but frequently the whole of the stuff broken off the lode is sent up for dressing. At the surface, cleaning is commenced by separating the large from the small stuff, for "spalling." This operation, which is commonly done by men, consists in breakng the large stones into pieces of 2 or 3 lb. weight. The resulting broken mineral is divided into 3 parcels, viz., "best ore," "poor ore," and " attle," or worthless matter, which is discharged from the dressing-floors. The " best ore " of this operation, with the smaller fragments of the pile, are " cobbled " by young females - a repetition of the spalling process-reducing the pieces still smaller; they are again sorted into 3 parcels: but the best ore is conveyed direct to the crushing-mill, or placed aside for that purpose. The poor ore of the spallers is subjected to the cobbling process, and is similarly divided. If careful dressing is pursued, the poor ores of the several cobblers are again sorted into the 3 divisions.
The ores of the miner, other than the smallest, are thus reduced to 2 qualities, and a considerable portion of the matrix is rejected in this early stage. The treatment of the 2 qualities of ore is essentially different.
Formerly the best was invariably crushed under flat hand-hammers, or cobbling-hammers, on iron plates or old stampheads, by females; and in mines producing but little ore, this method is still pursued. The cast hammer has a square face, and weighs about 4 lb.; with this primitive instrument, the ore is crushed so as to pass through sieves of 4 to 64 meshes or holes to the sq. in. If the ore is of more than average richness, it is broken to pass through the fewer meshes; if poor, to pass through the larger number. Crushing is now almost universally effected by roller-mills or stamp-batteries, as described at length in Andre's " Mining Machinery." The best ore is passed through a coarse riddle, and taken from the mill to the pile for sale. Poor ore is passed through a riddle with small meshes, and from the crushing-mill is taken to the dressing-floors, where it undergoes a succession of operations in order to partially free it from extraneous matter, thereby increasing the average percentage of metallic copper before offering it for sale.