COPPER, a metal which has been known to and used by the human race from the most remote periods. Its alloy with tin (bronze) was the first metallic compound in common use by man kind, and so extensive and characteristic was its employment in prehistoric times that the epoch is known as the bronze age. By the Greeks and Romans both the metal and its alloys were indif ferently known as XaXrcos and aes. As, according to Pliny, the Roman supply was chiefly drawn from Cyprus, it came to be termed aes cyprium, which was gradually shortened to cyprium, and corrupted into cuprum, whence comes the English word cop per, the French cuivre and the German Kupfer.
Copper is a brilliant metal (symbol Cu, atomic number 29, atomic weight 63.57) of a peculiar red colour which assumes a pinkish or yellowish tinge on a freshly fractured surface of the pure metal, and is purplish when the metal contains cuprous oxide. Its specific gravity varies between 8.91 and 8.95, according to the treatment to which it may have been subjected; J. F. W. Hampe gives 8.945 (0°/4o) for perfectly pure, compact copper. Ordinary commercial copper is somewhat porous and has a specific gravity ranging from 8.4 to 8.9. It takes a brilliant polish, is in a high degree malleable and ductile, and in tenacity it only falls short of iron, exceeding in that quality both silver and gold. By different authorities its melting-point is stated at from i,000° to 1,200° C; C. T. Heycock and F. H. Neville give i,o80.5°; P. De jean gives i,o85° as the freezing point. Its boiling point is vari ously given as between 1,98o° and 2,31o° C. The molten metal is sea-green in colour, and at higher temperatures (in the electric arc) it vaporizes and burns with a green flame. G. W. A. Kahlbaum suc ceeded in subliming the metal in a vacuum, and H. Moissan (1905) distilled it in the electric furnace. Molten copper absorbs carbon monoxide, hydrogen and sulphur dioxide; in addition, to all appearances, it decomposes hydrocarbons (methane, ethane) in absorbing the hydrogen. These occluded gases are liberated when the copper cools, and so give rise to porous castings, unless special precautions are taken. The gases are also ex pelled from the molten metal by lead, carbon dioxide or water even when in a fine state of division; but on heating, copper sul phate, cuprous sulphide and sulphur dioxide are formed in the first case, and cuprous chloride and hydrogen in the second. Con centrated nitric acid has also very little action, but with the dilute acid a vigorous action ensues. The first products of this reaction are copper nitrate and nitric oxide, but, as the concentra tion of the copper nitrate increases, nitrous oxide and, eventually, free nitrogen are liberated. Many colloidal solutions of copper have been obtained. A reddish-brown solution is obtained from solutions of copper chloride, stannous chloride and an alkaline tartrate.
Occurrence.—Copper is widely distributed in nature, occurring in most soils, ferruginous mineral waters and ores. It has been discovered in seaweed; in the blood of certain Cephalopoda and Ascidia as haemocyanin, a substance resembling the ferruginous haemoglobin, and of a species of Limulus; in straw, hay, eggs, cheese, meat and other foodstuffs; in the liver and kidneys and, in traces, in the blood of man and other animals; it has also been shown by A. H. Church to exist to the extent of 5.9% in turacin, the colouring-matter of the wing-feathers of the Turaco.
Native copper, sometimes termed by miners malleable or virgin copper, occurs as a mineral having all the properties of the smelted metal. It crystallizes in the cubic system, but the crystals are often flattened, elongated, rounded or otherwise distorted. Usually the metal is arborescent, dendritic, filiform, moss-like or laminar. Native copper is found in most copper-mines, usually in the upper workings, where the deposit has been exposed to atmospheric influences. The metal seems to have been reduced from solutions of its salts, and deposits may be formed around mine-timber or on iron objects. It is not infrequently found in serpentine and in basic eruptive rocks, where it occurs as veins and in amygdales. The largest known deposits are those in the Lake Superior region, near Keweenaw Point, Mich., where masses upwards of 400 tons in weight have been discovered. The metal was formerly worked by the Indians for implements and orna ments. It occurs in a series of amygdaloidal dolerites or diabases, and in the associated sandstones and conglomerates. Native silver occurs with the copper, in some cases embedded in it, like crystals in a porphyry. The copper is also accompanied by epidote, cal cite, prehnite, analcite and other zeolitic minerals. Pseudomorphs after calcite are known and also after aragonite, the latter being found at Corocoro, in Bolivia.
Ores.—The principal ores of copper are the oxides cuprite and melaconite, the carbonates malachite and chessylite, the basic chloride atacamite, the silicate chrysocolla, the sulphides chalco cite, chalcopyrite, erubescite and tetrahedrite. Cuprite (q.v.) occurs in most cupriferous mines, but never by itself in large quantities. Melaconite was formerly extensively worked in the Lake Superior region, and is abundant in some of the mines of Tennessee and the Mississippi valley. Malachite is a valuable ore containing about 56% of the metal; it is obtained in very large quantities from South Australia, Siberia and other localities. Frequently intermixed with the green malachite is the blue car bonate chessylite or azurite (q.v.), an ore containing when pure 55.16% of the metal. Atacamite (q.v.) occurs chiefly in Chile and Peru. Chrysocolla (q.v.) contains in the pure state 3o% of the metal; it is an abundant ore in Chile, Wisconsin and Missouri. The sulphur compounds of copper are, however, the most valu able from the economic point of view. Chalcocite, redruthite, copperglance or vitreous copper (Cu,S) contains about 8o% of copper. CoPPer pyrites, or chalcopyrite, contains 34.6% of copper when pure; but many of the ores, such as those worked specially by wet processes on account of the presence of a large proportion of iron sulphide, contain less than 5% of copper. Cornish ores are almost entirely pyritic; and indeed it is from such ores that by far the largest proportion of copper is extracted throughout the world. In Cornwall copper lodes usually run east and west. They occur both in the "killas" or clay-slate, and in the "growan" or granite. Erubescite, bornite (q.v.), or horseflesh ore is much richer in copper than the ordinary pyrites, and con tains 56 or 57% of copper. Tetrahedrite (q.v.), fahlore (fahl erz), or grey copper, contains from 3o to 48% of copper, with arsenic, antimony, iron and sometimes zinc, silver or mercury.
Alloys.—Copper unites with almost all other metals, and a large number of its alloys are of importance in the arts. The principal alloys in which it forms a leading ingredient are brass, bronze and German or nickel silver; under these several heads their respective applications and qualities will be found.
Oxides.—Copper forms two definite oxides, cuprous oxide, Cu20, and cupric oxide, CuO, both of which give rise to well defined series of salts. The other less definite oxides, Cu,0 and Cu30, do not possess this property, as is also the case of the hydrated oxides, Cu20,2H20 and Cui0,5H20, described by M. Siewert.
Cuprous oxide, Cu20, occurring as the mineral cuprite (q.v.), may be prepared artificially by heating copper wire to a white heat, and afterwards at a red heat, by the atmospheric oxidation of copper reduced in hydrogen, or by the slow oxidation of the metal under water. It is obtained as a fine red crystalline precip itate by reducing an alkaline copper solution with sugar. In its finely divided form it is of a fine red colour. It colours glass a ruby-red. The property was known to the ancients and during the middle ages; it was then lost for several centuries, to be rediscovered in. about 1827. Cuprous oxide is reduced by hy drogen, carbon monoxide, charcoal or iron, to the metal; it dissolves in hydrochloric acid, forming cuprous chloride, and in other mineral acids to form cupric salts, with the separation of copper. It dissolves in ammonia, forming a colourless solution which rapidly oxidizes and turns blue. A hydrated cuprous oxide, (4Cu20, H20), is obtained as a bright yellow powder from cuprous chloride and potash or soda. Cuprous salts are generally white, insoluble in water, and readily oxidized to cupric salts, but stable soluble cuprous salts, giving neutral aqueous solutions, are obtained by co-ordinating the cuprous radical with sulphur con taining agenda such as thiourea or ethylenethiocarbamide (Mor gan and Burstall, 1928).
Cupric oxide, CuO, occurs as the mineral melaconite, and can be obtained as a hygroscopic black powder by the gentle igni tion of copper nitrate, carbonate or hydroxide; also by heating cuprous oxide. It oxidizes carbon compounds to carbon di oxide and water, and therefore finds extensive application in analytical organic chemistry (q.v.). It is also employed to colour glass, to which it imparts a light green colour. Cupric hydroxide, Cu(OH),, is obtained as a greenish-blue flocculent precipitate by mixing cold solutions of potash and a cupric salt. This precipitate always contains more or less potash, which cannot be entirely removed by washing. A purer product is obtained by adding ammonium chloride, filtering and washing with hot water. Both the oxide and hydroxide dissolve in ammonia to form a beautiful azure-blue solution (Schweizer's reagent), which dissolves cellu lose, or perhaps holds it in sus pension as water does starch; ac cordingly, the solution rapidly perforates paper or calico (see