Copper is a metal of a reddish brown colour, hard, very malleable, ductile, and sonorous; of considerable tenacity, and of a specific gravity from 8.6 to 8.9, The good quality of copper is shown by its capability of alloying silver, without any diminution of its extensibility under the hammer or roller. For this reason the Swedish copper has a decided preference in the preparation of the alloys of gold and silver; and its superior purity has been found to render it far more durable when employed in the sheathing of ships, as well as for many other purposes. English copper is chiefly obtained from the mines of Cornwall and Devonshire, whence it is usually transported across the Bristol Channel to be smelted at the coal-works in South Wales. The ores consist chiefly of yellow copper ore, or copper pyrites, and the grey sulphuret of copper. The average produce in copper is said to be about 8 1/2 parts from 100 of the ore. The ordinary processes of smelting consist of alternate calcinations and fusions in reverbatory furnaces of the usual construction. The calcining furnaces are furnished with four doors or openings, two on each side, for the convenience of stirring the ore and drawing it out of the furnace.
They are commonly from 17 to 19 feet in length from the bridge to the flue, and from 14 to 16 feet in width, with a fire-place 5 feet by 3. The melting furnaces are usually about one-third the area of the calciners, but their fire-places are nearly as large, and they have only one door, and that in front. The charge of ore for a calcining furnace usually consists of about 3 tons, which is uniformly distributed over the bottom. When charged, the heat is gradually increased during twelve hours, until near the point of fusion, when the ore is discharged through holes in the bottom, and is suffered to lie underneath until it is sufficiently cool to be removed. With this calcined ore, which is in the form of a black powder, the melting furnace is charged, by spreading it over the bottom thereof. A few slags from the previous fusions are added, and the door is closed and luted. When brought into fusion, the liquid mass is well 6tirred, and the oxides and earthy matters which float at the top are skimmed off through the front door by the smelters.
When this first mass of metal has been freed from the last-mentioned impurities, the smelter lets down into the furnace a second charge of calcined ore, and proceeds with it in the same manner as with the first, repeating the charges until the furnace will hold no more, when a tapping-hole made in the side of the furnace is opened, and the metal runs through it into a pit filled with water, which causes the metal to become granulated, in which state it contains about one-third part of copper. The slags which have been cast in sand-moulds, are afterwards broken to ascertain if they contain any metal; the pieces that do are then remelted to separate it. When the ores are difficult of fusion, some fluor spar is added to the charge. The coarse metal obtained by the preceding process is then calcined by a similar treatment to that which the ore received. To oxidize the iron with which the copper is contaminated, the charge is allowed to remain twenty-four hours in the furnace, where it is repeatedly stirred and turned. The metal thus calcined is then to be melted again with some slags from the previous melting, to which may be added some pieces of furnace bottoms, which are impregnated with the metal.
The slags from this operation are also skimmed off; they have a great specific gravity, being composed chiefly of the oxide of iron; they fuse readily, and act as fluxes to reduce further portions of the ore. The metal obtained from the second operation of the melting furnace, after the slag has been separated, is tapped off either into sand-moulds or into water. In the former plan the product is collected in a mass, and is called blue metal; in the latter it becomes granulated, and is denominated fine metal. The quantity of pure copper these products contain is about 60 per cent. The subsequent calcination and fusion of these last products is conducted by a repetition of the operations described; and the result of this third melting are pigs containing from 80 to 90 per cent, of pure metal. The fourth process is, for the most part, an oxidizing, or, as it is called, a roasting of the pigs obtained in the third. For this purpose the furnace is filled with the pigs to the extent of 25 or 30 cwt. which are exposed to the action of the air, while the heat is gradually increased to the melting point, which facilitates the expulsion of the volatile matters, and oxidizes the iron or other metallic substances that may remain.
This operation is continued from 12 to 24 hours, according to the degree of purity of the pigs when put into the furnace; and when completed, the metal is brought to a state of fusion, and run out into sand beds. The ebullition of the metal arising from the extrication of the expanded air from the sand under the metal, caused the latter to assume in cooling a honey-comb texture internally, with a blistered surface; and, from this latter appearance, such copper is denominated blistered copper. The copper in this stage being freed from its more abundant contaminations is ready for the refining furnace, which is of a similar construction to the melting furnace, only that the bottom, instead of being of fire-brick, is a bed of sand, laid to incline to the furnace door, near to which a pool is formed for the purpose of lading out the refined copper. The furnace is charged with from three to five tons of the blistered pigs; the heat applied to them is at first moderate, and air is admitted so as to continue the oxidation, should the metal not be quite fine. When this charge is fused, and the slags skimmed off, an assay is taken by the refiner with a small ladle, and the metal broken in a vice.
From the appearances of the metal both in and out of the furnace, the refiner judges whether it is in a sufficiently forward state to undergo the toughening process; previous to which the copper is of a deep red colour, inclining to purple, with an open crystalline structure. In the process of toughening, the surface of the metal is first covered over with charcoal; a pole, commonly of birch, is then held in the liquid metal in the furnace, which causes considerable ebullition, owing to the evolution of gaseous matter; and this operation of poling is continued with the occasional addition of fresh charcoal, until the quality of the assays, which the refiner takes from time to time, attain the required degree of purity indicated by the polished silky appearance of the metal when cut half way through, the light red colour of it when broken, and the closeness of the grain. After this a trial of the malleability of the copper is made by taking out a small quantity in a ladle, casting it in a metallic mould, and then hammering it out upon an anvil; if it does not crack at the edges by this operation, it is deemed in a fit state to be withdrawn from the furnace, which is done by lading it out and casting it into rectangular cakes, 12 inches wide by 18 inches long.
In preparing copper for the manufacture of brass, it is usual to granulate it by pouring the hot metal through a colander placed over a cistern of cold water, which causes the drops of metal to assume by contact with the cold fluid, a ragged appearance, and to which the name of feathered shot is given in the trade. For making brass that is to be drawn into wire, the same process as the last is employed, except that the metal is poured into hot water; the drops, in consequence, take an oblong form, and are hence termed bean shot. In granulating copper, it should be poured into the water by small quantities at a time to prevent explosions. A considerable quantity of copper is exported by the East India Company under the denomination of Japan copper. It is cast in small sticks about six inches long, and weighing about half a pound each. It is of a rich red colour, caused by throwing the cast sticks, as soon as they have become solid in their moulds, into water. Copper unites readily with several of the metals. Its combination with tin is effected at a less heat than is necessary to melt the copper; on which circumstance is grounded the method of tinning copper vessels. For this purpose they are first scraped or scoured, and afterwards rubbed with sal ammoniac.
They are then heated (usually over a charcoal fire), and sprinkled with powdered resin, which defends the clean surface of the copper from acquiring the slight film of oxide that would prevent the cohesion of the tin to its surface. The melted tin is then poured into the vessels, and spread about. An extremely small quantity adheres to the copper, which is supposed to be sufficient to prevent the noxious effects of the copper. When tin is melted with copper, it forms the ancient compound termed bronze, the specific gravity of which is remarkable for being greater than would be deduced by computation from the bulks and specific gravities of its component parts. The uses of this hard, sonorous, and durable composition, in the fabrication of cannon, bells, statues, and other articles, are well known. The ancients made cutting instruments of this alloy. A dagger analysed by M. Hielm consisted of 83J copper, and 16 1/8 tin. Copper unites with bismuth, and forms a reddish white alloy. With arsenic it forms a white brittle compound called tombac; with zinc it form. the important compound called brass; and it is distinguished by various other names, according to the proportions of the two ingredients.
It is not easy to unite these two metals in considerable quantities by fusion, because the zinc is volatilized at a heat inferior to that which is required to melt the copper, but they unite very well in the way of cementation. There are various ways of uniting granulated or small pieces of copper with zinc, but the following is the ordinary method adopted in this country. Calamine, which is an ore of zinc, is calcined in a kiln, or made ret hot, then ground to powder, sifted fine, and mixed with ground charcoal, as the calamine is apt to clod and prevent an uniform admixture. About seven pounds of calamine are put into a melting pot of about a gallon content, to which are added about five pounds of the granulated copper; the calamine must be mixed with as much charcoal as will fill the pot, and the copper must lie uppermost. This is let down with tongs into a wind furnace, 8 feet deep, where it remains 11 hours. One furnace usually holds eight pots arranged in a circle round a grate. By the heat the zinc becomes revived, rises in vapour, and combines with the copper, which it converts into brass. Towards the end of the process the heat is suddenly raised, which causes the brass to melt and occupy the lower part of the crucible.
After melting it is cast into plates or ingots for use, as it is required. The following method of making brass, mentioned by Cramer, is recommended by the scientific Dr. Ure. The powdered calamine being mixed with an equal quantity of charcoal, and a portion of clay, is to be rammed into a melting vessel, and a quantity of copper, amounting to two-thirds of the weight of calamine, must be placed on the top, and covered with charcoal. By this method the volatile zinc ascends and converts the copper into brass, which flows into the rammed clay; consequently, if the calamine contain lead, or any other metal, it will not enter into brass, the zinc alone being raised by the heat. Copper unites readily with antimony, a compound of a beautiful violet colour. It does not readily unite with manganese. With tungsten it forms a dark brown spongy alloy, which is somewhat ductile. See Alloy.