If the poling has not been continued long enough, the metal will be brittle, as also if the poling has been continued too long. In the latter case, the metal is restored, but with some difficulty, by clearing the surface of the metal, and allowing heated air to pass over it, apparently for the purpose of decarbonizing the metal. When the metal has been overpoled, it becomes exceedingly bright and brilliantly clear, so that the roof of the furnace may be seen reflected on its surface. Before poling, the copper is in a peculiar condition, termed the "dry state," probably consisting of much copper oxide and oxygen in contact with the metal. In this condition it has a very strong action on the iron tools used in working the charge. The desired point having been attained by the refiner, the charge is again skimmed, a smoky flame is produced in the furnace to prevent oxidation, and the metal is taken out in ladles covered with a wash of fire-clay, and cast into moulds, varying according to the quality or form required for the market.
Various qualities of copper are met with in the market; they are known as "best selected," "tough copper," and "tile copper." These vary in quality, according to the choice of products for the tenth or refining operation.
When the copper is to be rolled into sheet, about 1/2 part of lead per 100 of copper is added just before skimming the surface, the whole being then well stirred, so that the action of the air may produce a lead oxide, which combines with the oxides of the foreign metals present to form a slag, which can be skimmed off before casting. The oxidation of the added lead must be complete, as even 1/10 per cent. will injure the quality of the copper.
The various modifications of this process, introduced of late years in England, are mainly designed with one common object - the utilization of the sulphur for making sulphuric acid. Economy of fuel is another consideration.
As modified, the series may be stated thus :-
1. Roasting in special furnaces (Spence's and Gerstenhofer's), which allow of the collection of this sulphurous acid in leaden chambers for conversion into sulphuric acid.
3. Roasting the matte in heaps or in a reverberatory furnace.
4. Fusion for black copper in a blast furnace.
5. Fining and refining in a reverberatory furnace.
In brief, the reverberatory furnace is reserved for operations needing an easy access of air simultaneously with a pretty intense heat; and the blast furnace is preferred for fusion, as in it the contact with the fuel permits a more complete utilization of the heat generated. (For details of Spence's, Gersten-hofer's, and other improved furnaces, as well as the manufacture of sulphuric acid, the reader may refer to Lock's 'sulphuric Acid,' and the art. Acid, Sulphuric, in Spons' ' Encyclopaedia/
The wet treatment can be employed alone or in conjunction with a certain number of the operations of the dry system. In any case, it is adapted only to those poor ores whose gangues are not attacked by the acids used. The solution of the copper is effected by different methods. Operations may commence by sulphatizing by means of a suitably conducted roasting, then dissolving out in water the sulphates .formed. Or the ore may be attacked directly by acids, either previously prepared or produced in contact with the ore. The acid treatment is applied to naturally oxidized ores or sulphides oxidized by roasting. Whatever the means adopted for the solution of the copper, the latter is precipitated by scrap-iron, affording the product known as " copper cement." The wet method is widely used. At Agordo, it is worked- successfully on sulphides containing only 1 1/2 per cent. copper. The process as there carried on deserves some further notice.
The characteristic phase of this process is the roasting of the poor ores; this operation is conducted with extreme slowness, and is arrested while there still remains a notable proportion of sulphur in the cores of the lumps. Owing to the gradual nature of the roasting, part of the sulphur is sulphatized, and part driven off in a state of vapour; the latter is condensed and collected, affording an article of commerce. But at the same time (and this is the important point), there is produced in the interior, even, of the lump of ore, a molecular movement of the copper towards the centre, when the sulphur is in excess; thus is created pieces whose external crust is sensibly impoverished, while the core is much richer in copper than the original ore. This earthy poor crust is removed by hand-breaking and sifting, and treated in the wet way; while the lumps are added to naturally rich ores for smelting in a blast furnace.
The process of the wet method applied to the poor portion comprises a series of very methodical successive lixiviations, terminating by a fresh roasting, and a final washing before rejecting the matters as waste. There is a production of waters charged with iron and copper sulphates, which are decanted and concentrated, and the copper is precipitated by iron or cast-iron; thus is obtained a pulverulent matter, which is dried, and passed on to the smelting for black copper. This material contains, according to Petitgand: 55 per cent. of copper containing 55 to 60 per cent. pure metal, 12 per cent. of 10-per cent. metal, and 33 per cent. of pure copper. As to the mother liquors, they are concentrated by natural evaporation in well-ventilated rooms, and afford crystallized iron salts.
The metallurgical treatment proper comprises crude smelting of the ore, roasting the matte afforded, smelting for black copper, fining, and refining. The first smelting is generally effected in such a furnace as that illustrated in Fig. 11. The characteristic feature is the inclination of the breast and of the back from rear to front, the object of which is to increase the period during which the matters remain, and to gradilate the effect of the heat. Towards the top the section it narrowed; and terminates in a sort of funnel which facilitates the charging. A single tube, supplying air under a pressure of 1 centim. of mercury, suffices to support the combustion in each furnace, while a flue conducts the gases from several furnaces to operation treats 15 or 16 tons of material per 24 hours; the materials consist of rich pyrites, enriched lumps, deposits of "cement," cinders and ashes of previous operations; the fuel employed Is wood charcoal, the consumption of which is about 42 bushels per too of copper produced.