COPPER 



927 



, opening for running off the liquid slag ; /, a small wall ; g, iron curb for keeping 

 the coals together. 



The hearth being heated with a bed of charcoal, | cwt. of Darrlinge are laid over it, 

 and covered with more fuel : whenever this charge is melted, another layer of the coal 

 and Darrlinge is introduced, and thus in succession till the hearth becomes full or 

 contains from 2^ to 2 cwts. In Neustadt 7| cwts. of Darrlinge have been refined in one 

 furnace, from which 5 cwts. of Gahrkupfer have been obtained. The blast oxidises the 

 foreign metals, namely, the lead, nickel, cobalt, and iron, with a little copper, forming 

 the Gahrslag ; which is, at first, rich in lead oxide and poor in copper oxide ; but at 

 the end, this is reversed. The slag, at first blackish, assumes progressively a copper- 

 red tint. This slag flows off spontaneously along the channel e, from the surface of the 

 hearth. The Gahre is tested by means of a proof-rod of iron, called Gahreisen, thrust 

 through the tuyere into the melted copper, then drawn out and plunged in cold water. 

 As soon as the GaJirspan (scale of copper) appears brownish-red on the outside, and 

 copper-red within, so thin that it seems like a net-work, and possessing sufficient tena- 

 city to admit of its being bent backwards and forwards several times without break- 

 ing, the refining is finished. The blast is then stopped ; the coals covering the surface, 

 as also the cinders, must be raked off the copper, after being left to cool. The surface 

 is now further cooled by sprinkling water upon it, and the thick cake of congealed 

 metal (ronddle) is lifted off with tongs, a process called Schleissen (slicing), or 

 Scheibenrdssen (shaving), which is continued till the last convex cake at the bottom of 

 the furnace, styled the kingspiece, is withdrawn. These ronddles are immediately im- 

 mersed in cold water, to prevent the oxidation of the copper : whereupon the metal 

 becomes of a cochineal-red colour, and gets covered with a thin film of red oxide. Its 

 under surface is studded over with points and hooks, the result of tearing the congealed 

 disc from the liquid metal. Such cases are called rosette copper. When the metal is 

 pure and free from oxide, these cakes may be obtained very thin, 5 \th of an inch, for 

 example. The refining of 2 cwts. of Darrlinge takes f of an hour, and yields l cwt. 

 of Gahr copper in 36 rosettes, as also some Gahrslag. Gahr-copper generally contains 

 from 1 \ to 2 J per cent, of lead, along with a little nickel silver, iron, and aluminium. 

 Most of the Mansfield copper is now refined in a reverberatory furnace, and sold as 

 Raffinadkupfer, and not as Gahrkupfer. 



Utilisation of the Copper Smoke. The application of the sulphur disengaged in the 

 calcination or roasting of copper ores to the production of liquid sulphuric acid, has 

 long been an important object of copper-smelters. In the case of a small proportion 

 of copper ores, containing a comparatively large proportion of sulphur, and in the 

 state of hard lumps, free from earthy matter, this is effected in the ordinary ' pyrites 

 burner ' of the sulphuric acid works. But copper ores, for the most part, are small, 

 poor in sulphur, with much earthy matter ; and are therefore calcined in reverberatory 

 furnaces with fire-grates. The gases from these calciners do not consist of more than 

 0-5 vol. of sulphurous acid in 100 vols., and are moreover largely mixed with the 

 products of combustion from the fire-place : they were therefore unfit for the vitriol 

 chamber, and were allowed to escape. 



An important invention, by which this loss is avoided, is the calcining furnace of 

 M. Gerstenhofer ; in which the ore, in a finely-divided state, is allowed to fall on a 

 number of fire-brick bars or bearers, placed in rows, at different elevations. The ore, 

 which is supplied in a continuous stream at the top, meets in its descent with a cur- 

 rent of air in the reverse direction. The furnace being moderately heated on starting, 



