34 Mr. J. Napier on Copper Smelting, 



we think very probable ; although from the above tabic, and 

 from many other analyses of coarse metal, almost eveiy charge 

 wbuld require a different formula. 



The slags skimmed from the charge are removed to an open 

 yard, and when cool are broken up into small pieces and examined 

 by the eye for small shots of metal which may have been drawn 

 out by the rake, and which are laid aside to be returned to the 

 fusing-fumace. The average quantity so returned from one 

 charge is about 2 cwt., even with the most careful workman and 

 best fused charges. The examination of the slags is generally 

 the work of old men, which from long practice they do with 

 astonishing accuracy ; nevertheless there are often slags with a 

 small quantity of copper chemically diffused through the mass 

 which cannot be detected by the eye, but which may be easily 

 and quickly tested by the methods described under assaying, 

 namely, taking a few test-bottles of equal size, containing each 

 h \i ^y h ^^^ 1 grain of copper dissolved and converted into 

 ammoniuret. A sample of the slag is ground fine, and 100 

 grains digested at a boiling heat in nitromuriatic acid for about 

 15 minutes; a little water is added, and then ammonia in excess, 

 after which the solution is filtered into a bottle of similar dimen- 

 sions with the others and the colours compared. Should it be 

 the same as the one containing \ grain of copper, then the slag 

 has 1 per cent., and so with the others. The careful testing of 

 the slags in this way, apart from the picking out of the shots, 

 becomes an important guide to the whole management of the 

 works. Imperfect fusion, whether caused by bad coal, bad fur- 

 nace, or bad workmen, never gives a perfect slag ; and that small 

 causes often produce great and important results, we may illus- 

 trate by the following circumstance. In a work where the 

 maximum quantity of copper in the slag allowed to be cast away 

 was i of a per cent., two furnaces working alongside each other 

 at the same rate produced what was practically termed clean 

 slag ; but the one contained seldom less than \ per cent., the 

 other seldom above ^ per cent. They fused 40 tons per week, 

 producing each about 25 tons slag, being a weekly loss in copper 

 in the one furnace over the other of nearly ^3, equal to the 

 wages of two first class men. The cause of the difference in this 

 instance was in the furnace, an imperfection in the draught, 

 which would not have been observed but for testing the slags. 



Clean slag is and ought to be a primary object to the smelter ; 

 but it often happens that a desire to increase the make of copper 

 induces a heavy charge being put into the furnace, or the build- 

 ing of larger furnaces, which in all the instances we have seen 

 where the slags were tried strictly by testing, proved of little or 

 no benefit. There is a maximum size of furnace and charge 



