Mr. J. Napier on Copper Smelting. 193 



in the metallic portion or mat exceeds 50 per cent., the slag or 

 scoria obtained will contain some copper, and there will therefore 

 be a loss. The mat should not contain more than 40 per cent. 

 of copper in order to get clean slag. The assayer therefore 

 arranges his samples according to their quality as determined 

 by the eye. Those full of mundic, having much sulphur and 

 iron, and containing arsenic, are kept apart, in order to be sub- 

 jected to a dull red heat for a short time to expel a portion of 

 these impurities before adding flux. 



A portion of the sample to be assayed is weighed off. The 

 assayers have special weights divided into 100 parts, tei-med cents 

 or centners, with i, \, ^, x^^h ; 100 parts or cents are generally 

 taken ; and if no excess of sulphur or arsenic be present to re- 

 quire a slight roasting previously, the ore is mixed with a quan- 

 tity of flux composed of lime, borax, fluor-spar, a little salt and 

 nitre; occasionally a little soda or potash is used, and ground 

 window-glass. The nitre is termed the operating flux, as it 

 purifies the ore by supplying oxygen to the arsenic and other 

 impurities present ; the quantity of the flux added is not very 

 precise, generally about twice the weight of the ore used, but 

 sufficient to cause the perfect fusion of all the silica present. 

 The flux and ore are intimately mixed and put into a clay cru- 

 cible having a clay cover, then placed in a furnace and brought 

 to complete fusion, in which state the mass is kept for about 

 ten minutes ; the whole should be perfectly liquid, and should 

 exhibit no effervescence. The crucible is then removed from the 

 fire, and the contents poured into an iron ladle and allowed to 

 cool : some operators immerse the ladle and contents into cold 

 water. When taken from the ladle, the metallic portion forming 

 the mat is found as a button at the bottom, the slag or scoria 

 on the top ; they are separated, and the slag carefully examined 

 for any metallic particles before being thrown away ; but, as 

 above observed, if the mat contain less than 40 per cent, of 

 copper, the slags are generally free. The contents of the cru- 

 cible, instead of being poured into a ladle, may be allowed to 

 cool in the crucible, which is afterwards broken and the slag and 

 mat separated. The crucible being seldom fit to use again, no 

 loss is occasioned ; howevei', it is seldom practised by the Cornish 

 assayers. 



The regulus or mat, when separated from the slag, is finely 

 ground and put into a clean crucible, taking great care that none 

 of it is lost ; it is then placed in a slow fire, and gradually brought 

 to a dull red, the powder being stirred constantly with an iron 

 rod to prevent it caking ; the point of the rod is examined from 

 time to time to sec whether there is the slightest tendency to 

 cake, which is evinced from the ]iarticles adhering to the rod, in 



Phil. May. S. 4. Vol. 4. No. 24. Sept. 1852. O 



