Mr. J. Napier on Copper Smelting. 489 



underpoled copper crack in rolling ; the former cracks along the 

 surface as well as at the edges, the latter only on the edges, the 

 surface remaining smooth. 



In making what is termed best select copper, the refining is 

 performed in the manner described, but no lead is added. This 

 quality of copper is used for the manufacture of fine alloys, such 

 as Muntz's yellow metal ; copper a little overpoled is generally 

 preferred for this purpose. Overpoled metal expands in setting, 

 giving the ingots a roundness on the top. Select metal is cast 

 into ingots about 8 inches long and 3 thick. The moulds are 

 fitted upon a frame placed over a pit or tank of water, so that 

 whenever the metal becomes solid, the mould which moves upon 

 a pivot is turned over and the copper ingot drops into the water, 

 which takes ofi^ all oxide from the surface. Each of these ingots 

 weighs about 15 lbs. The ingot-moulds are sometimes made of 

 iron, but these soon give way, crack upon the surface, and dis- 

 figure the ingot ; they also retain the heat so much as to give 

 the copper moulded in them a spongy appearance. Moulds 

 made of copper suit best, and are most generally used. 



We have said that the operation of poling is to effect the 

 reduction of the oxide and suboxide of copper dissolved in the 

 metal, but what constitutes overpoling is not so easily defined. 

 Mr. Vivian, in his paper on copper smelting, published in 1823, 

 gives it as his opinion that carbon is taken up by the copper, 

 forming a carburet, which remains dissolved in the metal as the 

 oxides do. In our inquiry we have never been able to detect 

 the slightest trace of carbon in overpoled copper, even in samples 

 where that operation had been purposely continued, until the 

 metal was, in the language of the workmen, rotten, being so 

 brittle. The following are a few observations upon this inter- 

 esting subject : — 



1st. In refining, the purer the copper is, the more difficult it 

 is to keep in the proper pitch. 



2nd. Overpoling causes the copper to expand when setting. 



3rd. Overpoled copper effervesces, which is no doubt produced 

 by the escape of gas, and which causes a shower of minute glo- 

 bules of solid metal coated with suboxide. 



These favour the idea that, whatever be the substance present, 

 it exists in the copper in a gaseous state, and is not permanently 

 combined with the metal. The following are the appearances 

 which copper in different states of poling have under the micro- 

 scope, with their specific gravities : — 



1st. Copper ready for poling. Red-brown colour, vitrified, 

 sandy fracture. Spec. grav. 8*491. 



2nd. After a little poling. Colour redder than last, sandy 

 fracture, not so vitreous-looking. Spec. grav. 8*526. 



Phil. Mag, S. 4. No. 35. Suppl. Vol. 5. 2 K 



