Assaying Ores of the Precious Metals. 61 



resorted to whenever the value of the ore was to be deter- 

 mined. 



A brief sketch of the furnace assay will not be out of 

 place. A few grammes of the ore, finely powdered and 

 mixed with many times its weight of pure lead, is placed 

 in a refractory earthen cup already in a furnace and glow- 

 ing with a bright red heat. Quickly the lead melts and 

 the ore may be seen floating upon its surface. A current 

 of hot air sweeping over the surface of the bath, speedily 

 oxydizes the ore and a portion of the melted lead. The 

 litharge thus formed decomposes the ore ; forms a fusible 

 slag with its baser materials, but refuses to touch the pre- 

 cious metal, which, being soluble in melted lead, enters at 

 once into the glowing bath of that substance in the centre 

 of the cup. At the end of this process most of the lead as 

 litharge together with nearly all the baser constituents of 

 the ore, constitute a fluid slag while the rest of the lead 

 with every particle of precious metal will remain behind 

 in the form of a red hot melted button upon which the 

 lighter slag is floating. The fluid is then poured into 

 another dish and allowed to cool. A few strokes of a 

 hammer afterwards clean away the slag, and a few more 

 shape the button into proper form for future use. 



By the process of scorification, just described, the precious 

 metal in the ore is obtained as an alloy with lead from 

 which it is afterwards to be separated by cupellation. The 

 principle in this last process is simple enough. It is this : 

 Oxygen at a high temperature has a strong attraction for 

 lead, and almost none at all for silver or gold. The ele- 

 gance of the process itself rivals the simplicity of its prin- 

 ciple. A little porous cup of bone-ash — the cupel — is 

 placed in a furnace and heated to redness. The button of 

 alloy, being then introduced, quickly melts, while a current 

 of air passing over its dull red surface furnishes oxygen 

 for the chemical action which at once begins. The oxygen 



