COPPER, SILVER, AND GOLD 



417 



Silver occurs in nature, both in a native state and in certain com- 

 pounds. Native silver, however, is of rather rare occurrence. A far 



' pickling ') after being worked up. This consists in removing the copper from the surface 

 of the article by subjecting it to a dark-red heat and then immersing it in dilute acid. 

 During the calcination the copper on the surface is oxidised, whilst the silver remains 

 unchanged ; the dilute acid then dissolves the copper oxides formed, and pure silver is 

 left on the surface. The surface is dull after this treatment, owing to the removal of a 

 portion of the metal by the acid. After being polished the article acquires the desired 

 lustre and colour, so as to be indistinguishable from a pure silver object. In order to 

 test a silver article, a portion of its mass must be taken, not from the surface, but to a 

 certain depth. The methods of assay used in practice are very varied.. The commonest 

 and most often used is that known as cupellation. It is based on the difference in the 

 oxidisability of copper, lead, and silver. The cupel is a porous cup with thick sides, 



FIG. 97. Portable muffle furnace 



made by compressing bone ash. The porous mass of bone ash absorbs the fused oxides, 

 especially the lead oxide, which is easily fusible, but it does not absorb the unoxidised 

 metal. The latter collects into a globule under the action of a strong heat in the cupel, 

 and on cooling solidifies into a button, which may then be weighed. Several cupels are 

 placed in a muffle. A muffle is a semi-cylindrical clay vessel, shown in the accompanying 

 drawing. The sides of the muffle are pierced with several orifices, which allow the access 

 of air into it. The muffle is placed in a furnace, where it is strongly heated. Under the 

 action of the air entering the muffle the copper of the silver alloy is oxidised, but as the 

 oxide of copper is infusible, or, more strictly speaking, difficultly fusible, a certain quan- 

 tity of lead is added to the alloy ; the lead is also oxidised by the air at the high tem- 

 perature of the muffle, and gives the very fusible lead oxide. The copper oxide then 

 fuses with the lead oxide, and is absorbed by the cupel, whilst the silver remains as a 



