LEAD 



Summary of Lead Ore, Lead, and Silver Produce of 1872. 



would be liable to vegetate, by which a portion of the metal might be thrown off, and 

 a certain amount of loss be thereby entailed. To prevent this, the cupel in which the 

 assay has brightened should be immediately covered by another, kept red hot for that 

 purpose. The two are now gradually withdrawn together, and, after having suffi- 

 cently cooled, the upper cupel is removed, and the globule of silver detached and 

 cleaned as follows : 



The globule is now laid hold of by a pair of fine pliers and flattened on a small 

 steel anvil, by which the oxide of lead which may have attached itself to it becomes 

 pulverised, and is removed by rubbing with a small hard brush. The flattened disc 

 is then examined, in order to be sure that it is perfectly clean, and afterwards weighed 

 in a balance capable of turning with ^^th of a grain. 



The cupellations should be conducted at the lowest possible heat at which they can 

 be effected. The temperature best fitted for this operation is obtained when the 

 muffle is at a red heat, and the vapours which arise from the assays curl gradually 

 away, and are finally removed by the draught. When the muffle is heated to white- 

 ness, and the vapours rise to the top of the arch, the heat is too great : and when, on 

 the contrary, the fumes lie over the bottom, and the sides of the openings in the 

 muffle begin to darken, either a little more fuel must be added or the draught in- 

 creased. If an assay has boon properly conducted, the button of silver obtained is 

 round, bright, and smooth on its upper surface, and beneath crystalline, and of a dead 

 white colour ; easily removed from the cupel, and readily freed from litharge. 



When the ores of lead, in addition to silver, contain gold, the button remaining on 

 the cupel is an alloy of these metals. For the method of estimating the gold, see 



For commercial purposes the silver contained in any given ore is estimated in 

 ounces, pennyweights, and grains troy upon the statute ton avoirdupois of 2,240 Ibs. 

 It is customary to return the silver obtained from lead ores upon the ton of lead as 

 yielded by assay, and not upon the ton of ore. For a Table to facilitate calculation, 

 Bee SILVEB. 



