SILVER ASSAYING 



823 



lead must be, in all cases, as free as practicable from silver ; otherwise errors of the 

 most serious kind would be occasioned in the assays. 



The assay is then placed upon a cupel which has been previously heated to the 

 proper temperature in the muffle, and the door closed. Fusion immediately occurs, 

 and the cupellation begins when the uncovering, or removal of the black skin of oxide 

 of lead takes place. The oxidation of the lead proceeds rapidly, and the spots of oxide 

 formed on the surface are rapidly absorbed by the cupel. Near the completion of the 

 assay the oxide forms thin films or bands ; and when the surface of the silver becomes 

 bright and immoveable , as the brightening occurs, the cupellation is finished. The 

 cupel is now allowed to remain in the muffle until the temperature is diminished and 

 the silver is solid. It is now removed from the muffle, and when cold the button of 

 silver is detached from the cupel by a pair of pliers squeezed or hammered on the side, 

 the under surface cleaned by means of a hard brush, and finally weighed. An assay 

 is thought to be good when the bead is of a round form, if its upper surface is brilliant 

 and crystalline, its lower stirface granular and dead white, and if it separates readily 

 from the cupel. "When copper is present the oxide of copper produced forms a fusible 

 compound with the oxide of lead and passes into the cupel. The proportion of lead 

 added varies with the amount of copper present in the alloy operated on. 



Quantity of Lead to be employed for Cupellation of Alloys of Copper and Silver 

 (M. D'Arcet). 



Bismuth may be used as a substitute for lead in cupellation ; two parts of it being 

 nearly equivalent to three of lead. But its higher price prevents its introduction 

 among assayers. 



During the process of cupellation, a portion of the silver is absorbed by the cupel, 

 varying in amount according to the temperature and the quantity of lead employed. 

 This loss is estimated and added on to the weight of the button of silver obtained. 

 The results are returned on 1,000 parts or on the pound troy. It is also customary 

 to report the assays in relation to standard. For example, English standard silver 

 contains 925 parts of silver in 1,000 of alloy. If the resxilt obtained was 920, it would 

 be reported 5 "w., or 5 parts in 1,000 worse than standard, and 930 would bo reported 

 5 B., or 5 parts better than standard. 



An assay may prove defective for several reasons. Sometimes the button or bead 

 sends forth crystalline vegetations on its surface with such force as to make one sup- 

 pose a portion of the silver may be thrown out of the cupel, technically called 

 ' spitting.' When the surface of the bead is dull and flat, the assay is considered to 

 have been too hot, and it indicates a loss of silver in fumes. When the tint of the 

 bead is not uniform, when its inferior surface is bubbly, when yellow scales of oxide 

 of lead remain on the bottom of the cupel, and the bead adheres strongly to it, by 

 these signs it is judged that the assay has been too cold, and that the silver retains 

 some lead. After the lead is put into the cupel, it gets immediately covered with a 

 coat of oxide, which resists the admission of the silver to be assayed into the melted 

 metal ; so that the alloy cannot form. When a bit of silver is laid on a lead-bath in 

 this predicament, we see it swim about a long time withoiit dissolving. In order to 

 avoid this result, the silver is wrapped up in ci bit of paper ; and the carlmfetted 

 hydrogen generated by its combustion reduces the film of the lead oxide, gives the 

 bath immediately a bright metallic lustre, and enables the two metals readily to 



