ASSAYING. 



authority, metallic lead is always used, 

 probably from the ease it is supposed to 

 afford in determining the weight of the 

 different ingredients by calculation. The 

 lead in the process first becomes oxyded. 

 Then yields some of its oxygen to the 

 other imperfect metals, and afterwards 

 becomes vitrified, in conjunction with the 

 other oxydes so formed, and carries them 

 off along with it, leaving the perfect me- 

 tals pure. The above operation is called 

 cupellation, and is performed on a flat 

 round cake of bone ash, compressed 

 within an iron ring, that is named a cupel; 

 this is placed in a vessel called a muffle, 

 which resembles an oven in miniature, 

 that is fixed in a furnace capable of giving 

 a heat sufficient for the fusion of gold, so 

 that its mouth may come in contact with 

 the door, at the side to which it is luted, 

 to separate it from the peal ; there are 

 small slits formed in the sides of the muf- 

 fle, to afford a passage for the air. 



When the muffle and empty cupels are 

 heated red hot, a little powdered chalk is 

 put on the floor of the muffle, to prevent 

 the cupels from adhering to it after the 

 operation. Cupels should be always of 

 size proportinate to the lead to be used, 

 as they cannot absorb a weight of litharge, 

 at the utmost, more than their own. 



The assay of silver is performed in this 

 country on a piece of metal not exceeding 

 thirty-six grains, if the alloy appears con- 

 siderable ; which piece is laminated, and 

 weighed with extreme accuracy in a very 

 sensible balance. It is then wrapped up 

 in the requisite quantity of lead, rolled 

 out into a sheet, which is revived from 

 litharge, that it may be free from the silver 

 which lead in general contains naturally. 



The silver and lead are put on the cu- 

 pel when it and the muffle are red hot. 

 The metal immediately melts, and begins 

 to send off dense fumes, and a minute 

 stream of red fused matter is seen per- 



Eetually flowing from the top of the glo- 

 ule, down its sides, to the surface of the 

 cupel, where it sinks; the fume consists 

 of lead in vapour, and the red stream of 

 vitrified lead which carries down with it 

 the copper, or other alloy of silver, in- 

 to the cupel. As the cupellation acl- 

 yances the melted button becomes round- 

 er, its surface becomes streaky, with large 

 bright points of the fused oxyde, which 

 move with increased rapidity ; the last 

 portions of the litharge on the surface 

 quickly disappear, shewing the melted 

 metal with bright .yidescent colours, 

 which directly after becomes opaque, and 

 then suddenly appears brilliant, clean, 



and white, as if a curtain had been with- 

 drawn from it ; at which time the assay - 

 ers say it lightens. The silver is now left 

 pure, and the cupel is allowed to cool 

 gradually till the globule of silver is fix- 

 ed, when it is taken out while still hot, 

 and, when cold, weighed with as much ac- 

 curacy as at first. The difference be- 

 tween the weight of the globule, and that 

 of the silver first put in, shews the quan- 

 tity of alloy. If the globule is cooled too 

 quickly, the outward surface contracts so 

 suddenly as to force out the fluid metal 

 at the centre in arborescent shoots, by 

 which some portion is lost, and the assay 

 spoiled. 



In the assays for the mint in this coun- 

 try, two assays are always made of the 

 same mass of metal, and no sensible dif- 

 ference between the weights of the but- 

 tons is allowed to pass in scales that turn 

 with the T ^ part of a grain, troy. If 

 they differ, the assay is repeated. 



The process is considered as well per- 

 formed, when the button adheres but 

 slightly to the cupel ; when its shape is 

 very considerably globular above and be- 

 low, and not flattened at the margin ; 

 when it is quite clear and brilliant, and not 

 folded or spotted with any remaining li- 

 tharge ; and especially when the surface 

 is disposed in minute scales, the effect of 

 a hasty crystallization, which gives it a 

 play of light very different from that of 

 a perfectly even surface of a white metal. 

 The scales are of a pentagonal form, 

 slightly depressed at the centre. When 

 any alloy remains in the silver, the sur- 

 face appears, under the microscope, 

 smooth, as if varnished, and scarcely at 

 all scaly in texture. 



In the common assays of plate, either 

 gold or silver, copper is the alloy usually 

 met with ; if the fine metal be nearly 

 pure, the cupel round the bottom is only 

 stained yellow by the litharge ; if copper 

 is contained, it leaves a brown stain. The 

 other metals, except bismuth, scarcely 

 penetrate the substance of the cupel, but 

 remain on the edges of its cavity, in the 

 form of coloured scoriae ; of which iron 

 is black, tin grey, and zinc a dull yellow. 



The management of the fire is a pointof 

 great consequence in cupellation. When 

 silver is kept in fusion in a very high heat 

 a portion of it is volatilized, as Mr. Tillet 

 found that a button of pure silver, kept 

 in a very high heat, lost a twentieth part 

 of its weight ; which loss would cause a 

 great error in assaying. On the other 

 hand, when the fire is too slack, the lith- 

 arge is not absorbed by the cupel, but 



