ASSAYING. 
perfect metal, platina, is but seldom found 
united to them. 
The basis of the method of separating 
gold or silver from the imperfect metals is 
founded on the facility with which the latter 
imbibes oxygen ; and the process is calcu- 
lated to accelerate this operation as much 
as possible ; hence the oxide of lead or 
litharge, is generally considered as the most 
powerful purifier of the perfect metals, from 
the ease with which it parts with its oxygen 
to the imperfect metals united with them,; 
but of late, oxide of manganese has been 
found superior to it in several instances for 
this purpose. In the chemical analyses of 
metals, the oxide of lead is generally prefer- 
red for the above purpose ; but in the 
assays, performed by authority, metallic 
lead is always used, probably from the ease 
it is supposed to afford in determining the 
weight of the different ingredients by cal- 
culation. The lead in the process first be- 
comes oxyded, then yields some of its oxygen 
to the other imperfect metals, and after- 
wards becomes vitrified in conjunction with 
the other oxides so formed, and carries 
them off along with it, leaving the perfect 
metals 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 suffi- 
cient for the fusion of gold, so that its 
mouth may come in contact with a door at 
the side, to which it is luted, to separate it 
from the peel; there are small slits formed 
in the sides of the muffle 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 opera- 
tion. Cupels should be always of size pro- 
portionate 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 cupel 
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 perpetually flowing 
from the top of the globule 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 into the cupel. As the cupellation 
advances the melted button becomes 
rounder, its surface becomes streaky with 
large bright points of the fused oxide, which 
move with increased rapidity ; the last por- 
tions of the litharge on the surface quickly 
disappear, shewing the melted metal with 
bright iridescent colours, which directly 
after becomes opake, and then suddenly 
appears brilliant, clean, and white, as if a 
curtain had been withdrawn from it ; at 
which time the assayers say it lightens. The 
silver is now left pure, and the cupel is al- 
lowed to cool gradually till the globule of 
silver is fixed, when it is taken out while 
still hot, and when cold weighed with as 
much accuracy as at first. The difference 
between the weight of the globule and that 
of the silver first put in, shews the quantity 
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 por- 
tion is lost, and the assay spoiled. 
In the assays for the mint in this country, 
two assays are always made of the same 
mass of metal, and no sensible difference 
between the weights of the buttons is allow- 
ed to pass in scales that turn with the 
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 con- 
siderably globular above and below, and 
not flattened at the margin ; when it is quite 
clear and brilliant, and not folded or spotted 
with any remaining litharge ; 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 pentago- 
nal form, slightly depressed at the centre. 
When any alloy remains in the silver, the 
surface appears under the microscope 
smooth as if varnished, and scarcely at all 
scaley in texture. 
In the common assays of plate, either 
gold or silver, copper is the alloy usually 
