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ASSAYING. 
met with ; if the fine metal be nearly pure, 
the cupel round the bottom is only stained 
yellow by the litharge ; if copper is contain- 
ed, 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 colour- 
ed scoria; ; of which iron is black, tin grey, 
and zinc a dull yellow. 
The management of the fire is a point of 
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 
fhe fire is too slack, the litharge is not ab- 
sorbed by the cupel, but lies on the surface 
as a red scoria. The heat is known to be 
too great when the cupel can scarcely be 
distinguished from the muffle, and the as- 
cending fume can scarcely be seen for the 
dazzling heat. Towards the end of the 
operation the fire should be gradually in- 
creased, for in proportion as the lead is ab- 
stracted from the alloy, it becomes less easy 
of fusion ; and at last an heat fully equal to 
the melting of pure silver is required. 
As the cupellation requires a free access 
of air as well as an high degree of heat, the 
stopper of the muffle is always removed as 
soon as the metal is put into the hot cupel, 
to allow a current of air to pass through the 
muffle : but to prevent this from cooling 
the muffle too fast, several round pieces of 
charcoal are heaped lip in front of the muf- 
fle, on an iron plate placed there to hold 
them, which burn with sufficient force' to 
heat the air as it passes to the cupels. The 
furnace should be made so that the heat of 
the fuel within may be readily increased or 
diminished, but at the same time so that it 
can be kept up with steadiness. 
The time taken up in making one assay 
of silver is generally from 15 to 25 minutes. 
The proportioning of the lead to the sup- 
posed alloy in the silver to be assayed, is of 
great importance ; if too little is employed 
some of the alloy will remain in the mass; 
but if too much is used some of the silver 
will be wasted; for Mr. Tillet has found 
that when the proper quantity of lead is 
used it carries down a portion of the silver 
into the cupel, which he has ascertained by 
accurate experiments to amount to of the 
lead in the cupel ; whereas the natural ad- 
mixture of silver in lead is only T J SI - But 
when an excess of lead is employed for 
J* VOL. I. 
cupellation, this loss of silver is somewhat 
greater, though it does not increase in the 
ratio of the excess of lead, for ten parts of 
lead to a given alloy will not' carry down 
twice as much silver as five parts, .though 
the difference of loss will be very sensible. 
When the litharge carried into the cupel is 
reduced to reguliue lead, on being cupelled 
a second time it will yield a button of silver 
fully equal to the loss of this metal in the 
first assay. In all these reductions the silver 
appears equally distributed through the 
lead, for Mr. Tillet found that separate glo- 
bules of the lead spurted out by accident 
upon an empty cupel in the muffle, each left 
a minute atom of silver lying upon the spot 
where the globules had scorified. 
Bismuth will serve the same purpose as 
lead in cupellation ; but besides being 
dearer, it is found to carry down with it 
into the cupel somewhat more of the silver 
than the same quantity of lead does. 
To estimate the quantity of alloy in silver, 
the ancient assayers used touch-needles, or 
small slips of silver, alloyed with known 
proportions of copper, in a regularly in- 
creasing series from the least to the greatest 
proportion ever used, The silver to be 
assayed was compared with these, and its 
alloy estimated by that of the needle to 
which it shewed the closest resemblance. 
But an experienced assayer is at the present 
time able to judge of the alloy with suffi- 
cient exactness, by the ease or difficulty with 
which the silver is cut, by the colour and 
grain of a fresh cut surface, the malleability, 
the change of surface when made red hot, 
and the. general appearance. 
The assay of gold is more complicated 
than that of silver. The baser metals may 
be separated from it by cupellation in the 
same manner as from silver, except copper, 
which has so strong an affinity for gold 
that it can scarcely he overcome by this 
method, unless silver is first combined with 
the mass ; and this makes the second opera- 
tion necessary, mentioned before, namely 
the parting of the gold from the silver. 
The process of parting is performed by 
the aquafortis of commerce, which dissolves 
the silver and leaves the gold untouched. 
But in this operation it is found that when 
the gold exceeds a certain proportion in the 
mixture, it so much protects the silver from 
the acid, as more or less to prevent its 
action. Therefore when the gold is in ex- 
cess it becomes necessary to add so much 
silver as to give this metal the predomi- 
nance. The proportion of silver generally 
C c 
