GOLD.— Assaying.
Alloy.
Quantity of
Quantity of
Proportion of
(Told.
Copper.
Lead required.
1000
0 .
1 part,
900
100 .
10 “
800
200 .
10 “
700
300 .
22 “
600
400 .
24 “
500
500 .
26 “
400 to 0 ....
. 600 to 1000 .
34 “
Kandelhaedt
gives somewhat different propor-
tions, as is shown
in the following table :—
Alloy.
Quantity of
Quantity of
Proportion of
Gold.
Copper.
Lead required.
1000
. 0
. 8
980 — 920 ..
. 20—80
. 12
920 — 875 ..
.... 80—125
. 16
875 — 750 ..
. 125—250
. 20
750 — 600 ..
. 250—400
. 24
600 — 350 ..
_ 400 — 650
. 28
350 — 0 .
.... 650 — 1000
. 32
ft must be observed that the numbers in the thirdcolumn indicate multiples of one part of the alloy.It may appear surprising that, at and below the pointat which the alloy contains 600 parts of copper, thesame proportion of lead is required, whatever maybe the proportion of the copper. This fact, however,has been fully verified by experience. It requires,in short, as much lead to remove the copper froman alloy containing 600 parts of that metal, as itwould to carry off a quantity of copper equal to theweight of the copper and gold together. It shouldbe added, that when the assayer knows approxi-mately the composition of the alloy, he should insurethe presence of about two and a half times as muchsilver as gold, which is not only useful in the cupel-lation, to guard against the loss of gold, but isnecessary, as already stated, for the subsequentoperation of parting.
The assays having been placed in the cupels, theopening into the muffle should be loosely closed byan additional piece of hot fire-brick placed above thefirst, or by pieces of charcoal. After a short intervalall the assay pieces will be melted, and in each cupelwill be seen a round mass of molten metal, on thesurface of which appear bright patches of litharge;these are produced by the oxide of lead which isconstantly forming, and passing off from the centreto the edges of the assay, where it meets with andsinks into the porous matter of the cupel, leavingalways a new surface exposed to the current of airwhich is carried through the muffle. At the sametime there rises, and passes through the openings ofthe muffle, a white vapour which is caused by thevolatilization of a portion of lead. When this va-pour is very thin and rises with great rapidity, itis an indication that the heat is too great, and partof the draught must be stopped; on the other hand,when it is thick, and heavy, and hangs sluggishlyover and around the cupel, the temperature is toolow, and the draught must be increased.
According to Makins, the draught should be soadjusted that no appearance of fumes is manifest,and thus the cupel should be found to have increased I
119
in weight by the weight of lead together with itsequivalent of oxygen.
As soon as the greater part of the lead has beenthus converted into litharge and absorbed, theremaining bead of rich alloy suddenly becomesopaque and glowing, and is traversed by iridescentbands produced by the extremely thin films offluid litharge resulting from the last traces of lead.This is a sign that the process is on the point ofcompletion, and moans must now be taken to in-crease the heat of the furnace for a short time, soas to insure the expulsion of the last remainingportions of lead. But if silver be present in theassay, and if it be desired to estimate also theamount of that metal, the increase of temperaturetowards the end of the process must be of shortduration, otherwise a considerable portion of thesilver would soon volatilize. Immediately after thedisappearance of these bands the globule becomessteady, after which the peculiar action known as“brightening” takes place, when the metal is ob-served to emit a brilliant flash of light, and itimmediately solidifies. If the cupellation has beenwell performed, the gold or the mixture of the twoprecious metals is now in a state of almost chemicalpurity.
When the cupellation is terminated, the mouth ofthe muffle ought to be closed, and the whole allowedto cool until the buttons become solid; or, whenonly a few assays are performed at a time, the cupelscontaining the buttons of pure metal may be.drawnto the front of the muffle, and allowed to coolslowly. This refrigeration must not be too rapid,otherwise there may be very sensible loss by “ spit-ting, sprouting, or vegetation ”—an effect, however,only produced when a large proportion of silver ispresent, and which has been already explained inconnection with the process of cupellation on thelarge scale. To insure gradual cooling, it is occasion-ally convenient to invert over the cupel containingthe bead another heated cupel. Some assayersremove the cupels from the furnace while themetal is in a molten state, but the practice cannotbe recommended, for in addition to the danger ofupsetting a cupel and the “ spitting ” of the assay,the buttons are found to be more malleable whencooled gradually.
The button is detached from the cupel after cool-ing by a pair of steel forceps, cleaned with a brushformed of stiff bristles, and flatted as presentlydescribed. If the process has been successful thebutton should be round and bright upon its uppersurface, but somewhat rough or crystalline on thepart by which it was attached to the cupel, fromwhich it ought to be removed without difficulty.
The presence of iron, tin, nickel, or zinc in thealloy under assay materially interferes with the suc-cess of the operation, as these metals will not passinto the cupel by the aid even of a large quantity oflead; hut from their rapid oxidation will eithervolatilize, or their oxides will accumulate on the sur-face as a sort of slag, in which particles of preciousmetal may be entangled. In this case the assay