At the same time the cupel will assume a yellow, brown, or blackish colour, according to the quantity and nature of the scoria imbibed by it.
Care must be taken to chuse a cupel of a proper capacity.
Take a cupel capable of containing one third more matter than you have to put into it: set it under the muffle of a furnace, like that described in our Theoretical Elements, as peculiarly appropriated to this sort of operation.
But the cupel must still be left a minute or two under the muffle, and then drawn slowly out with the iron hook towards the door of the furnace.
Mr. Geoffroy could not collect a sufficient quantity thereof to cupel them, and satisfy himself whether or no they were Silver.
As soon as your Copper comes to this state, cover it with charcoal-dust conveyed into the cupel with an iron ladle: then take the cupel out of the furnace and let it cool.
All the processes on Silver already delivered, whether for extracting it from its ores, or for refining it, either by the Cupel or by Nitre, are applicable to Gold also.
When you see the matter in the cupel in a violent ebullition, and emitting much smoke, lower the fire by the methods formerly prescribed.
Then in order that the cupel may be equally heated by the fire, turn it around with a small iron hook, whose handle is likewise made of iron and is a foot and a half long.
If you also wish to know what portion of silver is contained in the lead, melt the lead in the cupel until all of it exhales.
Now the lead which has absorbed the silver from a metallic ore is consumed in the cupel by the heat in the space of three quarters of an hour.
Geber (prior to the 14th century) are the first where adequate description of the cupel itself can be found.
The gold which remains is re-heated with stibium, and when this is exhaled the gold is heated for the third time in a cupel with a fourth part of lead, and then quenched.
The button that settles is transferred to the cupel and re-melted.
The process is not tedious, and in experienced hands it may rival the cupel in rapidity; it has the advantage over the cupel of being more within the reach of ordinary operators, and of not requiring a long apprenticeship.
The cupel furnace above described, must be slowly heated, in order to dry the cupel without causing it to crack, which would infallibly be produced by sudden evaporation of the moisture in it.
The test or cupel must be heated before the impure silver and soft lead are put into it.
Lead resembles bismuth very much; the cold cupel is of a lemon-yellow colour.
The silver cake receives a final purification at the Mint, in a cupel on a smaller scale.
It has been proposed to substitute bismuth for lead in assaying silver, as a smaller quantity of it answers the purpose, and, as its oxide is more fluent, can therefore penetrate the cupel more readily, and give a more rapid result.
The arched roof of the furnace is 12 inches above the cupel near the fire-bridge, and 9 inches near the flue at the other end.
A cupel may be regarded, in some measure, as a filter permeable only to certain liquids.
The cupel thus prepared is placed in the refining furnace.
A cupel or cupelling hearth in which precious metals are melted for trial and refinement.
The process consist in exposing the cupel containing the metal to be assayed or refined to a hot blast, by which the lead, copper, tin, etc.
The cupel has the remarkable power of absorbing the oxides of metal, and so in an hour or so there is nothing left in it save a little bead of silver and gold.
Then the cupel is put into a very hot furnace so arranged that a current of air passes over it.
In three cupellations of 1 gram of gold with 20 grams of lead made purposely at a very high temperature the cupel absorbed 6.
Its position here should correspond to that it will occupy in the muffle and eventually in the cupel tray.
For when this is done the button can be loosened from the cupel by merely touching it with the point of a pin, and is then safely and easily transferred to a watch glass by touching it with the head of a pin which has been moistened.
The lead buttons should be hammered into discs with rounded edges, and be freed from slag; if too big for a cupel they may be scorified together in a small scorifier, but it is better to cupel them separately.
The cupel is got out by pressing up the disc of metal forming the false bottom; the removal is more easily effected if the mould is somewhat conical, instead of cylindrical, in form.
The cavity of the cupel is about 1/4 inch deep, and something of the shape shown in fig.
If the button is a small one the cupel is withdrawn at once and placed on that square of the cupel tray which corresponds to the position it occupied in the muffle.
When the cupel shows signs of the presence of these metals in objectionable quantity, it is well to repeat the assay and scorify so as to remove them before cupellation.
Add to these the estimated cupel loss, and calculate the result.
Cupel the buttons of lead; weigh; make the necessary corrections, and calculate to ounces to the ton.
Remove thecupel from the muffle immediately the operation is finished.
The goldsmith, not content with the touchstone and the application of aqua fortis, exposed the metal on the cupel with lead, and fused it with antimony, but it sustained no loss.
One half of the lead thus obtained is put into a dry cupel of bone ash, and placed in the cupelling furnace, and treated as described in the article on assaying; the metallic button left on the cupel is then detached and weighed.
If the quantity of lead to be absorbed is more considerable, another cupel may be turned topsy-turvy, and the cupel in which the assay is to be made may be placed upon it.
Silver is volatile at a high heat, but when the muffle is almost white, the metal well fused and clean, the fumes rising slowly, and the cupel a cherry red, all is going smoothly.
Cupellation, on the other hand, collects them in the bone ash, of which the cupel is composed.
The cupelis made of the ashes of burnt bone, and it is better to make them on the spot, as the bone ash may be carried anywhere without damage, whereas the cupels are very fragile.
Take the cupel slowly from the fire to avoid "spitting," by which portions of the buttons are lost.
The bone ash is moistened with water, stamped in a cupel mould, and allowed to dry slowly.
Thus a singlecupel will often last 48 hours, and 6 or 7 tons of lead may be oxidized upon it.
The size of the cupel should always be regulated according to the quantity of foreign matter to be absorbed, it being generally understood that the material of which it is formed takes up double its weight of lead.
By introducing a cylindrical piece of wood to the lower aperture of the steel ring, the cupel can be removed without difficulty.
The cupel must be cooled with very great care, in order to prevent the silver from sprouting; which if allowed to take place would result in considerable loss, besides destroying the accuracy of the assay.
When the cupel has received the above proportion of metal, the addition of the alloy ceases, and the silver is allowed to purify.
To make a cupel the space in the ring is nearly filled with the moistened bone-ash, and pressed down by the hand, and afterwards by the die, the latter being driven into the ring by the application of a wooden mallet (Fig.
When the bone-ash has been sufficiently compressed, the die is withdrawn, and the cupel removed from the ring.
This interesting process is performed in a reverberatory furnace of a very peculiar construction, the cupel employed on the large scale differing somewhat from the ordinary one, being considerably larger and varying also in form.
I put this button into another cupel in the same furnace, observing to turn it, by which it only lost twelve grains in two hours; its colour and form were very little changed.
