Basic Info

Liquid silver has the peculiarity of dissolving a large volume of oxygen (22 times its volume in oxigen) , which is expelled upon solidification. The solidification of large beadsstarts at the surface. When the center of silver beads solidifies, the oxygen is sometimes expelled violently, spewing forth some of the interior silver and forming a cauliflower-like growth on the bead. This mechanism is known as “sprouting.”

Sprouted beads should be rejected if there is any reason to believe that, any particles of metal have been lost.

To avoid sprouting, all large beads should be cooled slowly, either by leaving them in the, furnace after the blick, and until certain that, they have cooled sufficiently, or by moving them to a slightly cooler zone in the furnace and then covering them with a very hot cupel before they have completely solidified.

The hot cupel melts the outer crust and allows the bead to solidify slowly, so that the oxygen can escape from the interior without violence. When the bead contains more than one-third of its weight in gold, it does not sprout; hence beads known to be of this composition may be removed from the furnace as rapidly as desired.

Sprouting of silver beads is considered a sign of purity, but this indication is of no practical value to the assayer, as sprouting should always be avoided. (Shepard p. 63)

Bugbee states that as little as 0.004% of rhodium in silver beads causes a distinct crystallization, which is more apparent at 0.01% Rh, and that 0.03% Rh causes unavoidable sprouting of silver beads. (Shepard p. 71)

Surcharge — Net Losses and Gains — The net sum of the (…) losses and gains incurred in the various operations is called the “surcharge” since, in the assay of high-grade bullion, the silver retained is usually in excess of the gold lost, so that the cornet will weigh more than the gold originally present in the assay piece; if the reverse is the case, the result is considered by some assayers to be less accurate. When there is a gain in weight it is referred to as a “plus surcharge“, while a loss in weight is designated a “minus surcharge“.

With material rich in gold, the silver retained more than compensates for the loss, and, as just stated, the surcharge is positive; but with a decrease in the proportion of gold the loss is greater and the surcharge is negative.

Thus the surcharge will usually amount to about 0 (nil) for bullion of about 700 to 800 fine; above that there will be a “plus surcharge” and below that a “minus surcharge“. The surcharge is reported in parts per 1000 in the same way as the fineness of bullion. (Smith p. 276)

Inquartation. – When the bead contains too little silver to part, it is necessary to alloy it with more silver. This process is called inquartation. It originated from the custom of the old assayers of adding silver until the gold was one-quarter of the whole. They considered a ratio of 3 parts of silver to 1 of gold to be necessary for parting. At present, in assaying gold bullion, a ratio of only 2 or 2¼ parts of silver to 1 of gold is used, mainly to avoid all danger of the gold breaking up in the boiling acid.

In this case some little silver remains undissolved, even though the alloy is rolled out to about 0.01″ in thickness. (Bugbee p. 121)

Many assayers, when working for both gold and silver and suspecting an ore to be deficient in silver, add silver to the crucible or to the leadbutton before cupeling, part directly and then run separate assays to determine the silver in the ore. (Bugbee p. 122)

Microcupellation consists of doing the cupellation process but with a much smaller sample.

The sample size for cupellation is tipically 250mg, but with microcupellation it is only about 10 mg. This allows not to “destroy” the jewelry piece to analyse as the sample can sometimes also be taken from hidden places of the jewelry article.

An additional advantage of microcupellation is that it takes less time than conventional cupellation.

The cupel is a shallow, porous dish made of bone-ash, Portland cement, magnesia or other refractory and non-corrosive material. (Bugbee p. 89)

Cupels should not crack when heated in the muffle and should be so strong that they will not break when handled with the tongs. Good cupels give, a slight metallic ring when struck together after air-drying. It is best to heat cupels slowly in the muffle as this lessens the chance of their cracking.

A good cupel should be perfectly smooth on the inside, and of the right porosity. If it is too dense, the time of cupellation is prolonged and the temperature of cupellation has to be higher, thus increasing, the loss of silver. If the cupel is too porous it is said that there is danger of a greater loss, due to the ease with which small particles of alloy can pass into the cupel. The bowl of the cupel should be made to hold a weight of lead equal to the weight of the cupel.

The shape of the cupel seems to influence the loss of precious metals. A flat, shallow one exposes a greater surface to oxidation and allows of faster cupellation; it also gives a greater surface of contact between alloy and cupel, and as far as losses are due to direct absorption of alloy, it will of course increase these.

The writer, using the same bone-ash and cupel machine, and changing only the shape of the cupel, has found shallow cupels to give a much higher loss of silver. In doing this work it was found harder to obtain crystals of litharge with the shallow cupel without freezing, and it was very evident that a higher cupellation temperature was required for the shallow cupel. The reason for this is that in the case of the shallow cupel the molten alloy is more directly exposed to the current of air passing through the muffle, and consequently a higher muffle temperature has to be maintained to prevent freezing. T. K. Rose* also prefers deep cupels on account of smaller losses. French found shallow cupels less satisfactory on account of sprouting. (Bugbee p. 92 – 93)

Magnesia cupels are very hard, which is an advantage in that they do not suffer so much breakage in shipment. They are always factory-made and are decidedly more expensive than bone-ash cupels, which may be home-made. Certain brands of magnesia cupels give an apparently lower loss of silver in cupeling than can be obtained with bone-ash cupels but it is a question how much of this is real and how much due to an increase in the amount of impurities retained in the silver beads.

Magnesia cupels behave quite differently from ordinary bone-ash cupels, and the assayer who is accustomed to bone-ash cupels will have to learn cupeling over again when he starts using those made of magnesite. This difference in behavior is due mainly to the different thermal properties of the two materials. Both the specific heat and the conductivity of magnesite are decidedly greater than those of bone-ash, so that with cupels of both kinds running side by side, the lead on the magnesia cupel is comparatively dull while that on bone-ash is very bright. This is due to the greater conductivity of magnesite, which allows a more rapid dispersion of the heat of oxidation of the lead, with the result that magnesia cupels require a higher muffle temperature than do bone-ash cupels. An especially high finishing temperature is required for magnesite cupels, to insure the elimination of the last 1 or 2% of lead. A bone-ash cupel will finish in a muffle, the temperature of which is sufficient to cause uncovering, but this is not true of the magnesia cupel, because in this case the heat of oxidation of the lead is diffused too rapidly and is not conserved to help out at the finish.

Magnesia cupels absorb about two-thirds of their own weight of litharge, those of cement about three-fourths of their weight of litharge.  (Bugbee p. 116 – 117)

Summary of Cupellation–temperature Cycle. — Characteristic temperature cycles in cupellation are shown on Fig. 8, based on measurements taken with a pyrometer ½” above the muffle floor just behind the front row of cupels. The curves apply particularly to buttons from 20 to 25 g in weight and with beads weighing 50 mg or more, cupeled with feathers.

The indicated temperatures are subject to corrections based on the furnace draft, heating and cooling lag, and the pyrometer position.

The minimum temperature allowable during the driving depends also upon the type of cupel used and the size of the button and bead. Large buttons that are cupeled rapidly in bone-ash cupels with the ordinary shallow cup frequently permit minimum muffle temperatures as low as 790°C and will finish at 820°C if the beads are small.

Magnesia cupels under similar conditions require minima of 830°C in the driving trough, and 840° to 850°C to finish.

The formation of feathers of litharge can be observed readily with bone-ash or bone-ash-cement cupels and serves to indicate proper driving temperature, but when copious feathers form on magnesia cupels the temperature is dangerously near the freezing point.

Finishing temperatures greatly in excess of 900°C, as measured in the manner indicated above, should be avoided in all cases, as higher temperatures cause greatly increased losses of gold and silver with all types of cupels and all variations in button and bead weight. (Shepard p. 66 – 67)

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  Figure 8: Cupellation Temperature Cycle