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Essential Fire Assay Terms



as used in Bibliography

"Cupels" :    Magnesia    Bone Ash     Cement       Behaviour




The cupel is a shallow, porous dish made of bone-ash, Portland cement, magnesia or other refractory and non-corrosive material. (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. (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. (p. 116 - 117)



Bone-ash cupel, mean specific heat between 15º and 100º C is 0.185. Magnesia cupel, mean specific heat for same temperatures is 0.215. A bone-ash and magnesia cupel of identical volumes weigh respectively 22 and 29 g. The heat conductivity of magnesia cupels is very much greater than that of bone-ash cupels. When the two types of cupels are heated to 90º C in a steam bath, at the end of 14 minutes the magnesia cupels are at 90º C and the bone-ash cupels at only 60º C. During cupellation of lead at the end of 6 minutes from the addition of the button the magnesia cupel showed practically the same temperature in the cupelling lead as in the bottom of the cupel, viz. 920º C, while the bone-ash cupel in the same muffle showed a temperature of 990º C for the cupelling lead, and only, 932º C in the bottom. The total heat capacity of a magnesia cupel is more than 50 %, greater than that of a bone-ash cupel of the same volume, so that on cooling the two types of cupel the magnesia cupel retains a higher temperature somewhat longer than the bone-ash cupel in spite of its greater diffusivity of heat. From this data the reason of the behavior of magnesia and bone-ash cupels during cupellation is apparent. It will be noted:

(1) That in magnesia cupels the lead is less bright and hence at a lower temperature than in bone-ash cupels, although the muffle temperature is the same. This is due to the fact that the extra heat generated by the combustion of the lead is diffused as rapidly as generated by the superior diffusivity of the magnesia cupel and hence cannot serve to raise the temperature of the lead, as is the case in the bone-ash cupel. Hence for the same "muffle temperature" the actual cupellation temperature of the lead in the magnesia cupels is 50º to 60º C lower than in the bone-ash cupels. To this fact is due the lower losses of precious metal in magnesia than in bone-ash cupels. From the discussion under "cupellation temperature" it will have been noted that with bone-ash cupels, if once the muffle has attained a temperature sufficiently high to cause the uncovering of the button, the rise in temperature of the lead due to its oxidation, is sufficient to carry the cupellation to a finish provided the muffle temperature is not lowered at the end of the operation. This is not the case with magnesia cupels for now obvious reasons  and it will be necessary to raise the muffle temperature toward the end of the operation or what amounts to the same thing, push the cupel to the hotter part of the muffle. Assayers who are used to bone-ash cupels, therefore, have some difficulty at first due to "freezing" of buttons when using magnesia cupels.

(2). Magnesia cupels retain a higher temperature longer than bone-ash cupels when withdrawn from the furnace or moved to the cool part of the muffle, and hence silver buttons show a lesser tendency to sprout, due to the slow cooling they undergo.

The lead in magnesia cupels seems to open somewhat more readily and cupels slightly faster than in bone-ash cupels. (p. 102 - 104)


Cupels. — A cupel is a porous cylinder or inverted-cone frustum of refractory material with a cupped depression in the upper end for retaining the lead button. In modern practice, cupels are made of bone ash, cement, bone-ash-cement mixtures, or magnesia. Magnesia cupels are purchased as a finished product, but the others are usually made at the assay office. (p. 46 - 47)


Bone-ash cupels absorb a weight of litharge about equal to their own, cement cupels absorb slightly less than their weight, and magnesia cupels absorb three-fourths of their weight. Magnesia cupels are denser than bone-ash or cement cupels, hence a magnesia cupel of a given volume absorbs as much litharge as a bone-ash cupel of the same volume. (p. 47)


Magnesia Cupels.   ...

Magnesia cupels have a higher heat capacity and thermal conductivity than bone-ash or cement cupels, and hence the heat of oxidation of the dwelling lead is abstracted more rapidly. The alloy is therefore maintained at a lower temperature than with bone-ash or cement cupels, but higher muffle temperatures must be maintained throughout the cupellation cycle. Largely on account of lower alloy temperature near the finish of cupellation the loss of silver by cupel absorption is greatly decreased and is usually less than half of the loss obtained with bone-ash cupels, under analogous cupellation conditions. The gold loss with magnesia cupels is the same as with bone-ash cupels. (p. 51)


The heat of oxidation of the lead causes the temperature of the button to rise considerably above that of the cupel and the muffle, in the case of bone-ash or cement cupels, but only slightly above the cupel temperature with magnesia cupels. Therefore with bone-ash or cement cupels the muffle temperature should be lowered during the driving period in order to keep the lead temperature from rising more than the minimum necessary for the reaction to proceed.


With magnesia cupels the temperature gradient from lead to cupel to furnace is not so great as with bone-ash or cement cupels, because of the greater heat capacity of magnesia compared with the other materials. The button will appear slightly hotter than the cupel, but the observable difference is by no means so great as with the bone-ash or cement cupels. Since the minimum lead temperature must be the same in all cases the muffle temperature with magnesia cupels must be higher during the driving period than with other types.

Muffle temperatures of 870 to 880°C, as measured ½" above the muffle floor behind the cupel, are recommended for the driving stage of cupellation in magnesia cupels, and temperatures as low as 830°C may often be successfully used during the period of most active driving. (p. 58 - 59)


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. (p. 66 - 67)


For example, the temperature of the cupeling alloy in a dense magnesia cupel does not rise so far above muffle temperature as it does in a bone-ash cupel, because the magnesia cupel is a better conductor of heat. This is the principal reason for good magnesia cupels giving lower losses than bone-ash cupels.       (p. 231)


Magnesia CupelsMagnesia was introduced a few years ago, as a substitute for bone-ash and a large number of brands of so-called "patent" cupels made with a magnesia base are now on the market. The magnesia (MgO) is produced by strongly calcining crude mineral magnesite (magnesium carbonate, MgO,CO2). The cupels require to be made under high pressure and should be baked at a high temperature before use. They are then very hard and firm and of a brown colour, resembling fireclay. Owing to the high pressure required (usually hydraulic), magnesia cupels cannot satisfactorily be made in the laboratory like bone-ash cupels. (p. 49)


Cupellation on Cupels of Material other than Bone-ash — Since, as stated on page 49, a large number of cupels made of materials other than bone-ash (mainly magnesia) are now supplied for assay purposes, it is well to point out that during cupellation there is a considerable difference in the behaviour of these so-called "patent" cupels as compared with bone-ash cupels, due to differences in the thermal properties of the materials used.1

The diffusivity of heat and the specific heat of magnesia cupels are greater than those of bone-ash cupels. If similar cupellations be conducted in bone-ash and in magnesia cupels side by side, a marked difference will be seen in the behaviour of the lead. The lead on the bone-ash cupel during the cupellation is very bright, whereas the lead on the magnesia cupel is comparatively dull during a considerable part of the operation and is not so hot, although the muffle temperature is the same for both. "This is due to the fact that the extra heat generated by the oxidation of the lead is diffused as soon as it is generated, owing to the superior diffusivity of the magnesia cupel and hence cannot serve to raise the temperature of the lead, as is the case in the bone-ash cupel. Hence for the same 'muffle temperature' the actual cupellation temperature of the lead in the magnesia cupels is 50° to 60º C lower than in the bone-ash cupels".

It has been pointed out on page 164 that the heat generated by the oxidation of the lead is sufficient to carry the cupellation to a finish with bone-ash cupels, provided the muffle temperature is not lowered at the end of the operation; but, for the reasons stated above, it is necessary in the case of magnesia cupels to employ a slightly higher temperature during the cupellation and to raise the temperature towards the end of the operation.

The difference between the temperatures of the lead during cupellation on magnesia and on bone-ash cupels, which can be noticed by observation, is much more marked at the beginning of the cupellation, and, in fact, is hardly discernible at the very end of the operation.

It will be noted also that magnesia cupels retain heat longer than bone-ash cupels, consequently silver beads take longer to solidify and to spit on magnesia than on bone-ash cupels, after being withdrawn from the same muffle temperature. Silver beads are also much less liable to spit on magnesia than on bone-ash cupels and the nature of the spit is different in the two cases, the spit in the former case generally taking the form of a frosty appearance only instead of the well-known "vegetation" obtained with bone-ash. These important differences in the properties of magnesia and bone-ash cupels are not always recognised by assayers.

An assayer, when asked to test magnesia cupels, usually puts half a dozen in the muffle with half a dozen bone-ash and cupels under conditions suitable for the bone-ash, with the result that he forms an unfavourable opinion of the magnesia cupels. When using magnesia cupels under these conditions the lead is very liable to "freeze" and the results are unsatisfactory; but if the proper conditions for magnesia cupels are employed, the results are quite as satisfactory as those obtained with bone-ash. Although a somewhat higher temperature is required for cupellation on magnesia cupels, the tendency is to have a very much higher temperature than is necessary, in order to make the cupelling lead look like that on bone-ash cupels; and this is a great mistake, whereby one of the most important advantages of the magnesia cupels is lost. The loss of silver due to absorption is usually less with magnesia than with bone-ash cupels (see p. 177).

Portland Cement Cupels — The behaviour of Portland cement cupels during cupellation is very similar to that of bone-ash cupels, although, as shown on page 177, the loss by absorption is slightly higher. When using cupels made entirely of Portland cement for gold assays, especial care must be taken to thoroughly clean the buttons, otherwise when subsequently parting in nitric acid insoluble silica will remain adhering to the cornet and be weighed as gold. This difficulty may be overcome by "facing" the cement cupel with bone-ash. In this case the cupel mould is filled about two-thirds full with cement and bone-ash added to fill the mould, the cupel then being finished .in the ordinary way.

Cement cupels are also made from mixtures of Portland cement and bone-ash, the proportions being usually equal parts of each. (pages 166-167)


At present there is little evidence as to the comparative variations in the absorption of silver and gold by bone-ash and magnesia cupels respectively, but it would appear from the data published that the absorption is less with magnesia cupels than with bone-ash. Whatever cupels are used, the absorption should be tested frequently. (p. 176)


With ordinary gold-bullion assays the absorption of gold is stated by S. Smith 1 to be about 0.5 parts in 1000 for bone-ash cupels and about 0.3 parts in 1000 in the case of Morganite cupels. It was found that this absorption difference for cupels of these materials was practically constant. (p. 178)


Bone ash:  Made from calcined sheep bones. Its low thermal conductivity allows lower furnace cupellation temperatures. The oxidation of lead generates heat. Temperature, at the lead surface, increases over the general furnace temperature.

Cement: Raw material is very cheap. When cement looses water of combination physical imperfections are produced & they can trap the doré.

Magnesia: Give more accurate silver assay values. More tolerant of the effects of cupelling dirty buttons. The type more commonly used in F.A. labs










bone ash


bot as

cendre d'os

cenere d'ossa

ceniza de hueso

костяная мука

cupel / cupels

Kupelle / (~)n


coupelle /coupelles

coppella / coppelle

copela /  copelas

капель / капели

cupel (to)

kupellieren /
treiben/ ab(~)

cupelleren /





cupellation /

Kupellation /

cupellatie /

coupellation /

coppellazione /

copelación /

купелирование / микро(~) /

капельный метод














окись магния, магнезия







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