cupellation on Cupels of Material other than Bone-ash-cs ceramic

Cupellation on Cupels of Material other than Bone-ash ¡ª 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¡ã 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cupels, consequently silver beads take longer to solidify and to spit on magnesia than on bone-ash cupels, after being withdrawn from the same muffle temperature.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 ¡ª 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

magnesium oxide and bone ash cupels	magnesium oxide and bone ash cupels	Magnesium sand cupel	Ceramic Couple
Plate cupel	Fire assay cupel	magnesia boat crucible	magnesium oxide crucible

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