CUPELLATION IN CUPELS OF DIFFERENT MATERIAL-cs ceramic co.,ltd

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 amagnesia 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 cupellationtemperature 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 theuncovering 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.