| Antimonite | 8 | 8 | 8 | 8 |
| Quartz | — | 12 | — | 12 |
| Nitre | — | — | 4 | 4 |
| Lead got | 29.0 | 26.0 | 13.0 | 13.0 |
| Galena | 10 | 10 | 10 | 10 | 15 | 20 |
| Quartz. | — | 15 | — | 15 | 5 | — |
| Nitre | — | — | 3 | 3 | 3.5 | 7 |
| Lead got | 17.0 | 19.0 | 8.0 | 8.0 | 18.5 | 18.5 |
A similar set of experiments, with 80 grams of litharge instead of 80 grams of red lead, gave:—
| Pyrites | 4 | 4 | 4 | 4 | 7 | 10 |
| Quartz | — | 15 | — | 15 | 13 | 10 |
| Nitre | — | — | 5 | 5 | 12.5 | 20 |
| Lead got | 46.5 | 40.5 | 25.5 | 24.5 | 27.0 | 26.5 |
| Copper Pyrites | 5 | 5 | 5 | 5 | ||
| Quartz | — | 15 | — | 15 | ||
| Nitre | — | — | 5 | 5 | ||
| Lead got | 44.5 | 32.5 | 23.0 | 25.0 | ||
| Blende | 5 | 5 | 5 | 5 | 10 | |
| Quartz | — | 15 | — | 15 | 10 | |
| Nitre | — | — | 5 | 5 | 15 | |
| Lead got | 41.5 | 38.5 | 21.5 | 22.5 | 21.6 | |
| Antimonite | 5 | 5 | 5 | 5 | 10 | |
| Quartz | — | 15 | — | 15 | 10 | |
| Nitre | — | — | 5 | 5 | 10 | |
| Lead got | 31.0 | 32.5 | 11.5 | 12.5 | 18.7 | |
| Galena | 10 | 10 | 10 | 10 | 15 | 20 |
| Quartz | — | 15 | — | 15 | 5 | — |
| Nitre | — | — | 5 | 5 | 7.5 | 11 |
| Lead got | 33.5 | 33.5 | 13.0 | 14.0 | 19.5 | 22.7 |
The variation in some of these experiments, in which we might have expected similar results, is due to the fact that the sulphur, and in some cases the metals, are capable of two degrees of oxidation. For example: theoretically 1 gram of iron pyrites (FeS2) would yield 8.6 grams of lead if the sulphur were oxidised to sulphurous oxide (SO2), and the iron to ferrous oxide (FeO); whilst if the sulphur were oxidised to sulphate (SO3), and the iron to ferric oxide, 12.9 grams of lead will be thrown down. Similarly the yield with copper pyrites would be 7.5 or 11.6; with blende, 6.4 or 8.5; with antimonite, 5.5 or 8; and with galena, 2.6 or 3.4. As regards the metals, the lower oxide will always be formed if the assay is carried out properly (fused under a cover, and with a sufficiency of reducing agent). But the proportion of sulphur oxidised completely will vary with the conditions of the assay. With a slag containing much soda the tendency will be to form sulphate, and, in consequence, a big reduction of lead; whilst with an acid slag containing much quartz the tendency will be for the sulphur to go off as sulphurous oxide (SO2). In a fusion with litharge alone all the sulphur will be liberated as the lower oxide, whilst with much soda it will be wholly converted into sulphate. For example: 3 grams of an ore containing a good deal of pyrites and a little galena, gave, when fused with litharge, 16.5 grams of lead. A similar charge, containing in addition 20.0 grams of soda, gave 22.5 grams of lead.
It will be noted from the experiments that 1 gram of nitre kept up on the average 4 grams of lead; the range being from 3.2 with acid slags to 5.3 with very basic ones. These facts serve to explain some apparently irregular results got in practice.
CUPELLATION.
The process is as follows:—The cupels, which should have been made some time before and stored in a dry place, are first cleaned by gentle rubbing with the finger and blowing off the loose dust; and then placed in a hot muffle and heated to redness for from 5 to 10 minutes before the alloy to be cupelled is placed on them. The reasons for this are sufficiently obvious: the sudden evolution of much steam will blow a cupel to pieces; and, if the whole of the water has not been removed before the cupel is filled with molten lead, the escaping steam will bubble through, and scatter about particles of the metal. If some particles of unburnt carbon remain in the bone ash, a similar result will be produced by the escape of bubbles of carbonic acid as soon as the fused litharge comes in contact with them. The cupels having been prepared are arranged in a definite order in the muffle, and the assay buttons are arranged in a corresponding order on some suitable tray (cupel tray, fig. 41); the heat of the muffle being at bright redness. Then with the help of the tongs (fig. 42) the assay buttons should be placed each in its proper cupel; a note having been previously made of the position it is to occupy, and the door of the muffle closed.
