Cupellation separates the gold and silver from the lead. This consists of an oxidizing fusion in a porous vessel called a cupel. The objective of cupellation is the conversion of Pb and other base metals such as Cu that are in the lead button into oxides. The oxides are separated from the Au, Ag and other precious metals because they do not oxidize under cupellation conditions.
One should check furnace temperature and inspect cupels for smoothness & foreign matter. Blow debris from cupel with low pressure air canister. Place empty cupels in furnace for 10 minutes in order to expel all the moisture and organic matter. The pounded lead buttons are placed into the cupel in the furnace and the lead is oxidized to PbO, which is absorbed into the cupel. If the cupels are thoroughly heated, the lead will melt at once and become covered with a dark scum. If the temperature is correct this will disappear within minutes. The lead will become bright once this has happened and it is said that the button has opened up or uncovered. This signifies that the lead has begun to oxidize. Close vent & door for 5 minutes to make sure that all the buttons have opened up. The door is opened slightly to admit a supply of air to promote oxidation of the lead. The air oxidizes the Pb to PbO, 98.5% soaks into the porous surface of the cupel and about 1.5% volatilizes. Continue to cupel for 35 – 40 minutes. If the temperature of the furnace becomes to low when the door is opened to allow the supply of air for the oxidation, the litharge will not be absorbed into the cupel and the lead will solidify. When this occurs, the button is said to have frozen. The assay should be rejected, as the results are usually low. One can try to bring them to the driving temperature with a popsicles stick, this sometimes works. The oxidation proceeds at a rate of about 1 g of lead per minute so 30 gram buttons should take about 30 minutes for all of the lead to be oxidized. The Au, Ag and the other precious metals do not oxidized and are left as a small bead in the bottom of the cupel.
The precious metals in the original sample have been concentrated into the 2 or 3 mg bead. If the fluxing, fusion and cupellation are done properly, the Au, Pt, Pd found in the bead will represent well over 99% of that in the sample. There are small losses to slag and the cupel but they are not usually significant. Silver losses are usually more significant and have to be taken into account when reporting Ag analysis based on fire assay work. Silver losses are determined by the cupellation of a known amount of Ag using the same mass as is expected in the sample. The weigh difference of the control silver before and after cupellation is added to the Ag of the sample as a correction.
There are losses in cupellation and the amount depends on factors such as the nature and shape of the cupel, the temperature of cupellation, the proportion of lead and silver, the amount of the impurities and the draft through the furnace. The losses of the precious metals are from sprouting, absorption of bouillon by the cupel, oxidation and absorption of gold and silver and volatilization of silver and gold. With good cupellation the losses of precious metals is due mainly to oxidation and absorption of the oxides. The small losses are due to volatilization. Silver is more readily oxidized than gold and the bead is higher percentage of silver. The most important single factor relative to cupel loss is the temperature. The cupellation should always be conducted at the lowest practicable temperature. There several things that can make the cupellation easier. After cupels are driving, open vent and furnace door to speed up the process. Do not overload Pb buttons directly from the moulds to the cupels. For large beads that are high in Ag, one should slowly remove from the furnace or cover the cupel with an empty hot one to prevent sprouting. Increased time is required when loading buttons into cold cupels.
Indications of Metals present: Zinc burns as brilliant greenish white flame and emits dense white fumes.
Tin oxidizes quickly and forms SnO2 which covers the lead with infusible yellow lava fragments and stops the cupellation.
Iron gives brown or black lava fragments, as does cobalt and manganese. Small amounts of iron oxide will stain the cupel dark red.
Nickel will deposit dark green lava fragments and stain the cupel green. Large amount of nickel will cause the button to freeze.
Copper is absorbed in the cupel and gives the cupel a stain that ranges from dirty green almost to black according to the amount present.