The lead buttons are placed into a cupel. Cupels are made of bone ash and cement or magnesium oxide. The cupels are loaded into the cupellation kiln at 1750°F.
The lead buttons are allowed to "open", that is to become molten. When the cupellation is "driving" or the lead is being absorbed into the cupel or driven off as lead oxide, the furnace is opened slightly to provide a draft. The lead is absorbed leaving behind a tiny doré bead containing the gold and silver.
The cupels are allowed to cool. The doré is then removed, brushed to remove any adhering cupel material and flattened using an anvil and bead hammer. At this point the doré is weighed.
The doré is then finished gravimetrically or by wet chemical digestion and analysis using Atomic Absorption Spectroscopy.
Parting and Annealing
If your bead appears to be relatively pure gold, no parting is necessary. If the presence of a significant amount of silver and/or PGMs is indicated, proceed as follows:
To part the silver from the gold, the button must contain at least 3 times as much silver as gold.
If it does not, add sufficient assay silver to make this ratio. This is called "inquartation.
If the analysis is to be finished gravimetrically the flattened doré is placed in a porcelain parting cup. Nitric acid (15% v/v) is added to the parting cup and heated on a hotplate. The nitric acid dissolves the silver leaving behind a gold sponge. The sponge is washed three times with de-ionized water and allowed to thoroughly dry.
Then proceed to "anneal" the button as follows:
Anneal by repeatedly hammering the bead flat and heating it in a flame until it's red hot. Repeat heating after every couple of blows with the hammer.
The annealed strip is then rolled into a loose coil or "coronet," placed in a parting cup and covered with a parting solution, a mixture of chemically pure nitric acid and distilled water, 6 parts water to I part acid, or a 6/1 ratio.
Heat the flask gently. The coronet will blacken as the hydrogen bubbles off. Do not allow it to boil too fast.
If the coronet becomes immediately black, add a little more distilled water to slow the action.
Finish Gravimetric Instrumental: GA-AAS
Silver correction factor (c.f.) = blank inquart wgt. (mg.) /blank doré wgt. (mg.)> Gold (oz/st) = gold (mg.) x (29.167/sample wgt.) Silver (oz/st) = (((doré (mg.) - gold (mg.)) x c.f.)- inquart wgt.) x (29.167/sample wgt.)