After cupelling the lead with all the base metal impurities, the resulting bead if there is one, contains the precious metals that were in the ore. The most common precious metals contained in the bead are gold and silver. In some areas the platinum group metals called (PGMs are also found in the bead. Most often this is the case when the assayer used one of the electronic methods of assaying and all of the metals are noted.
In the Oatman, Arizona area, the gold and silver may be equal in percent. In the Sierra Madre's where we mine for both gold and silver, silver is the higher percentage with gold usually measured in a few grams (1 gram = .03215 troy ounce). The base metals, copper, lead, and zinc have been dissolved in the lead and absorbed by the cupel cup.
The next step is to part the gold and silver. We weigh the bead on the assay balance. This weight is recorded. The bead is then flattened (and if large enough, rolled into a spiral called a cornet) and dropped into a warm nitric acid solution contained in a parting cup of white porcelain. The object here is to dissolve the silver into the nitric acid forming silver nitrate while the gold which is impervious to the nitric acid remains as black specks in the cup. To give one an idea of the sizes and weights involved, let's look at an example.
We have a bead left from the cupelling that weighs on the assay balance, 2 milligrams. This indicates (Remember the assay ton) two ounces of precious metals may occur in one ton of the ore. After parting, we find that we have a weight of gold of .5 milligrams. Subtracting .5 from the original weight of two milligrams leaves us with 1.5 troy ounces of silver and .5 troy ounce of gold. The 1/2 troy ounce of gold will make this ore very profitable to mine remembering the 60 to 1 ratio between the price of gold versus silver.
Of the many references I have in my library, there is not a lot of agreement on the strength of the nitric acid parting solution, with some saying 5 parts of water to one part of acid, and some as much as 9 parts of water to one part of acid. In fact, the stronger acid solutions work faster and may cause the bead to break into pieces as it dissolves in the solution, especially if the temperature of the solution is too high.
The right temperature and mixture is important and only found after much experimenting. I use a hot plate with silica sand as a medium to transfer the heat to the parting cup. As the silver reacts and begins to go into solution, the flattened silver bead slowly becomes spongy. The trick is to get the strength of the solution and the temperature which will keep the sponge together in a mass.
In our part of Mexico, most of the time, the entire bead dissolves indicating there is no gold in the ore. If there is gold, it will be found as a small black sponge, or if too much agitation has occurred, excessive heat or acid, tiny black specks will be rolling around in the bottom of the cup under any solution that has not boiled away.
After all action has stopped, usually less than ten minutes, the solution is siphoned or poured off and the remaking specks or sponge is washed very carefully with distilled water. There are several ways to handle the remaining sponge or specks. One assayer puts the cups on top of the still hot assay oven door to dry the remaining distilled water and brighten the gold. Another may use a small torch to carefully heat the cup and the remaining value. Either way requires great care as one is usually dealing with very small pieces. The heat turns the black specks or sponge if you are lucky, into bright yellow gleaming gold of high purity.
The last part of the assay is to weigh these specks or sponge on the assay balance. This is quite tricky and any carelessness on the assayer's part will result in a lost assay. Once the gold is on the assay scale, the weight is recorded and subtracted from the total weight of the original bead from the cupelling. The resulting difference is the weight of the silver in the bead.
In order to increase the size of the bead, it is common to perform a practice called in-quartation which adds a known weight of silver to the bead to give it more body. The fine details of the parting I leave to the reader to explore on their own should the above simple explanations increase one's desire to know more about this method.