Table of Contents
Drilling Bullion Bar for Sampling
Bullion submitted for assay is usually brought in the form of bars, bricks, slugs or buttons, and a sample of bullion in this form is best obtained by drilling into the brick or slug with a small bit 3/16″ in diameter (Fig. 32). Drill enough so that the shavings or drillings will yield material for at least four assays of an original weight of from 300 to 300 milligrams. When extra care is observed in bullion assaying a certain amount of sample is taken (say 0.5 or 1.0 gram exactly weighed) and the check samples are all of the same weight as the original. The object of this is to make the check and original assays under exactly the same conditions. There is a slight silver loss through volatilization, and most technical works on assaying give tables showing the percentage of loss by volatilization. It is useless to go into this matter in a brief treatise intended merely for beginners and for others who prefer rapid methods with reasonable accuracy. In case the bullion is very base a larger quantity of the original sample should be taken so that after alloying, cupellation, and partition, the fine gold obtained will be sufficient to weigh.
Gold and Silver Fineness
The percentage of gold in bullion is described as so many thousandths “fine,” and is written decimally. Percentage of silver in bullion is expressed in the same manner, and is called silver fineness. A bar containing 900 parts of gold to the 1000 would be gold fine 900, and providing the other parts were silver, the latter would be expressed as silver fine 100.
Weighing Sample Drillings and Quartation
Knowing the bar to be of a fineness in excess of 600 (see page 72, touch stone and test needles), take drillings amounting to from 300 to 500 milligrams carefully weighed on a button scale (Fig. 23, page 38) and make a weight record on a card of the number of milligrams and fraction thereof expressed decimally. This card should accompany the assay throughout its various stages. Assuming the bar from which the sample was taken to be from 600 to 800 gold fine, add silver foil so that after cupellation there will be in the bead approximately three times as much silver as there is gold. Assuming a sample weighing 300 milligrams 700 gold fine, the amount of gold present would then be 700 x 300 or 210 milligrams, and the silver to be added would be 210 x 3 or 630 milligrams. The reason for alloying or adding this amount of silver is that after cupellation the bead of gold and silver may be rolled out into a thin strip.
In order to make a perfect separation of the gold and silver it is necessary that the strip (which, after being bent or coiled, is termed a “coronet”) should contain at least 2½ parts of silver to each part of gold. It would be difficult to part the silver from the coronet without breaking it up should it contain more than 3 parts of silver to each part of gold.
Cupellation
Having added the required quantity of pure silver to the sample as above described, tear off a strip of chemically pure lead foil about 7 inches long, and fold it end to end, making the sheet of lead double the original thickness and about square. Enclose the weighed sample and the added silver in this sheet of lead by carefully folding the lead over the sample; or form the double sheet of lead into a cornucopia, placing the sample and alloy in it, and then close the cornucopia tightly around the sample. The lead foil must be compactly folded around the sample in order that it may be easily handled with the cupel tongs, and of such shape that it will be contained in the cupel without overhanging the edges.
Bring the muffle to a bright red heat, and place the cupels to be used m the muffle at least ten minutes before using them so that they will be at the same temperature as the muffle. With the cupel tongs place the folded lead foil containing the drillings and silver alloy in the cupel, in the muffle; use great care while conveying it with the tongs to the cupel so that it does not fall onto the bottom of the muffle, as such a mishap would not only necessitate weighing and alloying another sample, but should the lead melt before it could be scraped out, it would make a weak place in the floor of the muffle which would soon give way and have to be replaced. Close the muffle with the plug and leave closed until the sample and lead are fused together and have become slightly molten. This may take five or six minutes, when the plug must be removed so that fresh air may be admitted for oxidizing purposes. The fumes of the volatilizing lead carry all the base metals contained in the bullion sample, and will be seen rising like smoke from the cupel.
The cupellation is finished when the gold and silver bead has “set,” which is shown by the fact that no more fumes come from the cupel, and a solid hemispherical bead remains. The distinction between a “set” and “frozen” bead is explained under the head of cupellation (Chapter II).
Rolling Coronets and Annealing
Remove the cupel containing the gold and silver bead from the muffle with the proper tongs. The bead may then be taken from the cupel with a pair of bullion bead pliers (Fig. 33) and must be thoroughly cleaned of any adhering bone ash with a stiff brush (see Fig. 22).
Roll (or hammer if the operator has no coronet roll, Fig. 34) into a thin strip or disc having less than half the thickness of a ten cent piece. This hammering must be done on a smooth faced steel block. After every few blows pick up the bead with a pair of forceps
and hold it in the flame of an alcohol lamp or Bunsen burner until the bullion is red hot (Fig. 35). This is called annealing, and expands the metal by opening the pores that have been closed by the hammering or rolling. This is necessary as otherwise the bullion would crack, and small pieces would chip off and be lost. When the bullion has been repeatedly rolled or hammered and annealed until sufficiently thin, it is then, after a final annealing, ready to “part.”
Parting
The trolled or hammered strip is then bent or coiled into a coronet so that the acid will have a chance to act on the maximum surface. The final annealing leaves the pores of the metal open to the action of the acid.
Place the coronet in an Erlenmeyer flask (Pig. 36) of 6 or 8 ounces capacity, and cover with ½ ounce of
chemically pure nitric acid and distilled water, about 5 parts water to 1 part acid. Put the flask on the sand bath (Fig. 37) and apply very gentle heat. The acid will soon begin to act, as will be shown by the small bubbles of hydrogen and by the blackening of the coronet. Should the action of the acid be too violent and the bullion immediately become black, add a little more distilled water, as when the operation of partition is performed too rapidly, there is danger of breaking up the coronet, which will leave the gold in fine particles or pieces which cannot be washed and transferred without loss. It is also desirable to have the gold coronet left in one piece as then there is less danger of loss in transferring the gold to the scale pan. Should the acid fail to act upon the coronet after a few minutes, add chemically pure nitric acid, a few drops at a time until action commences. Then when the action subsides add more nitric acid a few drops at a time. When this latter addition of acid causes no giving off of fine bubbles and the coronet is black, it is safe to assume that the silver has mostly been dissolved out of the coronet, leaving a sponge of gold. 
Now pour off the diluted acid into a receptacle for containing silver, cover the sponge-like coronet with strong chemically pure nitric acid, and boil until no more red fumes are given off. The partition is then complete. Pour the acid into the receptacle and wash the black sponge-like coronet three or four times with hot distilled water. This is to remove any trace of the silver nitrate that may be left in the pores of the coronet.
Transfer the coronet from the Erlenmeyer flasks to an annealing cup (Fig. 38) which is best done by holding the flask in one hand and the annealing cup (Fig. 39) in the other, submerging both hands in water, allowing both cup and flask to fill, and while both are under water inverting the flask into the annealing cup, when the coronet by gravity will slowly drop down into the cup. This prevents any dust from blowing away, and is the only safe way of transferring from the flask to the annealing cup should the gold be in a broken-up state. Remove the cup carefully from its submerged position, drain off all the water and carefully dry on the sand bath, applying the heat very gently until all the moisture is evaporated. When thoroughly dry brighten or anneal the gold coronet either by placing the annealing cup in the muffle and heating red hot, or by holding the cup in the flame of a Bunsen burner (Fig. 40) or alcohol lamp (Fig. 41) until the cup and gold are red hot. This operation restores the gold from the brown or black color to its natural golden hue. Cool to the temperature of the atmosphere, and when cool carefully weigh and note the weight on the card which contained the original weight of the sample.
Calculating the Gold Content of Bullion
The percentage of fine gold contained in the sample is found by dividing the net weight of the assay (the weight of pure gold) by the gross weight, i.e., the weight of the original sample of the drillings. Assuming the original weight to have been 300 milligrams, and the weight of the coronet after partition to be 210 milligrams, the gold fineness would be 210 ÷ 300 = 700 gold fine.
Determine the Silver Fineness or % of Silver in Gold Bullion
Weigh accurately from 300 to 500 milligrams of the drillings. Note the weight on the tag or card which bears the number of the assay. Do not alloy, but wrap the weighed sample in the lead foil and cupel exactly in the manner just described.
When the cupellation is properly finished all the base metals have been carried off by the volatilized lead, and the “set” bullion bead contains the gold and silver alone. Carefully clean the bullion bead and weigh when cold, noting the result. The weight of the gold and silver divided by the weight of the sample before cupellation will give the percentage of gold and silver together contained in the sample. Determine the gold fineness of this sample in the manner just described in the operation previous to this one. Subtract the gold fineness from the gold and silver fineness, and the remainder will represent the silver fineness, or the percentage of silver contained in the sample.
In the operation previous to this one, the gold value was found to be 700. 
If after wrapping the sample of 300 milligrams in lead without adding silver and cupelling, the weight of the button is 270 milligrams, we find by dividing 270 milligrams by 300 milligrams, the percentage of gold and silver together contained in the sample is 900. By subtracting the gold fineness, which we found to be 700, from the gold and silver fineness 900, we have 200, which is the silver fineness of the sample.
Should the bullion contain much base metal such as copper, for example, the button might freeze or solidify in the cupel before the copper had all been carried off by the volatilizing lead. This is remedied by rolling up a piece of lead foil (5 or 10 grams) and dropping it with the cupel tongs into the cupel containing the frozen button. When this is done close the muffle with the plug, and leave it until the frozen button and lead have fused together and have become brightly molten. Then remove the plug and continue the cupellation until the bead “sets.” The difference between a frozen button and a set bead is readily discerned. The frozen button has a peculiar coated appearance, while the bead which has perfectly set appears lustrous and clean when all the base metals have been carried off with the lead.