How to Refine Gold Dust
Gold dust is produced in several operation of the jewelry process and this material may be collected, melted and refined. It is well known that refine gold is not a simple task and must be practiced by skilled people. The gold dust must first be collected and melted to produce a gold alloy. In order to separate the alloy from the gold, the composition will have to be submitted to the action of nitric acid; but as the gold alloy consists probably of some impurities, the nitric acid cannot effectually do its work with efficiency. If this is the case, therefore it will be necessary to re-melt the bar of gold dust, and to add to it, if of 9-Karat quality, one-half of its weight of silver, a little potassium nitrate or charcoal being used as a flux. When the mixture is properly incorporated, to effect which it should be well stirred, it is poured from some height into a deep vessel of water, being briskly stirred in a circular direction during the operation.
The granulation of the gold can be performed skillfully, by the operator standing upon a stool and carefully pouring the molten metal into a vessel beneath. The gold alloy will then be found at the bottom of the water in small grains, which must be carefully collected in order to avoid waste. However, a little is almost sure to happen, even with the greatest care. These grains are then taken and placed in a suitable flask, which should be quite free from lead, and treated with nitric acid. It is common to use a solution of nitric acid (one part of acid and two parts of water).
The solution must stand for some hours. At the end of the operation, it will be necessary to raise the temperature in order to promote the chemical action so that all the components of the mixture, except the gold will become dissolved, and the latter will be in the form of a dark brown powder at the bottom of the glass. In order to remove all the base metals, pour the solution of nitric acid, containing the silver, copper, and other metals into another vessel, for subsequent treatment. Fresh nitric acid should now be added to the gold and heat applied, in order to remove any percentage of base alloy, which would be the means of making the gold brittle. When the acid is found to have no action of any kind upon the gold, it must be again poured off, and the gold well washed with hot water, to remove any trace of alloy that may be discovered in its interstices.
At this stage the gold is pure, or at all events sufficiently pure for every practical purpose, requiring now only to be melted with borax or potassium nitrate. Unless every particle of alloy is removed from the gold, it is very likely to be brittle when it comes to be worked up again; for that reason, if for no other, it is better to entrust it to the practical refiner, for, it never pays the working jeweler to do it himself. Even refiners best gold from the gold dust bar never works up like the refined native gold. Gold collected from the other waste material of jewelers may be treated in this manner if preferred and is important to know the chemical composition of the material in order to select the appropriate addition of the chemical compounds.