The solutions obtained from wet-coloring processes contain gold in variable proportions and its recovery is practiced in several places. Basically, in wet-coloring, the waste water in which the work has been rinsed, as well as the exhausted coloring-salts are collected in special vessels for the purpose, as they all contain gold; and without such precautions, where a large manufacturing trade is being done, the loss in the course of a year would be very great. Many metallurgists have noted that all the gold can be recovered from these waste waters by a very simple and at the same time effective process, entailing little or no expense.
A basic procedure requires the preparation of a solution made of ferrous sulphate, typically the solution contains 15% of the salt and the other part is water. Mix together, and when the salt is all dissolved add it to the solution in the vessel, and stir it well; the gold will then begin to precipitate; this must be repeated each time after coloring, and as the vessel becomes full a little more of ferrous sulphate must be added, and the contents well stirred. If this produces no effect upon the solution, the gold has all been precipitated. It should then be allowed to settle, when the supernatant water may be decanted or poured away; but care should be taken not to disturb the precipitate, which consists of a dark spongy mass at the bottom. The sediment must then be well washed several times with hot water to free it from the acid; thoroughly dried, and afterwards melted with a quantity of dry sodium carbonate and sodium chloride, with a proportionate part of silica in a crucible.
In order to prepare the flux, reduce all the ingredients to a fine powder, and well mix them together; great heat will be required in the fusion of the mixture, in order to effect the complete reduction of the gold; to assist which, a small portion of potassium nitrate may be occasionally added to the contents in the pot; but this must be done with care. More silica may be used in the reduction of the above mixture, if preferred, to refine it more thoroughly, as either will draw the iron or other impurities into the flux, leaving the button of gold at the bottom of the crucible.