The recovery of gold from sludge or sediment which accumulates in the scratch-brush box is practiced in several places. Basically, this material contains a considerable quantity of gold and silver, removed by the brushes from articles which have stripped in parts owing to defective cleaning of the work. This waste, with other valuable refuse of a similar description, should be collected every few months, and after being dried, should be mixed with a little dried sodium carbonate and fused. The resulting button, being an alloy of gold, silver, copper, and may be treated as follows to separate the gold and silver: Remelt the alloy, and granulate it by pouring the molten metal into cold water. Place the grains in a flask and pour on them a mixture of 2 parts nitric acid to one part water, and apply moderate heat, when all but the gold will be dissolved, the latter remaining as a brown powder at the bottom of the flask.
The liquid must then be carefully poured into another vessel, and strips of clean copper immersed in it, which will cause any silver present to be thrown down in the metallic state. The gold and silver deposits must afterwards be well washed with warm water and after drying, be mixed with dried sodium carbonate, and fused as before. In fusing these fine deposits, after they have been intimately mixed with the dried alkali, which is to act as a flux, the mixture should be compressed as much as possible when placed in the crucible, or melting pot, by means of an iron pestle or other suitable tool, and the heat allowed to progress slowly at first and after a short time this may be increased until the contents of the, crucible assume a semi-fluid condition.
When in this state, the heat should be moderated, to allow the metal to gather, as it is termed, by which the molten globules will gradually subside and unite in the form of a liquid mass at the bottom of the pot. It is very important at this stage to keep the fused mass in as liquid a state as possible, taking care also not to apply too great heat, or the contents may rise up and overflow. Should this be likely to occur, a pinch of dried common salt may be thrown into the pot, which will cause the fused mass to subside. When the operation is complete, the pot is to be withdrawn from the fire and placed aside to cool; the pot is afterwards broken at its lower part, by a blow from a hammer, and the button extracted. This may then be plunged into a dilute sulphuric acid pickle to remove any flux that may attach to it.