Copper plates may be freed from gold by heating them over a fire and causing the quicksilver to evaporate slowly. The plates, after being cooled, are rubbed with dilute muriatic acid and covered with damp cloths for one night. They are then rubbed with a solution containing salt peter and sal ammoniac, and once more heated over some hot coals, but not allowed to get red hot. Soon the gold scale rises in blisters; the plates are then removed from the fire and scraped. Those parts of the plates that have not yielded up their gold must be re-treated and fired until they do so. All these scales of gold are then collected in a porcelain dish, the base metals are dissolved out with nitric acid, and the gold is then smelted. Corrosive sublimate should be placed in the crucible as long as any blue flame is seen to come from it. Some mill men prefer to amalgamate their copper plates with silver amalgam, claiming that silver-coated plates save a higher percentage of gold. To amalgamate in this way take some silver bullion, or silver coin, and dissolve in weak nitric acid, only just strong enough to act upon the silver. (If you use too much nitric acid you will waste mercury and make the amalgam harder than it should be for the best results.) After crystals have formed, quicksilver must be added, heating gently meanwhile, until a thick, pasty amalgam has formed. Let this new compound stand for some hours, and squeeze through chamois as usual. The proportion of silver may be about 1 ounce to the square foot of copper to be plated. In facing new copper plates with this amalgam, they should be washed first with dilute nitric acid; then in clear water; the ball of amalgam being rubbed over their surfaces, some little force being applied. Plates should not be used for 24 hours after coating. Porous copper plates of the best quality, and not too heavily rolled, should be used. Follow the amalgam with a swab, and rub the alloy well into the plate. Zinc amalgam (preferable when mine water containing sulphuric acid is used in the battery) is applied to the plate after it has been cleaned with a moderately dilute mixture of sulphuric acid and water. The zinc-quicksilver ball is rubbed in and applied while the plate is still wet. Zinc amalgam is prepared as follows: Cut zinc-sheet into small pieces; wash in weak sulphuric acid; and dissolve in mercury. When the quicksilver will take no more zinc, squeeze through chamois and rub in. Zinc-coated plates should stand a week before being used. Very weak sulphuric acid will always clean these plates of any scum that may form before they have received a gold coat.