Whether amalgamating on plain copper plates or on those that have been silver plated it must be kept in mind that an amalgam retains metallic particles better than mercury alone, the practical application of which is that all amalgamated surfaces should be covered with a layer of amalgam of the right consistency to catch and hold the metals amalgamated. When the plates are cleaned or amalgam taken off, leave enough on to form a good surface for future work. If bare copper plates are used, the amount of amalgam left to cover the plate must be greater than when the plate is electroplated with silver, because copper is more readily oxidized and the oxidized compounds of copper form a scum on the surface of plates which prevent amalgamation.

An excellent rule to follow is to avoid the use of chemicals on plates. There are a few compounds, such as soap (mineral) and water, sal ammoniac and weak washes of soda, that might be of benefit and are harmless, but the majority of " dopes" used on plates are of temporary benefit only and when their effect wears off the plates are in worse condition than formerly. Most chemicals and acids, while not particularly affecting copper or mercury, may affect the constituents of the ore and with them forms compounds that are far from desirable.

Verdigris is a salt of copper which appears on copper plates when the covering of silver or amalgam is too thin to protect the plate from corrosion. It may be due partly to the effect of sulphates in the ore or to the corrosion of copper in the amalgam. The remedy is to clean off the coating with chamois leather, rub in amalgam frequently until the spot is thickly coated and let this amalgam remain on the plate, taking off the excess only, every day. If the verdigris still persists in coming to the surface, follow the procedure outlined above and in addition let water flow over the plate immediately after the plate is brushed to prevent the air from reaching the surface. If gold amalgam is not available prepare silver amalgam by dissolving silver in nitric acid and after evaporating to dryness add mercury and a few bright nails and stir to a smooth paste. If the amalgam is prepared from a copper coin when the coin is dissolved in nitric acid, evaporate and fuse to render the copper oxide insoluble, then dissolve the nitrate of silver in water and proceed as in the first instance.

The inclination of copper plates used for amalgamating should be such that the pulp flows gently over the surface in a thin layer. There must never be a bounding stream nor should the plates be exposed to the air. If the pulp is thick, the grade should be greater than for thin pulp, the extremes being 21/2 and 11/2 in. to the foot.