Plates which have been once used give a larger quantity of verdigris in a second operation, which is to be ascribed to the fact that they possess a larger surface than unused plates. New plates are made more susceptible to the further action of the acid by dipping in strong vinegar, by which they become covered with a layer of copper acetate. The verdigris obtained by this method consists of crystalline scales of a pale bluish-green colour, which produce a pale blue powder. In crude verdigris grape seeds are often found, and occasionally stalks and pieces of metallic copper. These admixtures are a consequence of the method of manufacture and cannot be regarded as adulterants. Gypsum is to be regarded as an adulterant. Verdigris often contains basic copper carbonate. Verdigris behaves in a peculiar manner with water. In contact with a small quantity it swells to a bluish-green mass, which becomes quite warm. Neutral copper acetate and the basic salts 2Cu(C₂H₃O₂)₂.Cu(OH)₂.5H₂O and Cu(C₂H₃O₂)₂.2CuO.1½H₂O are formed, the latter of which is insoluble. By the addition of a larger quantity of water the basic compounds are decomposed, neutral copper acetate dissolves, and a mixture remains of the compound Cu(C₂H₃O₂)₂.2CuO.1½H₂O and a brown basic acetate containing still less acetic acid. On account of this peculiar behaviour of verdigris towards water some care is required when it is used as a water colour. A very dilute solution is not green, but has an indefinite shade. =Distilled or Crystallised Verdigris= consists of neutral copper acetate, Cu(C₂H₃O₂)₂.H₂O. It can be made from blue verdigris by treating with the amount of acetic acid required to completely neutralise the copper oxide, or by decomposing copper sulphate with the acetate of a metal which forms an insoluble or difficultly soluble sulphate.