Crystallised verdigris is very simply made from ordinary verdigris; the latter, whilst still moist, but containing only a small quantity of water, or the acetic acid would be too largely diluted, is brought into a pan and strong acetic acid poured over. Strong pyroligneous acid may be used, its empyreumatic odour is without influence on the quality of the product. The pan is heated until the contents almost, but not quite, boil. They are frequently stirred so that the particles at the bottom are brought in contact with the acid. A dark green solution is formed; when its colour no longer increases in depth it is allowed to stand until suspended solids sink to the bottom. The clear solution is then drawn off and rapidly evaporated in shallow copper pans. It is important that the liquid should not contain an appreciable quantity of free acid. When a crystalline skin begins to form on the surface of the solution it is drawn off into the crystallising vessels. These are made of glazed earthenware; wooden rods are placed in them, on which the crystallisation takes place. In order that large crystals shall be obtained, it is necessary to maintain a regular temperature in the room in which the crystallising vessels are placed. It is heated at the commencement, and the temperature is allowed to sink a little towards the end of the operation. From twelve to fourteen days are required to obtain large crystals. The mass of verdigris crystals adhering to each rod weighs 2·5 to 3 kilogrammes. The residual liquid is a saturated solution of verdigris, and is used in the next operation. It is only necessary to heat it with the residue from the first operation and some acetic acid in order to obtain a crystallisable solution of verdigris. The residue in the pan contains metallic copper, grape stems and seeds, and basic copper acetate. The copper is extracted by moistening with acetic acid and exposing to the air, when verdigris is formed, which is added to a later batch.