The grape residues are effective on account of the sugar, which is transformed into alcohol and then into acetic acid. Residues which have been extracted with water after pressing should not be used in the manufacture of verdigris; they contain but little sugar and hence undergo a very feeble acetic fermentation. When such residues are used the verdigris is often accompanied by black spots of copper sulphide upon the plates; this is due to incipient decomposition of the residues. As a practical test for the termination of the process a strip of copper is plunged into the heap and left for some hours. It should be covered by a uniform coating of verdigris; if it is covered with small drops that is a sign that the process is not completed and the heaps must be left some time longer. The course of the process can be simply and safely followed by means of a thermometer. This is placed in a perforated copper tube and plunged into the interior of the heap. A continuous rise of temperature indicates a steady increase of chemical action; the temperature of the room is regulated in accordance. When the temperature in the interior of the mass begins to fall the process is nearing the end. More heat is now supplied to assist the chemical action. The process is at an end when, in spite of external heat, the temperature of the heap decreases and approaches the temperature of the air of the room. Verdigris can also be made by placing sheet copper and grape residues in layers in pots, which are deposited in a room of fairly uniform temperature, such as a cellar. This method has the advantage that the formation of the verdigris is finished in a shorter time, which is more than counterbalanced by the labour and expense of filling the pots. The copper plates, when removed from the heaps or pots, are covered by a crust of thin needle-shaped crystals of a bright green colour. They are shaken to free them from the adhering grape skins and seeds, and then treated with weak vinegar, in which they are dipped; they are then left standing on edge for several days. This treatment with vinegar has the object of converting the neutral salt formed on the plates into basic salt. The dipping into vinegar and exposure to air are repeated six to eight times, when the originally pure green crust on the plates is gradually changed into bluish-green verdigris. These processes are continued until the plates are covered uniformly by a layer of verdigris about 5 centimetres thick. The crust is then scraped off by copper knives and stirred with water to a paste, which is pressed in leather bags in rectangular moulds. The lumps are then slowly dried in the air.