Verdigris is little used as a pigment, but is important to the colour maker, because copper colours are made from it. In wine-growing countries it is made from the marc at small expense and without much labour. In commerce several kinds of verdigris are distinguished; they differ in physical and chemical properties. All verdigris consists of copper acetate, either alone or combined with varying quantities of copper hydroxide. =Blue Verdigris= is made in large quantities in France and is commonly known as French verdigris (_vert de gris naturel_). It has the following formula:— Cu(OH)(C₂H₃O₂).2½H₂O. The process adopted in southern France, especially in the neighbourhood of Montpellier, is as follows: copper plates are placed in layers in heaps of the freshly pressed grape residues, which always contain a certain quantity of must even when the most powerful presses are used. The process is conducted either in large heaps or in pots; in the first case care must be taken that the heaps are in a room whose temperature does not lie below 15° C. It is important to maintain this temperature, because the formation of acetic acid takes place with sufficient rapidity only at temperatures not below 12° to 15° C. Too high a rise of temperature in the heaps must also be avoided or a considerable quantity of acetic acid will be volatilised. The heaps should therefore not be made too large, otherwise the temperature cannot be kept within the proper limits. The residues contain a considerable quantity of sugar, which can be transformed by fermentation into alcohol. If air has free access, the alcohol is at once oxidised to acetic acid, which is recognised by the acid smell given off. In order to make possible the entry of air into the interior of the heaps, rectangular wooden bars are introduced in piling up the heaps. These are carefully withdrawn when the heap is finished, so that it is traversed by passages through which air can enter. By the simultaneous action of air and acetic acid copper acetate is formed, and since copper is always in excess, the acetate produced is a basic salt. In consequence of the chemical processes, the temperature of the heap rises to 35° to 40° C.; it is endeavoured to attain this temperature by artificial heat. It has already been stated that too great a rise of temperature is accompanied by a considerable evaporation of acetic acid. The process is then finished in a short time, but the loss of acetic acid is remarkable. When the temperature is so regulated that it varies between 25° and 30° C. the process takes a normal course—four to five days may be regarded as its duration.