The dried lees are carbonised in iron tubes, protected from the fire by a thin coating of clay mixed with chopped hair so that the coat more readily adheres to the iron. Old stove pipes or cast-iron gas or water pipes may be used. The tubes are about one metre long; they are closed with well-fitting covers, in one of which is a small opening through which the gases produced on heating can escape. These covers are fastened on air-tight with clay. The process begins by affixing the unperforated lid, and packing with a wooden rammer the dry lees as tightly as possible into the tube; the second lid is then luted in place. The tubes are placed near one another in a suitable furnace and first slowly heated at the back, _i.e._, the end closed by the unperforated cover. At the commencement of the operation heat must be gradually applied. With too large a fire the products of dry distillation might be evolved in sufficient quantity to force the covers from the tube; air would then enter and the contents be burnt. When the hinder part of the tube is red-hot, the heating is conducted forward, and finally the whole length of the tube is brought to a good red heat. The termination of the operation is shown by the disappearance of the pointed flame of the products of distillation which protrudes from the opening in the cover of the tube. When this occurs, the fires are drawn; the tubes are left to cool in the furnace until they can be taken out by the hands protected by wet cloths. The covers are at once taken off and the hot contents emptied into a large tub filled with water, which is thus soon raised almost to boiling point. The time required for washing is thus considerably shortened. The charcoal falls from the tubes in lumps, which soon fall to a fine powder in the water. Wine lees consist of a mixture of yeast cells and small crystals of tartar (a mixture of the tartrates of potassium and calcium). On ignition these salts are converted into carbonates, by which the particles of carbon are united. Potassium carbonate is very readily soluble in water; in contact with the warm water it dissolves in a very short time, the carbon particles then fall apart. When the charcoal has settled to the bottom of the vessel the liquid is drawn off. It can be utilised in a colour works, since it is a fairly strong solution of potassium carbonate. The charcoal is mixed with calcium carbonate, arising from the decomposition of the calcium tartrate, and with the other insoluble salts which yeast ashes contain in some quantity. These salts would prevent the proper grinding of the charcoal; they are therefore removed by treatment with hydrochloric acid after the water has been drawn off as completely as possible. A small quantity of hydrochloric acid, diluted with an equal volume of water, is poured over the charcoal, an effervescence of carbonic acid follows, the easily soluble calcium chloride is formed and the remaining salts are also dissolved.