The general procedure is as follows: The dye substances to be used are ground up to a powder in a mill constructed for the purpose, after which they are boiled with water in a copper-lined kettle or cauldron, heated from the outside by steam. The customary arrangement is to have a jacketed kettle, supported on a stand, and having taps and valves to enable the liquor to be drawn off, or pivoted, so that the kettle can be tilted, and the contents poured out. The use of the copper-lined vessel is to be preferred, as it is unaffected by any of the dye substances, and so cannot cause any rust stains. After the dyes have gone into solution and have cooled, the mordant chemicals, previously dissolved in water, are added, and the mixture stirred up. The dyeing in this instance is effected by the simultaneous application of dye and mordant. The dye mixture is now run off, or poured out in the proper quantity into a number of small vats of 25–30 gallon capacity, or into a paddle vat, which can be closed, while the paddle is rotating. The latter device is to be preferred because it permits the dye to retain its temperature better and for a longer period of time, but when lambs are being dyed only the open vats are used. The temperature of the dye mixture is between 40° and 45° C., for only at this temperature can the hair absorb the dye properly without injuring the leather. The killed skins are immersed in the dyebath for a time, usually overnight, after which they are removed, drained and hung up, with the hair-side exposed to the air, so as to permit the dye to develop, which takes place with the aid of the atmospheric oxygen. The dyebath is again brought to the proper temperature, and the skins are again entered, to go through the same process as often as is necessary to obtain the desired depth of shade. The dyed skins are thoroughly washed to remove excess dye, then dried and finished. The following are a few dye formulas used in the production of blacks: Logwood extract 100 grams Chestnut extract 14 c.c. Turmeric 38 grams Iron acetate 6° Bé. 50 c.c. Water 1200 c.c. or, Cutch 15 grams Soda 14 grams Logwood extract 120 grams Verdigris 19 grams Iron acetate 5° Bé. 16 c.c. Water 1200 c.c. A recently published formula for dyeing China goat skins black, is the following: