The skins are again thoroughly washed, and then hydro-extracted. KILLING WITH LIME Lime, calcium oxide, forms a white, amorphous, porous substance, which readily takes up water, giving calcium hydroxide, or slaked lime. Only the best grades of lime should be used, as it is very frequently contaminated with calcium carbonate and other inert materials. 10 grams of lime are dissolved in 1 liter of water. The skins are entered, and allowed to remain for a period of time which varies according to the nature of the fur. During the killing, the solution must be agitated, in order to evenly distribute the milk of lime, which has a tendency to settle out. After rinsing, the skins are “soured,” by treating with weak acetic acid solution, then thoroughly washed, and drained. KILLING WITH CAUSTIC SODA Caustic soda is used only on furs the hair of which is very hard and resistant to killing. Usually it is applied by the brush process, but in some instances, the dip method must be used. In order to reduce as far as possible, the action of the caustic soda on the leather, the weakest permissible solutions are used, increasing the time of treatment, if necessary. Caustic soda is a white, crystalline substance, occurring in commerce in lumps, but more conveniently in a solution of 40 degrees Beaumé, containing 35% of caustic soda. Various quantities, ranging from 4 to 25 grams of this solution per liter of water are taken, according to the character of the fur, and the skins treated for 2–3 hours, although weaker solutions may be used, and increasing the duration of the killing. By keeping the solution in motion, by means of a stirrer or any other method of agitation, the best results are obtained. After the skins are sufficiently killed, they are soured, and washed as by the other killing methods. Where the nature of the hair of the fur is such that the top-hair and the under-hair require different killing treatments, the skins are first killed by the dip process, with an alkali suited to kill the under-hair, then a brush killing with a stronger alkali is applied to the top-hair. The subsequent treatments are the same as for usual dip-killing methods. CHAPTER XI FUR DYEING MORDANTS