The first step in softening the skins is to get them thoroughly moistened, and this is variously done, depending on the nature of the skin. Lambs, for example, require the gentlest means of wetting them, while rabbits can stand soaking in water for several days. The manner and duration of moistening must be adjusted to the character of the pelt. For the putrefactive processes which were stopped by stretching and drying the skins, continue as soon as the pelt is again moistened. The progress of decay causes the evolution of certain gases, the simplest of which is ammonia, and eventually, if permitted to proceed, brings about the complete disintegration of the skin tissue. It has been found that a certain amount of gas formation is necessary to loosen up the fibres in order to get the best quality of leather after tanning. This process must be interrupted at the proper time and not allowed to proceed too far. Skins which have been preserved fresh by salting, require only a comparatively short time (about 2 hours) to become softened by soaking in clean, soft water. Most dried skins need a longer treatment before they are sufficiently flexible. The addition of certain substances to the water facilitates and accelerates the softening. In some instances salt water is used for soaking the pelts, the preservative action of the salt tending to prevent any loosening of the hair. A solution of 1/4% borax is very effective in rendering the skins soft, and clean as well. Borax has an exceedingly mild alkaline action, and causes a slight swelling of the skin tissue, which then absorbs the water more readily. Being also preservative and antiseptic, borax tends to prevent decomposition of the skin tissue. Another chemical of a different nature, but equally effective is formic acid, used in the proportion of 1.5–2.5 parts per 1000 parts of water. Formic acid also induces a swelling of the skin, the pelts being soaked in a short time, and the antiseptic action of the acid obviates the possibility of the hair becoming loose. The water used should be fresh and clean, and the soaking must be stopped as soon as the skins have become soft and flexible. Sometimes the skins are allowed to soak overnight in water, while in other cases, the pelts are just moistened by dipping in water until thoroughly wet, and then laying them in a pile for several hours, or overnight. Another method which is practised with certain types of skins is the use of wet sawdust or of sawdust moistened with salt water. The fur skins are either embedded in the sawdust or drummed with it for several hours, or until sufficient moisture has been absorbed to render them flexible. By this means there is no danger of the skins being over-soaked, or of the hair being loosened. When the skins have been properly wetted, they are drawn with the flesh-side across the edge of a dull knife-blade, in order to help loosen the texture of the skin.