When dry hides have become soft enough to bend without cracking, they are put into a machine and beaten and rolled, then soaked again until they are soft and pliable. It is very important that all the salt and dirt are removed during the process of soaking, as they injure the quality of the leather if they are not removed before the hides are unhaired. When the soaking process is completed, the lumps of fat and flesh that may have been left on by the butcher are removed by hand or by a machine, and the hides are then in condition to be passed along into the next process. The parts that cannot be made into leather, such as tails, teats, etc., are trimmed off before the hides are soaked. Large hides are cut into two pieces or halves, called “sides,” after they have been soaked.
For the purpose of taking the hair from the hides and skins, lime, sulphide of sodium, and red arsenic are used. Lime is sometimes used alone, but usually one of the other two chemicals is mixed with it. The lime is dissolved in hot water, a quantity of either sulphide of sodium or red arsenic is added to it, and the solution is then mixed with water in a vat, the hides being immersed in this liquor until the hair can be easily removed. The action of the unhairing liquor is to swell the hides, then to dissolve the perishable animal portion and loosen the hair so that it can be rubbed or pulled off.
There are several different processes of unhairing the hides. Each tanner uses the process that will help to give the leather the qualities that it should have, such as softness and pliability for shoe and glove leather, or firmness and solidity for sole and belting leather. This is one of the most important in the series of tannery processes, and if the hides are not unhaired properly and not prepared for tanning as they should be, the leather will not be right when it is tanned and finished.
There is also a process of unhairing, called “sweating,” which softens the hide and loosens the hair so that it can be scraped off. In this process the hides begin to decay before the hair is loose; it is therefore a dangerous process to use and must be carefully watched or the hides will be entirely spoiled. Sweating is never used for the finer, softer kinds of leather. It is applied chiefly to dry hides for sole, lace, and belt leather. It is an old-fashioned process and is not used as much nowadays as some years ago.