A Study of the Changes in Skins During Their Conversion into Leather

INTRODUCTION.

The use of hides both as skins and leather for protection against cold and rain, for weapons, or for ornaments, dates back to the remotest history of man.

While the hides were tanned in the earlier times with the hair on, methods were soon found to remove it and thus improve the product. The first substance used was probably wood ashes and this continued as the standard for some time. After tanneries were established, for up to this time the tanning was done only on a small scale, new substances were sought for, and lime, one of the oldest depilatory agents, was used. The method followed was to slack the lime in pits and soak the hides in a saturated solution of calcium hydroxide. This method although slightly modified has remained practically the same for centuries.

The tanning process was and is in general the following:

1. Hides are soaked to remove blood and dirt and to bring them back as nearly as possible to their original condition.

2. They are placed in pits containing milk of lime, bacteria being always present and sulphides being frequently added, for from 3 to 18 days until the hair slips easily, that is, can be easily removed.

3. They are then bated to remove lime and bring the skin into the desired physical condition. The bate may be either acid or bacterial.

4. The next step is the pickling process in which the skins are treated usually with salt and sulphuric acid.

5. Then follows the tanning process proper, which may be either a mineral or a vegetable tannage.

6. The last step is a finishing process.

The entire tanning process has thus far been outlined to show the dependence of the finished product upon the correct performance of each succeeding step of the process. It is only through tests on the finished leather that the effect of any alteration in any step of the process can be detected.

The liming process is the one studied in greatest detail in this paper but any changes due to this operation can be detected only in the finished product. Moreover, the method of soaking must, because of its influence, also be specified. The object of soaking hides is to cause them to resume as nearly as possible their original clean and pliable condition. This part of the process while not so important when green hides are used becomes a matter of great importance when dried hides are to be investigated. The length of soaking, number of changes of water and the acids or alkalies which may have been added, all have an effect on the final product. Should the hides be soaked too long or should the water not be renewed frequently enough, bacteria multiply and a part or, in extreme cases, all of the hide may be lost. On the other hand acids and alkalies cause swelling of the hide and if too much of either be added the hide will be plumped too much.¹ This, while not fatal to a good final product, has its disadvantages and as a rule causes trouble.

The object of liming is not alone the removal of the hair but also the loosening of the fiber bundles. More surface is thus exposed and hence the tanning agents are taken up more readily. In the vegetable tannage this makes a heavier leather. In the mineral tannage the loosening of the fiber bundles makes a more pliable leather.² The latter object is of great importance in the chrome tannage which is the method most used in manufacturing light or upper leathers. The hides after soaking are placed in pits in which an excess of calcium hydroxide is always present and to which some sodium or arsenic sulphide may or may not have been added. The hides are hauled, that is, taken out and the lime bettered once a day until the hair slips easily. It requires much practice and experience before one is able to tell exactly whether a hide is done or not, and the method is not only unsatisfactory but also erg unscientific, for the personal equation of the operator plays too important a part. The hides are then soaked in warm water, paddled and beamed. In the latter process the skin is placed over a piece of wood semi circular in cross section and the hair is removed with a blunt knife. The long hairs comes off very easily but the fine or ground hair and the pigment, especially in the case of black skins, cause some trouble. Part of the intercellular substance, corium, seud or gneiss and some lime soaps are also removed in this