Leather: From the Raw Material to the Finished Product

by K. J. Adcock

Other useful artificial products are: "Erodin," and "Puerine," a patented American product which consists of a weak organic acid and a small proportion of molasses. Possibly, malt enzymes or diastase could be utilised for the manufacture of an artificial bate or puer, although they would not be so effective as animal products.

Where hides and skins have been treated with Oropon, they may be transferred to the tan liquors after being rinsed in water, although it is better to submit them to the operation of scudding.

There are two tests to determine the end of the process of bating and puering, although the extent to which the reduction is carried depends on the kind of leather wanted. For this reason, it is not advisable to remove every trace of lime from hides which must possess a certain degree of firmness when finished into leather. Of the two tests, one is chemical and the other mechanical. In the former case, a cross section is made in the thickest part of the hide and a few drops of phenolphthalein are added to the cross section; if the whole of the lime has been removed, no coloration is given, but, if lime be present, a purplish colour is given, which varies in intensity according to the contents of lime in the hide. The other test is made by pressure with the thumb nail, and, if the impression be permanent, the hides or skins are in a sufficiently reduced, or, as it is technically known, "fallen" condition for all practical purposes. The latter test is really only useful when the process has to be carried to its fullest practical extent, as it affords no idea of the intermediate stage. The experienced workman can judge the progress made by appearance and touch.

In cases where skins have been puered with excrement it is often necessary, after scudding them, to submit them to a further process before tanning. This is known as "drenching," and consists in treating the skins in an infusion of wheaten bran or pea flour. The acid fermentation produced by these ingredients effectually cleanses the skins by neutralising the last traces of lime and scud, and prepares them in an ideal condition for the process of tanning. The combined processes of bating with Oropon and drenching in bran are extremely useful for skins to be dressed into glove leather.

Following the processes of bating, puering, or drenching, the skins are washed in water to remove all mechanical impurities, and are then in a perfectly clean condition for tanning.

CHAPTER VI
TANNING PROCESSES

The methods of tanning may be classified as follow: (1) Vegetable tanning processes; (2) methods of chroming; (3) tawing processes; (4) oil tannages; (5) formaldehyde tannages; (6) sundry tannages, chiefly mineral; and (7) combination tannages. The public is chiefly familiar with types of vegetable tannage in boot sole, bag, and portmanteau leathers; of chrome tannage, in box calf and glazed kid used for boot uppers; of the tawing process, in kid glove leather; and of the oil tannage in wash leather or "chamois."

Vegetable Tanning Processes

The methods of tanning with infusions of barks, leaves, and fruits of trees and plants containing tannin are much more numerous than they were a decade ago, and tanners have now to pay special attention to the selection and blending of the materials they use in order to produce the various qualities of leather required. Formerly, most of the sole leather was tanned in oak bark liquors, and, in the later stages of the process, valonia and gambier were added to quicken the process and give solidity to the leather. This tannage, which may be described as an oak bark tannage, is still used by a few tanners, but the wide choice of materials available has brought other tanning agents into prominence, chiefly because they possess more astringent properties, and, therefore, tan more quickly than oak bark. Moreover, the need of other materials than oak bark became a necessity several years ago, as the supply of the latter would be inadequate to produce the large quantity of leather now required, even allowing for the rapid development of mineral tannages. Further, it is amply proved that a good blend of materials (mixed tannage) produces quite as durable and a firmer leather than the old oak bark tannage; unfortunately, there are other blends, occasionally combined with extraneous weighting materials, which account for the poor quality of a great deal of modern sole leather.