Leather

Bating.

Bating, practised almost exclusively upon skins, has for its objects the further removal of lime; an emulsification of the natural grease and some hydrolysis of the collagen fibres to obtain the degree of stretch demanded in such leathers as glove leathers, etc., or a smoother, more elastic and cleaner grain sur face in • the case of calfskins, and to make the finished leather softer and more pliable. There is still some uncertainty as to the way the skin is modified by bating but there is a removal or breaking down of elastin fibres, a further degree of deliming and a certain degree of hydrolysis of the collagen.

Pickling.

A further preparatory process commonly employed upon sheep, goat and calf skins is that known as pickling. This consists in processing the goods, most commonly by drumming or paddling, in a solution containing a mixture of sulphuric acid and common salt. Pickling brings the skins to a uniform degree of acidity before tanning ; thoroughly cleanses the goods ; and effects a further separation of the fibres to enhance softness.

Tannin or tannic acid is abundantly distributed in many dif ferent forms of plant life. Whereas the older type of tanner processed his skins exclusively with oak bark, the modern tan ner uses tannin-containing materials from various sources and of different types. Tannins were originally classified as belonging to the pyrogallol or catechol class, but this classification has been superseded by more scientific grouping either according to Freud enberg by the chemical groups of (A) hydrolyzable tannins and (B) condensed tannins; or to Perkin in 3 groups (r) gallo-tannins, (2) ellagi-tannins and (3) phloba-tannins or catechol-tannins. For the practical tanner the latter grouping is of most interest.

The following is a brief table of the most common tannins showing the classification of Perkin ; and classified according to their behaviour in the tanning process, as to whether they deposit "bloom"—a crystalline deposition of ellagic acid—which con duces to firmness in the finished leather but is objectionable when goods are to be subsequently dyed.

Synthetic Tannins.

There are a number of synthesised products manufactured from phenols and hydrocarbons, such as naphthalene, which are converted into leather forming materials by sulphonation with sulphuric acid and subsequent condensation with formaldehyde. These products are being used to a consider able extent as re-tanning agents and for bleaching dark col oured leather. These products are not synthetic tannins in the strictly chemical sense for their chemical constitutions are en tirely different from those of the natural tannins.

Vegetable Tanning.

Vegetable tanning may be conveniently dealt with in two distinct groups, (a) the tannage of heavy leather —sole, belting, harness etc. and (b) the tannage of light leather —shoe upper leather, fancy, bookbinding, etc.

Tannage of Sole Leather.—The world's production of sole leather is estimated to aggregate half a million tons per annum, of which the United States produces 200 million pounds, Great Britain 175 million pounds, and Germany 13o million pounds. In Great Britain, preliminary to the tannage for sole leather, the hides in the limed condition are subjected to "rounding"; this consists in cutting the hide into several portions—butt or bends, shoulder and bellies. In the United States, Australia and else where, the tannage is very often done in sides, the hide being di vided into two portions by cutting down the line of the backbone.

The tannage of sole leather usually consists of three group operations:—s. Colouring, by suspending the goods in a series of weak liquors of gradually increasing strength. 2. Laying the goods in strong liquors and transferring them to stronger liquors as the tannage proceeds ; the operation being called handling and the series of pits being described as the "handlers." 3. Placing the goods for comparatively long periods in a strong liquor after they have been dusted (in layer pits, called "dusters") with a small quantity of solid tanning material between each piece of leather. The suspender tannage is accomplished in a series of pits contain ing weak liquors that have been used upon previous goods.

The goods are usually moved daily into gradually increasing strength solutions and their position is also changed in the same pit once or twice daily. The length of time and the number of pits used varies in different tanneries.

The goods are moved from the strongest suspender pits to the "handlers" or "floaters," being handled daily by being drawn up on the side of the pit, allowed to drain and then re-transferred to the pit in a horizontal position, moving the goods to pits of grad ually increasing strength of tannin. "Handling" becomes less nec essary as the tannage proceeds. In the latest stages of the handler round it is customary in Great Britain to sprinkle the goods with a little solid material ; Mimosa bark, ground myrobalans or va lonia, as a preliminary to the "layers," or "dusters." At the end of the "handler" round the goods are usually almost completely tanned and are then transferred to the "layers" which contain very strong liquor obtained from the leaches, of previously unused tannin; or by the addition of chestnut or oak wood extract.