Leather: From the Raw Material to the Finished Product

by K. J. Adcock

If the fleshing is not done directly after dehairing, the hides or skins should be placed in a weak lime liquor, and this method is to be recommended. The great point to be observed is to keep the limed hides, both before and after dehairing, away from contact with the air, as the chemical action of the carbonic acid on lime results in the formation of chalk, which tends to harden the hides and to roughen the grain, so that it is likely to be scratched in later operations. Where the hides are intended for sole leather, and not treated with an acid before entering the tanning liquors, the presence of chalk would lead to an uneven colouring of the leather.

Fig. 19

FLESHING KNIFE

Fleshing by hand demands great skill. The knife used (Fig. [19]) is similar to the unhairing knife, except that it has two edges. The cutting is done with the convex edge, which has to be kept very sharp. The concave edge need not be sharp, its use being limited to scraping off loose particles of flesh, while the parts not removed by this means are cut off with the sharp edge. The knife is held slantingly, with the blade almost parallel with the beam. The strokes should be short and in a semicircular direction, otherwise it is difficult to avoid cutting the skins. This manual operation is now largely superseded by machinery, and will, no doubt, soon be obsolete. The early types of fleshing machines were not a success, but the modern machine is very effective. There are several makes on the market, but in most cases the working tool is of similar construction and consists of a long cylinder to which spiral knives are fixed. (Fig. [18].) Half of these blades converge to the left and half to the right, the object being not only to cut away the flesh but also to stretch the hides outward, thus ensuring an evenly cut surface. The fleshings and useless pieces of skin are kept in a weak lime liquor until there is a sufficient quantity to send to the glue maker; although in some of the larger tanneries this offal, technically termed "spetches," or glue pieces, is converted into glue on the premises. In hot weather, a large accumulation of fleshings is liable to putrefaction, despite the use of plenty of lime water. While lime certainly arrests putrefaction of gelatinous matter for a time, decomposition afterwards sets in and serious damage may be done. To avoid this, it has been a common practice in Germany to use formaldehyde, but, while this acts as a preservative, it hardens animal tissues, and has a tanning effect, with the result that the pieces are rendered insoluble and cannot, of course, be reduced to a liquid gelatine by boiling. Glue makers have condemned the use of formaldehyde, but other preservatives, such as "lysol" (a cresylic compound) and "arasol," have no tanning effect and may be used with safety.

After the operation of fleshing, it is necessary to get rid of the lime in the hides, for, if they were put directly into tan liquors, the lime, being alkaline, would neutralise the acidity of the tan liquors and retard the beginning of the tanning process for a long period. The leather would ultimately be poor, thin, stained, and brittle.

Up to the process of deliming, there is not a great deal of difference in preparing the hides and skins for the large variety of leathers, but between the fleshing and the tanning processes the work varies considerably, and largely determines the character of the finished leather. When the hides or skins are in the limed state, they are gristly and firm in texture. A certain amount of this firmness is desirable in some leathers, such as sole and belting, and, therefore, it is deemed advisable to leave a small quantity of lime in the hides, although, to get them evenly coloured in tanning, it is essential that the lime should be completely removed from the surface of the hide. An old method, which is even in use to-day in some tanneries, is to wash the fleshed hides in a cubical or hexagonal drum for several hours in running water, which is conveyed through an iron pipe in the journal of the drum, and escapes through small holes in the drum. The effect of using a hard water for washing out the lime is shown in the interior of these washing drums, the sides of which become incrustrated with a thick, hard deposit. The surface of this incrustration is irregular, and small projections are sometimes formed which mark the hides and reduce their value, as the impressions are not removed in later processes.

The modern method of deliming hides intended for sole leather is to use a weak solution of acid, or an acid salt. For this purpose, sulphuric and hydrochloric acids are by far the cheapest, but require handling with great care, as any excess used has a corrosive and destructive effect on the fibres, which results in weak leather. Mild organic acids are much safer, and those generally preferred are lactic, formic, and butyric, although boracic and acetic acids are favoured by some tanners. Butyrate of ammonia is used for the same purpose by a number of French tanners. The same acid bath can be used for a second lot of hides, but sufficient acid should be added to raise the acidity of the liquor to its former standard. It is inadvisable to use one bath more than four or five times. The salt formed during the process by the combination of the lime with the acid—calcium lactate, formate, butyrate, acetate, or borate, according to the acid used—must be washed from the hides, either in a pit or drum, before they are ready for the tan liquors.