The Principles of Leather Manufacture

by H. R. Procter

Fig. 41.—Wilson’s Double-bed Butt Roller.

In the West of England, much heavy leather is still manufactured from South American hides, which are tanned with a large proportion of valonia; and which consequently are heavily bloomed. No attempt is made to remove this bloom, which would too much lessen the weight and firmness, but the goods, after a light oiling to preserve the colour, are hung up and partially dried, and are then laid in pile to temper. The grain side is now wet with soap and water, with which a little oil is often mixed, and the bloom is “struck in” with the pin or machine; a somewhat blunt pin being used, or a blunt tool in the striking machine; which is held at such an angle as to smooth and compress the grain without taking too much hold on it. After a little further drying, the striking is generally repeated, the goods are washed over with water, and rolled “on.” They are now coloured with a mixture of pigment colour, generally containing a large proportion of whitening, or sometimes of French chalk coloured with ochres, chrome-yellow and orange, or whatever may suit the tint preferred by the tanner, or best imitate the colour of a clean-scoured tannage, and usually mixed with size and oil, or sometimes with oil and tan liquor. This mixture is well rubbed in, and smoothed over with a cloth, and then polished by brushing, when the goods are “rolled off,” rapidly dried, and again brushed. If the work has been well done, it is not easy to distinguish from clean scouring, and is much cheaper.

Fig. 42.—Brushing Machine.

A method intermediate between this and the first described, and which was formerly much used in London, was to proceed as above, but using more water and holding the pin in the first striking so as to scour out as much bloom as possible, and assisting this by the free use of water and the brush. Instead of using an opaque pigment-colour, the goods were generally coloured either between striking and the first rolling, or between the two rollings, with a transparent colour, such as dissolved annatto, or a mixture of aniline dyes, so as to conceal the traces of bloom, and to render slight damages to the grain less conspicuous.

The principles of the manufacture have been fully explained in previous sections, up to the time when the goods are taken into the tanning liquors. At this stage complicated reactions take place between the lime in the butts, the free vegetable acids in the liquors, and the tannins; and on the right adjustment of these three factors much of the success of the operation, and indeed of the whole manufacture depends. If the lime is in excess of the acids present, it forms insoluble compounds with the tannins in the surface of the hide. If these are protected from the air, they are generally redissolved as they advance into more acid liquors, but they readily become oxidised into dark-coloured matters, which can no longer be removed. Their presence in the finished leather is one of the great causes of darkening in drying. If the hide in the limy condition has been exposed either to the carbonic acid of the air, or to free carbonic acid, or acid calcium carbonate dissolved in water (“temporary” hardness, [p. 94]), a precipitate of calcium carbonate will be formed in the surface, which is much more difficult to remove than free lime, and which is perhaps the most common cause of the stains and discolorations which are so serious a source of loss to the sole-leather tanner. These stains may, if not too much oxidised, be removed by treatment of the tanned leather with weak warm sulphuric acid, but this remedy brings other evils in its train, and should not be required. The great remedy is to keep the goods from the time of unhairing till they go into the liquors, under water in which there is always a trace of caustic lime, or which at any rate are free from carbonic acid. In deliming sole-leather with acids, it is best to give the full dose of acid required, at once, and not gradually, so that it may act most powerfully on the exterior, and remove any carbonates present, before it penetrates to and becomes neutralised by the excess of lime in the interior. This is exactly the reverse of what is advisable with dressing leather, where the object of the tanner is to remove lime as uniformly and completely as possible, without excessive acidity of any part. Of course hides should not, even in the case of sole-leather, be allowed to go into the liquors while any acid swelling of the surface remains, but this will soon disappear if the goods are suspended for a time in cold water after deliming, unless excess of acid has been used (cp. [p. 153] et seq.).

If the proportion of free acid in the suspender liquors is as it ought to be, it is probably rather advantageous than otherwise for a little lime to remain in the interior of the hide, as it keeps the pelt in a plump condition during the first stages of colouring, quickens the penetration of the tannin, and lessens the tendency to “drawn” or wrinkled grain, which arises when the goods go into the liquors in a flat or fallen condition. The causes of drawn grain are often a little obscure. Of course that case needs no elucidation in which the hides are submitted to the tanning liquor in a creased or wrinkled condition, which is simply fixed and made permanent. This may arise, either from carelessness in handling the goods before taking into the suspenders, or from the way in which they are slung to the sticks, which often draws them into long wrinkles, afterwards difficult to remove. Drawn grain in general, however, arises from the grain-surface becoming tanned and fixed in area, while the substance of the hide is in a more extended condition than that which it assumes as tannage proceeds. Hides in a flat and unswollen condition are thinner, the fibres are slenderer and looser than when swollen, and consequently the hide has a larger area. If, after the grain is tanned, the substance of the hide becomes contracted in the liquor, either by swelling with acids, or by the direct action of the tannin on the interior fibres, the grain is certain to be shrivelled, like the skin of a dried apple. A similar effect, produced in a mechanical way, may always be noted where a hide has been coloured hanging grain-side out over a pole, so that the surface is extended at the bend, on which long wrinkles are formed as soon as it is straightened.

A hide in a slightly alkaline condition colours, and even tans more quickly than one which is acid. In presence of a trace of lime, and deficiency of free acid in the suspender liquor, tannages of valonia and bark give the butt a sort of lemon-yellow colour, which is not in itself injurious, and which disappears as the hides advance into more acid liquors, but which is a sign of danger, as showing that no excess of acid exists in the suspender-liquors. Gambier gives pelt perfectly free from lime a pale buff colour, but where lime is present, the colour is always reddish, and much darker, and this coloration does not disappear so readily as that with valonia, so that if gambier is to be used in the first liquors, care should be taken to remove all lime from the surface. The only known tannin which gives no insoluble compound with lime is that of the babool pod (sometimes called “gambia-pod”), which is frequently used in India as a bate, and which would probably prove very useful in colouring liquors ([pp. 165], [288]).

When sole-leather first goes into liquors, it is generally swollen with lime to some extent. If the liquors contain, as they usually do, sufficient free acid (acetic, lactic) in addition to the tannins, these combine with and neutralise the lime, and the pelt, without absolutely becoming flat and thin, loses its firmness, and becomes soft and spongy. This is a favourable condition for the absorption of tannin, but care should be taken not to allow the pelt to be squeezed or pressed, or water will be squeezed out, and the pelt will not easily resume its plumpness. As the tannage proceeds, both the tannin and the acid of the liquors penetrate deeper into the pelt, the former tending to contract and the latter to swell the fibres. Thus a given quantity of acid will cause the greater swelling, the less tannin is present; and therefore in strong tanning liquors more acid is required. The presence of certain products of bacterial putrefaction has a great but unexplained effect in preventing hide from swelling with acids; and in hot weather, much better swelling is obtained by sterilising and deliming the hides with one of the coal-tar products mentioned on pp. 30, 162. Boric acid may also be satisfactorily used for this purpose, but should not be allowed to get into sole-leather liquors, as it tends to produce a soft and loose tannage, and from its inorganic and indestructible character, is apt to accumulate in a yard in which it is used. The same reasons render unadvisable its introduction into any liquors which are to be returned to the leaches even in the tannage of dressing leather, though its presence in the colouring liquors is otherwise very useful in lessening the astringency of the tannins (“mellowing the liquors”), and making a fine grain. Its mode of action is by no means clearly explained, but is in some way connected with its tendency to produce “conjugated acids” (L.I.L.B., pp. 37, 46).

The so-called “mellowness” of old liquors requires a word of comment. It is well known to practical tanners, that old liquors are much less liable to produce drawn grain, and a harsh surface, when used to colour green goods, than liquors, even equally weak, which have been made from fresh materials. This is probably due, in part at least, to more than one cause. Most natural tanning materials contain tanning matters of varied degrees of astringency and power of attaching themselves to the leather-fibre. It is obvious that if a tanning liquor is used, the most astringent and energetic tannins will be first removed from it, leaving those of a milder character. It is also known that the presence of neutral alkaline salts of weak acids has considerable influence in producing mellowness; the addition, for instance, of sodium acetate has a marked effect. This effect is probably due in the first place to the action of neutral salts in diminishing the energy of weak acids (see [p. 81]), and secondly to the fact, that their bases combine to some extent with the tannins; and that, as was perhaps first pointed out by the writer, such tannins are, as it were, partially paralysed in their action on hide ([p. 339]). Sodium sulphite acts powerfully in this way, and may perhaps prove of technical value in temporarily diminishing the astringency of liquors in quick tannage. Borax has a similar effect, but is too alkaline, and, unless used with extreme caution, spoils the colour of the liquors by causing oxidation. It is probable that similar causes explain the mellowness of palmetto extract, which contains large quantities of alkaline salts, and of some extracts which have been treated with sulphites, when used undiluted in drum tannage. The addition of free acid will generally restore these tannins to an active condition.