Further disadvantages arise from the fact that the chrome tannage is an "empty" tannage. Compared with the vegetable tannage, very little of the tanning agent is adsorbed. Hence there is little matter of any kind between the hide fibres isolated during tannage. The inevitable effect of this is that the leather has not the same solidity and firmness, and needs filling out with other materials. A commercial consequence is also that it is impossible to obtain the same yield of leather from any given quantity of raw material. In trade parlance chrome tannage does not give good "weight." Another consequence is that (even when well filled with greases in finishing) chrome leather tends to be "woolly" on the flesh side or at cut edges.
On the other hand, chrome tanning has very many advantages over the older process. The most obvious of these is the great saving in time. Many chrome tannages involve only a day or two, and none more than a week or two. A chrome leather factory therefore needs less capital on account of the quicker turnover. If, moreover, the market be unfavourable, a chrome tanner can stop or reduce his output in a very short time, whereas a vegetable tanner is committed to many weeks' supply of the goods he is manufacturing. Another notable advantage of chrome leather is its durability. In the finishing processes more grease is usually employed than in vegetable tannage, and this has a preservative effect upon leathers which often get wet. Chrome sole leather and hydraulic leathers are cases in point. Chrome leather will also stand changes of temperature and friction much better than vegetable tannages. The light chrome tannage results, further, in yielding a leather which has great tensile strength, and it is not surprising to find that chrome harness and chrome picking bands are highly thought of. The empty nature of the tannage necessitates the use of stuffing greases, but such large proportions of these may be used that chrome tannage becomes obviously suitable if one wishes to produce a waterproof leather. Hence the popularity of chrome tannage for waterproof soling and hydraulic leathers.
The advantages of the chrome process are very real, and very obviously such as will appeal to manufacturers. Chrome leathers have now been for some time in the forefront as far as boot uppers are concerned, especially for the best quality goods, in which the popular "box calf" and "glacé kid" are so largely employed. There seems little doubt that this will continue to be the case. It is an unfortunate fact that in this important branch of tanning, British manufacturers have not quite risen to the occasion. Their products have in the past been faced with very serious competition from Continental and American manufacturers of chrome uppers, and there can be no doubt that these competitors produced a better article, and produced it more economically. The disorganization of the Continental factories owing to the war should give British manufacturers a valuable opportunity of putting such businesses on a better basis. For sole leather also the chrome tannage makes constant headway, and the relative proportion of it becomes gradually greater. A great impetus to chrome sole leather has been given by the war conditions of Britain. Owing to the submarine campaigns of Germany the tonnage question became all-important, and the bulky imports of vegetable tanning materials became a serious item. British tanners were therefore encouraged to make more chrome sole and less vegetable sole. The urgent need of leather for our armies also assisted in the same sense. The production of chrome sole progressed therefore enormously during 1917 and 1918, and although some reaction will doubtless occur, there seems little doubt that chrome sole leather has taken a definite and permanent leap forward. Once the general public fully appreciate its qualities of waterproofness and durability its future will be assured.
On the whole the position and prospects of chrome tanning are good. The chrome tannages are making headway in all directions, and undoubtedly threaten the existence of many of the older processes of vegetable tanning.
REFERENCES.
Procter, "Principles of Leather Manufacture," pp. 198-220.
Bennett, "Manufacture of Leather," pp. 210, 355.
Bennett, J.S.L.T.C., 1917, 176.
Stiasny, Collegium, 1908, 117.
SECTION II.—GENERAL METHODS OF CHROME LEATHER MANUFACTURE
It has been previously pointed out that the chrome tannage is an "empty" one; the primary principle in the wet work of goods for chrome leather is to avoid anything which will make this feature more obvious. In the vegetable tannages relatively larger amounts of the tanning agents are used, and these fill the interfibrillar spaces; indeed, as we have seen (Part I., Sections III., V. and VI.), effort is made to increase these spaces and to fill them to their maximum capacity, thus yielding a leather of which 50 per cent. is the tanning agent. In chrome tanning, however, the tanning agent may only be approximately 5 per cent. of the finished leather, so that any trouble taken to split the hide fibres or to dissolve hide substance is usually not only superfluous, but also calculated to enhance the "emptiness." The governing principle of all the preparatory processes for chrome tannage is therefore the conservation of hide substance, and this principle determines the modifications of the processes of soaking, liming, and deliming, which are in vogue. Now, in most of these processes there is usually some loss of hide substance, and it is the particular problem of chrome tanning to reduce this loss to a minimum in each stage. Whether the loss of hide substance be due to alkaline or fermentive hydrolysis, or to solation of the hide gel, the effect is increased by swelling, and in the wet-work for chrome, therefore, any variations in the degree of swelling are objectionable. The preparatory processes should be carried out with as little change as possible in the volume and elasticity of the pelt. Whether also the loss of hide be due to hydrolysis or solation, it is increased by time, hence short processes are (other things being equal) much to be preferred. Fermentive hydrolysis is minimized by cleanliness, alkaline hydrolysis by avoiding strongly alkaline liquors, and solation of collagen is reduced by both, and also by avoiding, as far as possible, the presence of calcium and ammonium salts.
Soaking should be quick and clean. The use of the paddle or drum gives the greatest efficiency and also assists in procuring the softness so essential to the bulk of chrome leathers.
Liming chrome leather satisfactorily is almost an impossible ideal. Every conceivable arrangement has some objection to it. The time of the process may be shortened either by the use of sulphide or by the use of mellow or old limes. To shorten time by the use of sodium sulphide unfortunately involves the employment of more alkali than is desirable, with a consequent plumping effect and tendency to harshness. If sufficient sulphide be used to make the liming very short, then the grease is not "killed" (saponified or emulsified). If the harshness and alkalinity be removed by using also an excess of calcium chloride, then the lyotrope influence of this substance enhances the solation of the hide gel. On the other hand the use of old lime liquors avoids the plumping effect, but increases considerably the bacterial activity, and the bacterial enzymes produce both hydrolysis and solation of the pelt. In practice what is generally done is to shorten time by both methods and so to admit both disadvantages to a limited extent. This is theoretically more sound than would appear, for in mellow limes sulphide has less plumping power but is just as strong a depilatant; whilst, on the other hand, a mellow liming shortened by sulphide is less objectionable as there is some evidence that bacterial activity is relatively less in the first few days. Hence a mellow sulphide liming of 7-10 days is very common in practice, but sometimes a 3-4 days' process with more sulphide is also found satisfactory.