The production of a soft leather depends on the fibre being tanned in a fallen and unswelled condition. It is for this reason that bating is in many cases essential, though where somewhat firmer leathers are required, mere reduction of the swelling by removal of the lime is sufficient. For the same reason, no acid-swelling is permissible either before tanning, or in the liquors, and though liquors for soft leathers must be rather acid than alkaline, they are incapable of removing any large quantity of lime, and for the best results, the deliming must be complete before tanning. As mere bating or puering is mainly designed to reduce swelling by the action of bacterial products (p. 172), and is not a very efficient means of removing lime, it is desirable where it is employed, to supplement it by some more active deliming process. In the lighter leathers, drenching (p. 166) generally fulfils this purpose and many of the more intelligent tanners now give bated hides a bath in boric acid before tanning, which not only removes the last traces of lime without acid-swelling, but checks the bacterial fermentation, and prevents its introduction into the liquors. In gambier tannages, a decidedly better colour is obtained by this treatment (p. 228). In most cases the production of bloom is not desired in dressing leather tannage, and is prevented by relying chiefly on liquors, and avoiding the use of bloom-giving solid materials, which include most pyrogallol tannins. Dressing leather tannages can frequently be advantageously hastened by drumming: which by continuously bending the leather in all directions, constantly widens and contracts the spaces between the different fibres, and, as it were, pumps the liquor through the skin. The softness of dressing leathers is increased, and the hardening action of acids present in the liquors is prevented by the addition of salt, or of some sulphates (sodium, magnesium, ammonium) which exercise a sort of pickling action on the fibre, and prevent its swelling, but at the same time tend to light weight and a somewhat empty tannage. It by no means follows that a hide or skin which is thoroughly coloured through, is really fully tanned; as, though the fibres may be actually tanned or coated on the surface, time is required for the tannins to penetrate them to the centre. This incompleteness of saturation is often found in drum tannages. Such leathers are generally tough, and gain weight and softness in currying. In order to “carry grease” well, that is, to absorb a large quantity without appearing greasy, it is essential that the fibre-bundles should be thoroughly split up or differentiated; and the degree to which this is attained largely depends on the extent of liming. There is also considerable difference in different tannages, as to the amount of grease which they will carry.