Besides the direct use of mineral acid which has been described, sulphuric, or still better, oxalic acid may be very advantageously employed in precipitating lime from used bating liquids containing weak organic acids, or other lime solvents, so as to restore their original activity. Not only is the bate economised by being used repeatedly, but some of the organic products dissolved from the hide have themselves considerable power of removing lime. Putrefaction should not be allowed to take place; but many of the organic acids which have been proposed for bating belong to the aromatic series, and have considerable antiseptic power. Where organic acids are employed, the presence of their neutral lime-salts in the liquor, resulting from previous operations, will reduce the swelling action of the acid on the skin, without diminishing its power of removing lime (cp. [p. 81]).
In place of sulphuric acid, some tanners have employed a material advertised under the name of “boral.” This substance consists simply of sodium anhydrosulphate melted up with about one-seventh of its weight of boric acid, the quantity of which is, however, too small to have appreciable influence as an antiseptic, while it is said to form insoluble borates with the lime present, which are sometimes a source of subsequent trouble.
There is no reason why ordinary sodium bisulphate should not be used for the purpose, and its action is more mild than that of sulphuric acid itself, but great care must be taken that no nitric acid is present, as is frequently the case in the crude product obtained in the manufacture of nitric acid from sodium nitrate, and known in commerce as “nitre-cake.” The presence of a trace of sodium chloride would not be disadvantageous for dressing leather, but would tend to prevent plumpness in sole. Paessler and Appelius[88] have recently shown that raw hide absorbs sulphuric acid from sodium bisulphate, leaving the neutral sulphate in solution.
[88] ‘Wissenschaftlich-Technische Beilage des Ledermarkt,’ 1901, p. 107.
Boric (boracic) acid, though used to a slight extent for a number of years past, has recently come much into favour as a deliming agent, for which purpose it is in many respects particularly suitable. Sole-leather may be improved in colour by giving a short bath in 11⁄2-2 per cent. boric acid solution to remove surface-lime. In this case the acid is best applied just before the hide enters the suspenders. Boric acid may also be suitably employed on hides which have been bated. It then acts as a drench and removes traces of lime still left in the hides, so that the liquors have a more even effect on them. Experience has shown that the skins should never be allowed to lie for any length of time in the boric acid solution in a motionless condition, as this tends to produce patches of partially delimed skin, which cause irregular colour. It is best to keep the skins in fairly constant motion in a paddle or by frequent handling. Boric acid has considerable influence in preventing drawn grain in the early liquors, but if it gets into the forward liquors it renders the leather loose and light (cp. [p. 229], and L.I.L.B. p. 37).
Borax has also been suggested as a deliming agent, and as it is chemically an acid salt, it has naturally some deliming effect, but it cannot compare with boric acid in either price or efficiency.
Both boric acid and borax are antiseptics (see [p. 25]).
In the employment of either sulphuric, boric, or any other acid forming calcium salts of limited solubility, it must be borne in mind that if the solution is repeatedly re-strengthened, it will become saturated with the lime-salt, and although the acid will still combine with the lime and render it neutral, it will no longer remove it from the hide. Under these conditions, sulphuric acid may cause the deposition of crystalline calcium sulphate in minute nodules between the fibres. Calcium borate may be similarly deposited, and has the further disadvantage of becoming decomposed by the tanning liquors, which form dark compounds with the lime. In using sulphuric acid alone it is therefore best to renew the water each time. When it is used in conjunction with some other acid, forming very soluble lime salts, this danger is not to be apprehended, while oxalic acid precipitates the lime almost completely from the solution.
It is to be borne in mind that in all cases of using acids, any carbonate of lime present on the pit sides or elsewhere will be decomposed, and the carbonic acid will become dissolved in the liquor, and unless acid is used in sufficient quantity to remove the whole of the lime, may tend to fix the remainder as carbonate. In the case of dressing leather there is less danger of this, as warm water is generally used, in which little carbonic acid dissolves. It is probable that some of the coal-tar acids which have been advertised for bating dressing leather might be advantageously employed for sole. Hauff’s “anticalcium” (see [pp. 29], [163]), would appear to be very suitable for this purpose, and if the liquor were regenerated by the addition of sufficient sulphuric acid to neutralise the lime dissolved from the hide, might be used repeatedly, and would not then prove expensive; while its sterilising power would be very advantageous to the proper swelling of the butts in the handlers, since nothing tends to check plumping so much as putrefactive action.
Turning from sole to dressing leather, mineral acids are very successfully employed for “pulling down,” the goods being thrown into a paddle containing warm water of about 30°-35° C., and the calculated quantity of sulphuric or hydrochloric acid, previously largely diluted with water, is then added in two or three successive portions at intervals of perhaps ten minutes. The acid must in no case be sufficient to neutralise quite the whole of the lime. Goods treated in this way can be further bated, puered, or drenched as required by the ordinary methods, if they are not sufficiently soft. If too much acid has been used, and the skins show signs of swelling, they may be brought down by the addition of a little ammonia, borax, or even soda.