In many cases the addition of salt in small quantity to the acid liquor will tend to deplete the hides, and at the same time prevent any injurious action of the acid. Ammonium chloride may also be used with advantage (see [p. 159]). A solution containing about 15 per cent. of salt and 0·3 per cent. of sulphuric acid, with some molasses, has been a good deal used in the States as a bate, and seems to answer well on some classes of goods, but the acid and salt are apt, ultimately, to find their way into the liquors and destroy tannin. The process is well suited for chrome-leather, and may also be usefully applied in cases where goods have become “wind-blasted” or otherwise impregnated with carbonate of lime, since in presence of salt the acid can be used in sufficient excess to dissolve the carbonate. Vegetable acids may, of course, be used in conjunction with salt in the same way. The salt does not neutralise the acid, but simply controls the swelling of the skin, and if acid has been used in any material excess, the first part of the tanning must be done in salted liquors, or the acid neutralised with ammonia, sodium carbonate, or chalk, previous to tanning, as, otherwise, the goods will plump up in the liquors, and be tender when tanned (cp. [p. 91]).
Lactic acid has recently come largely into use as a deliming agent. It is best known as the acid which gives a characteristic taste to sour milk, and is the chief product of the lactic ferment. It may be very successfully used for neutralising the lime left in the skins after the depilation, but, if used in excess, it tends to plump or swell the leather very strongly, being one of the best plumping agents known. When used for deliming, a solution of 2 lbs. in 100 gallons is very suitable. It may, in many cases, be substituted for the bran-drench with advantage, and is much more rapid and less dangerous in hot weather, but the effect is not in all respects identical.[89]
[89] On the manufacture of lactic acid by fermentation, see Claflin, Journ. Soc. Chem. Ind., 1897, p. 516. Campbell states that practically pure cultures of the lactic bacteria are obtained by continued culture in milk. These cultures employed as a ferment for drenches have given good results in the Yorkshire College Experimental Tannery.
When lactic acid is used for bating, or drenching, the operation should always be conducted in a paddle, and the liquid works more satisfactorily if it is at a temperature of 30-35° C. As regards cost, it will be found that in practice it is not appreciably more expensive than dung or bran. About an hour’s paddling will generally suffice, if the right quantity of acid has been used, but in some cases it is best to add the acid in several portions and take more time.
The estimation of the amount of lactic acid in the commercial article may be carried out by diluting exactly 9 grms. with about ten times its volume of water, and then titrating it with normal caustic soda as described in L.I.L.B., p. 16, for acetic acid. As each c.c. of normal alkali is equivalent to ·090 grm. of lactic acid it will represent one per cent. of real lactic acid in the sample. If other acids are present, they are of course included. Commercial lactic acid is usually of about 50 per cent.
It is important that the lactic acid should be free from iron, a dilute solution should give no blue coloration on addition of either potassium ferrocyanide or ferricyanide. Acid perfectly free from iron is now easily obtained.
Formic acid in 60 per cent. solution, formed synthetically by the combination of carbon monoxide with caustic soda and the subsequent decomposition of the sodium formate so produced, has recently been brought into commerce at a cheap rate, and will probably form a satisfactory substitute for acetic acid in the deliming of hides and many other technical operations.
Instead of acids, many neutral salts may be used to neutralise lime, and in sole-leather, it is not generally disadvantageous to leave the lime in the hide, so long as it is in an insoluble and fixed condition, and combined with an acid which cannot be displaced by tannin. Thus phosphates, or oxalates of sodium or ammonium will convert the lime into insoluble phosphate, or oxalate, setting free sodium- or ammonium-hydrate which form soluble tannates and other salts which are easily washed out of the hide. Zinc sulphate will form sulphate of lime and zinc oxide in the hide, and seems worth further experiment for sole-leather, but must be free from iron. Alum, or sulphate of alumina, would similarly form calcium sulphate and alumina, but the tanning effect of alumina salts is too great to admit of their general use for bating. Ammonium sulphate will form calcium sulphate with liberation of ammonia.
For dressing leather, the use of ammonium chloride would be still more advantageous, and it is a powerful bating material, converting the lime into calcium chloride with the evolution of ammonia, which has but little plumping power, and which is easily washed out. Ammonium chloride has been very successfully used in calf-kid manufacture as a preparation for drenching, instead of puering, which was formerly in vogue. As, however, only about 3⁄4 oz. per dozen skins was employed, the cleansing must have mainly depended on the warm water with which it was used, and the free ammonia evolved.
The use of ammonium chloride as a bate was patented by Zollickoffer in 1838.