It will be noted that the quantity of acid used is only about one-tenth of that required to neutralize completely the lime in the skin. If a greater amount of acid be used the skins begin to absorb acid before the lime in the interior is neutralized; but, provided the lime on the surface of the skins is removed, that remaining in the interior is forced out during the course of the bating process (see Chapter [III].). If more acid were used, then it would require to be added so slowly that time would be wasted unnecessarily in doing the work.
With regard to the use of other acids for deliming, Lamb considers formic acid preferable to hydrochloric, and states that the removal of lime is more effectively accomplished by its use. He attributes a pulling down action to the calcium formate produced, so that less formic acid is required than would be equivalent to hydrochloric, merely regarded as a solvent for lime. We shall refer to this point again in Chapter [II]. It may be stated here that the author has obtained better results by the use of a mixture of formic and acetic acid, in equal proportions, than from either acid alone, but that the cost of working is considerably greater than with hydrochloric acid.
Lactic acid is in fairly common use as a deliming agent. In practice 1 per cent. lactic acid (50 per cent. strength) calculated on the wet weight of pelt, is used, added gradually in small quantities. This does not remove the whole of the lime, but sufficient for the skins to puer quickly.
Some tanners measure their acid per dozen skins. In a case which came under my notice, 1200 c.c. 90 per cent. formic acid were being used for ten dozens medium goat skins at a temperature of 35° C., time 1 1/4 hours.
The following table, due to Professor H. R. Procter,[8] gives the cost of dissolving 1 lb. of lime at present approximate prices of the various acids. The dissociation constant K shows their relative “strength;”[9] the equivalent, Eq., the weight in lb. of 100 per cent. acid required to dissolve 28 lb. of lime. Ordinary wet limed hides, unhaired and fleshed, contain only about 4 lb. lime per 1000 lb. wet weight.
Cost of Acids to remove 1 lb. Lime.
Acid | Eq. | K | Cost | Strength | Cost for | |
s. | d. | d. | ||||
| Hydrochloric | 36·5 | say 200 | 3 | 3 | 31·5 | 1·4 |
| Sulphuric | 49·0 | " | 4 | 0 | 95·0 | 0·8 |
| Oxalic | 63·0 | 0·1 | 30 | 4 | 99·0 | 8·1 |
| Formic | 46·0 | 0·0214 | 35 | 0 | 87·4 | 7·0 |
| Lactic | 90·0 | 0·0138 | 26 | 0 | 49·7 | 18·0 |
| Acetic | 60·0 | 0·0018 | 18 | 0 | 40·0 | 10·0 |
| Butyric | 88·0 | 0·00115 | 21 | 0 | 82·8 | 8·0 |
| Boracic | 62·0 | 0·00000001 | 27 | 0 | 99·0 | 6·5 |
It will be seen from the above table that sulphuric is the cheapest acid to use; but, owing to the insoluble nature of the calcium sulphate, it is best to use hydrochloric acid. Procter has suggested using a mixture of sulphuric acid and common salt in molecular proportions, so as to avoid the iron which is generally present as an impurity in commercial hydrochloric acid. The author has tried this, and found it perfectly satisfactory. For a complete discussion of chemical deliming, see Procter’s “Principles of Leather Manufacture,” chapter xiii.
Quite recently butyric acid, which is now being manufactured on a commercial scale by the process of Dr. Effront (Brussels), has come into use for deliming.[10] and is likely to become a useful acid, as it can be manufactured cheaply. According to Parker, it dissolves less skin substance than formic, acetic or lactic acids, which observation is confirmed by Thuau. The quantity required per 100 lb. of ordinary washed sheep skin is about 1/2 lb. of the 80 per cent. acid. It is used in exactly the same way as other acids, i.e. it should be added in small quantities, suitably diluted, at successive intervals of time.