------------+------+-----------+----------+----------+---------- | | | Cost | Strength | Cost for Acid | Eq. | K | per cwt. | per cent.| 1 lb. CaO ------------+------+-----------+----------+----------+---------- | | | _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. [10] See also L’Acide Butyrique dans la Tannerie, Urbain J. Thuau, Le Cuir, Aug. 1910. Also Collegium, 1910, pp. 347, 363. Acid salts are also used for deliming, and Procter suggested sodium bisulphate NaHSO_{4} (“Principles,” p. 155). More recently, sodium bisulphite (NaHSO_{3}) has been recommended;[11] the skins are tumbled for half an hour with a dilute solution, which combines with the lime according to the following equation-- (1) Ca(OH)_{2} + NaHSO_{3} = CaSO_{3} + NaOH + H_{2}O if sulphides are present the reaction is-- (2) Na_{2}S + 2NaHSO_{3} = H_{2}S + 2 Na_{2}SO_{3} [11] Le déchaulage des Peaux en tripe, Ettore Guisiana (Turin), Coll. 1910, p. 14.