In the cases of sole- and belting-leather no softening is desired, and formerly tanners usually contented themselves with a very perfunctory washing in water, trusting to the acids present in the liquors to complete the removal of the lime. Even pure distilled water effects this removal very slowly and imperfectly, owing to the strong attraction of the lime for the fibre; and if “temporary hard” water is used, the lime present in the hide combines with that present in the water and is precipitated as chalk in the surface of the hide. This may be prevented by previously adding a small quantity of lime or lime-liquor to the water before use to soften it (see p. 95); but unless this is very carefully done, the free lime present in the water prevents it from removing any from the hide. The safest way is not to add lime direct to the water, but to change the latter gradually, so as to allow the lime already present to soften the new portion of water. A much more efficient method is to suspend the butts in water to which small portions of diluted acid are successively added till the lime is nearly, but not quite, neutralised. If carefully used, sulphuric acid[86] is perhaps as good as any, but, of course, any excess will spoil the colour or “buff” of the leather. [86] The use of sulphuric acid for this purpose was patented by H. Belcher of Wantage (No. 14,943), but was used some years previously in several tanneries known by the author. Acetic, formic, and lactic acids are safer than sulphuric, but are somewhat costly, and must not be used in appreciable excess. Crude pyroligneous acid may be used, and it has a considerable antiseptic effect owing to the phenols, etc., which it contains. Hydrochloric acid is not suitable for sole-leather, on account of the bad effect of chlorides on plumping. Sulphurous acid[87] is perhaps the best, and its acid properties are so weak that slight excess does little harm, but the neutral calcium sulphite is insoluble, and to actually dissolve the lime the hydric sulphite must be formed, which can only occur in presence of excess of the acid. Unless such excess is used, the colour of the pelt in the early liquors is apt to be somewhat greyish. Probably a very good method would be to suspend the butts in a solution of sulphurous or some other acid of about N/20 strength, sufficiently long to remove all lime from the surface and slightly to plump it but not to penetrate to the centre of the hide, which should then be suspended in water until any excess of acid had been taken up by the unneutralised lime still present in the middle of the butt, which at the end of the operation should be rather alkaline than acid. The course of this, or any other bating operation can be followed by cutting the hide, and moistening the cut surface with alcoholic solution of phenolphthalein, which is turned red, or pink, by the least trace of free lime.