The time of tannage will of course vary with the thickness of the goods, and for calf-skins will usually extend over some days, though it can be much quickened by drumming. The tannage is generally best accomplished in the paddle, but can be carried out by frequent handling in pits or tubs, or, where very smooth grain is important, by suspension. When the goods come out of the final liquor, they may be allowed to lie in pile for twenty-four hours, or even for some days, with advantage, as the surplus chrome liquor is pressed out, and the tannage becomes more complete. They are then washed with plenty of warm water, till it ceases to be coloured with chrome. They may be kept for an almost unlimited time in a wet condition, as they do not bleed, and have little tendency to heat even in pile. They have now reached the stage at which we left the “two-bath” leather, and the subsequent treatment may be the same in both cases. Although by both processes, the chrome-salt fixed in the fibre is of a decidedly basic character, it still contains enough acid to act injuriously on the leather in course of time, and to lead to serious inconveniences in its subsequent treatment. Before proceeding further, this access of acid must be removed or neutralised, and it is not too much to say that most of the troubles experienced in the fat-liquoring arise from neglect or mistake in the washing and neutralisation. The difficulty in the process arises from the fact that while the acid should be reduced to a mere trace, it must not be entirely removed,[125] as chromic oxide itself does not seem capable of tanning, and at any rate the effect of excess of strong alkalies is at once to render the leather hard and pelty. Borax is one of the safest neutralising materials, about 3 per cent. on the wet weight of the pelt being required, in not more than ¹⁄₂ per cent. solution. Eitner recommends the use of silicate of soda, which, sold as a solution of S.G. 1·5, is somewhat stronger and much cheaper than borax. Hyposulphite of soda and whitening together neutralise more rapidly and completely than either alone. Other salts of weak acids may also be used, the acids exercising a regulating influence which prevents neutralisation going too far. Sodium carbonate or bicarbonate, or ammonia may also be used, but with these it is difficult to get even “neutralisation,” or to avoid the risk of carrying the process too far. Even a thorough drumming with a milk of “whitening” (calcium carbonate) is effective. With the latter there is no danger of overdoing the process, but in some cases the adhering whitening and precipitated calcium sulphate are troublesome in later operations. In any case the neutralising should only be carried so far that the skins show no acid reaction to litmus paper. [125] Procter and Griffith, Journ. Soc. Chem. Ind., 1900, p. 223.