The so-called “mellowness” of old liquors requires a word of comment. It is well known to practical tanners, that old liquors are much less liable to produce drawn grain, and a harsh surface, when used to colour green goods, than liquors, even equally weak, which have been made from fresh materials. This is probably due, in part at least, to more than one cause. Most natural tanning materials contain tanning matters of varied degrees of astringency and power of attaching themselves to the leather-fibre. It is obvious that if a tanning liquor is used, the most astringent and energetic tannins will be first removed from it, leaving those of a milder character. It is also known that the presence of neutral alkaline salts of weak acids has considerable influence in producing mellowness; the addition, for instance, of sodium acetate has a marked effect. This effect is probably due in the first place to the action of neutral salts in diminishing the energy of weak acids (see p. 81), and secondly to the fact, that their bases combine to some extent with the tannins; and that, as was perhaps first pointed out by the writer, such tannins are, as it were, partially paralysed in their action on hide (p. 339). Sodium sulphite acts powerfully in this way, and may perhaps prove of technical value in temporarily diminishing the astringency of liquors in quick tannage. Borax has a similar effect, but is too alkaline, and, unless used with extreme caution, spoils the colour of the liquors by causing oxidation. It is probable that similar causes explain the mellowness of palmetto extract, which contains large quantities of alkaline salts, and of some extracts which have been treated with sulphites, when used undiluted in drum tannage. The addition of free acid will generally restore these tannins to an active condition.