Abstract

Tanners who process pickled sheepskins which have been stored for a long time have probably noticed that some times the pelts are liable to swell a great deal after having been returned to the pelt state (for example, after being depickled to pH 7, and thoroughly washed). The published literature covering the deterioration produced on the skin by the preserving pickle does not refer to this swelling. There are many papers which deal with the attacks of acids (without salt) on collagen, and others which treat the acid pretreatment of collagen in order to obtain gelatin. Some of them mention that this kind of acidic attack has also been observed in the presence of common salt, with the consequent swelling at neutral pH. The conditions which produce this swelling have not been studied systematically as yet. In this paper, variations of both swelling at neutral pH and the shrinkage temperature of the pelts (Spanish sheepskins after a conventional beamhouse process) are investigated as a result of the conditions of the previous pickle (NaCl in the bath: 6 and 12°/Be; H₂SO₄: 1.3 and 9% on pelt weight; temp.: 20 and 40°C; and time 0, 1, 6, 8 and 34 days). It has been found that both the degree of swelling and the decrease of shrinkage temperature depend on the conditions of the previous pickle. Both increase when the pickle action has been more intense. Furthermore, the effects of the acidic attack produced by a strong pickle are studied specifically on: (a) variations in the percentage of swelling and shrinkage temperature at different pH values; (b) titration curves; (c) the amount of chrome taken up by the skin in a conventional tanning; and (d) physical tests of the leather obtained, measured in the Lastometer. According to the results, and relating them to the findings of other authors, some theoretical aspects of the possible causes of this swelling are discussed. In conclusion, a simple method to assist the sheepskin tanner is presented. This method aids in establishing the degree of deterioration of the skins caused by the combined action of the beamhouse process and the preserving pickle. The method does not require specialised personnel or instruments.