Aim is to prepare pelt for tanning by stopping enzymatic bating and adjust acidity and pH or its water content for subsequent process.
Excessive and unwanted swelling on the acid side of the isoelectric point is prevented by pickling, ie. the controlled addition of strong acids and their salts, prior to tannage. Dibasic acids (sulphuric) are more effective.
Pickling: The use of liquors containing acid and salt is referred as pickling.
The skins are paddled in the salt and water until all salt has dissolved and diffused evenly. Sulphuric acid (diluted at least 10 times) is added and run (about 2 hrs) until good penetration. Pickling will give pH 1-2 and there is ample salt present not only to prevent acid swelling, but to cause some decrease in thickness of the skins, which are now white, soft and flaccid. 3 % salt on the total volume of water present will prevent acid swelling..
The limed or bated skins must be adjusted to a suitable acidity for tannage by pickling. To pickle the pelt means to acidify it in such a way as to prevent it from simultaneous swelling under the action of acid. This is usually done by salt addition. The presence of acid in the medium suppresses dissociation of carboxylic grous of collagen hide chains. Solution of acid and salt outside the pelt is hypertonic relative to the water contained in the pelt, so it has a higher osmotic tension. This is a reason for the coming over of water molecules from pelt to the solution. Solution in the pelt surrounding becomess less concentrated, and owing to this the electrolytes in it become more dissociated, and diffuse into the interfibrillary spaces in collagen. This process stops when concentration of ions in pelt and in solution become equilibrated, and at the same time an interaction occurs between salt cations and collagen carboxyls, and between anions of salts and basic groups of collagen side chains. Ions remaining in the pelt keep their solvation water, thus the pickled pelt is hydrophilic and mellow.
Pickling slows down the chrome tanning process, does not allow the tanning agents to be bound to the external pelt layers, as this would stop their deeper penetration. Acid, contained in the pickle, reacts with the basic functional groups of collagen side chains and imparts to it a pH of about 1.5.
The first contact of chromium float with leather should occur at pH of the order 1.5-2.5. the pH of the common chromium floats is 2.5-3.5, so the acid used for pickling shall decrease the pH of the pelt to the lower value than that of the chromium float. The purpose of the salt, contained in the pickle in concentration of about 10% is to quench the tendency of collagen to swell in acidic medium. Accordingly, the salt is to be chosen which shall decrease osmotic swelling; that is, such a salt whose a cation and anion have possibly the lowest place in the lyotropic series, where ‘structural breaking’ is a prevailing feature. This breaking action of salts and acids, manifesting itself by dehydration of pelt, ie. removal of the structured water from it, makes the start of tanning easier.
Introduction of formaldehyde to the pickling bath, still containing organic acids, causes only slight tannage due to a low pH. Other tanning agents for pretanning pelts, may be as well introduced to the pickling. Introduction of organic acids into the pelt may be considered as a method of chrome salt masking.This method is based on introduction of a tanning agent into pelt as a nonactive molecule.