Delimed and bated pelts are no subjected to a PICKLING process. There are two and different objective for pickling pelts. The first is to condition the pelts for chrome tanning; the other to preserve delimed; and bated pelts either for tanning at a later stage or for export, It also serves another useful function - at this pickled stage, skins can be easily sorted for production of different types of leather.

Delimed and bated pelts have a pH between 7.5 to 8.5, indicator of continued alkaline condition. If they treated with chrome tanning agents without pickling, the acidity in the chrome tanning agent will be consumed to neutralize the alkalinity of the pelts, thereby making the chrome tanning agents more basic. Since chrome-tanning agents become highly basic and are therefore precipitated, they form heavy deposit on the surface of the pelt, which would altogether nullify the tanning action.

When the pelts are treated in the acid-salt pickle liquor they absorb acid which is partly utilized - a) to neutralize the free alkalinity and b) to combine chemically with collagen. The remaining acid is held up mechanically in the space between the fiber bundles of the pelt. By absorption of acid, pH of the pelt is lowered to about 2.0 to 3.5 depending upon the quantity of acid used in the pickle. In this way the pickling treatment partly or fully saturates the acid binding capacity of the collagen and places the pelts in an acidic environment ready to accept; chrome tanning agents.

The pickling process first calls for the addition of common salt or brine to the system. If acid is added alone it will cause swelling and plumping is not properly checked and controlled the pelts will be damaged. A certain minimum concentration of neutral salts is essential to prevent damage to the pelts. Salts attract and ties up excess moisture that would otherwise cause the fibers to swell. The Degree of swelling is a function of pH value of the pickle bath, where as the prevention o swelling is purely a function of salt concentration in the pickle liquor.

Since the required quantities of salt, acid and water are taken on the weight of the pelt, the water quantity should be strictly observed. The concentration of salt in pickle liquor should be between 6% to 10 % (on 100% float), depending upon the acidic condition of the bath, since any amount of salt less than this level will cause damage to the pelt by acid plumping. On the other hand, excessive amount of salt will be carried into the subsequent tanning bath, which may give flat and tinny leather lacking in fullness. The salt concentration in pickle is usually 7ºto 9º Be.

Sulfuric acid and common salt are generally used in the pickling system. Other acids like hydrochloric acid and formic acid are also used in tanneries. A pickling of sulphuric acid and common salt gives a fuller leather but also tends to make the grain more coarse; while a hydrochloric acid/common salt pickle gives less fullness but the grain is smoother. A smooth grained and lighter colored leather is obtained when formic acid is used in the pickle system alone or in combination with sulphuric acid. Some tanners are of the view that common salt fully or partially replaced by sodium sulphate in the pickle system gives tightness to the grain Common salts when partially replaced by buffer salts like sodium or calcium formate gives grain tightness and fullness to the final leather.

Pickling is usually carried out in drums. The drum is filled with the required quantity of water into which the appropriate amount of salt is dissolved. Pelts are introduced into this solution and drummed for ten minutes. Next, pre-diluted acid is added to revolving drum through its axle. Pickling time depends upon the substance of the pelt. Drumming is normally done for two hours and the pelts are then left overnight in the pickle bath. The starting pH (1.7 to 2.0) of the pickle bath gradually and automatically increases due to the absorption of acid from the pickle bath and, towards the end rises to pH 2.7 to 3.6. Over 90% of the original sulphuric acid or 95% of the hydrochloric acid is absorbed by the pelts however, absorb only a small quantity of salt, the bulk remaining in the pickle bath.

After completing the pickling operation, the pelts are either chrome or vegetable tanned. Chrome tannage can be carried out either in the pickle bath itself or in a fresh bath. If the pelts are to be vegetable tanned, a light pickling, followed by pre-tanning with a suitable syntan prior to the main vegetable tanning process is given to the pelts.

The pickling operation is a preservation technique in its own right. Raw hides and skins putrefy when kept in an open atmosphere for even a few hours. Proteolytic enzymes present in the hides or secreted by bacteria hydrolyze proteins. These enzymes and bacteria will only act when conditions of humidity, temperature and alkalinity are favorable for their putrefying action. A low pH or a highly acidic condition of the pickled pelts will prevent the enzymes from acting on the pelts.

The pelts, which are put through the pickling process for the explicit purpose of preservation, are usually treated with heavy doses of acid and salt. Mould growth on pickled stock is an ever present problem for all tanners engaged in making pickled leathers for export. Mould growth produces stain which cause discoloration in the finished leather or, if extensive, can even damage the grain.

Rohm and Haas Company, have a fungicide - KATHON LP -, which is effective to preventing mould growth in wet pickled stock. Kathon LP has a broad spectrum of fungicidal activity at a very low application level. It is supplied in liquid form readily dispersible in water. The used level of KATHON LP is four fluid ounces per 1000 lbs. (0.025%) of pelt weight. This level gives protection for months from any type of mould forming organisms. KATHON LP may be added directly to the pickle bath 10 minutes prior to completion of the process. It is less toxic than most of the other commonly used preservatives.