Bookbinding and the Conservation of Books
A Dictionary of Descriptive Terminology

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pickling

The process in leather manufacture of bringing hides and skins into a condition of equilibrium in preparation for tannage. Pickling, which is a process developed specifically for use in modern chrome tannages, takes place following LIMING ,UNHAIRING , and DELIMING (and BATING ), and is used particularly in the case of sheepskins. The skins are de-wooled (the wool being more valuable than the skins) and are then pickled, drained and packed, ready for transportation to the tannery. Pickling takes place in a solution of 12% salt and 1.2% sulfuric acid (previously diluted with water). The salt solution should remain above 10% and the acid above 0.8%. Furthermore, if the proportion of salt to acid is incorrect the skins may be damaged; consequently, both must be carefully controlled. Pickled skins also must not be allowed to become dry, as drying may cause the acid to weaken the skin structure, and the crystallization of salt on, or in, the grain may lead to SALT SPROUT .

The salt serves to preserve the skin, just as in the case of salting for the purpose of curing, while the acidity, if below a pH of 2.0, inhibits nearly all known putrefying bacteria. The treated skins, therefore, may be stored for several months, provided they are kept cool; however, at temperatures above 32° C., the acidity may cause damage to the skins. Pickling, while stopping bacterial damage, does not stop the formation of molds, which favor pH values of less than 5.0, and which can cause green, black, or white discolorations, as well as a loss of gloss or face on the finished leather. This is due to their attack on the grain structure, and often manifests itself in uneven dyeing. Mold growth can be prevented by the use of fungicides added to the pickle liquor in a concentration of about 0.001% of the weight of the liquor. A typical pickle fungicide is paranitrophenol. (248 , 306 , 363 )




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