Pickling

Abstract

Pickling is the further lowering of the pH after bating; most commonly it is a preparation for chromium tanning, but also applies to other tanning processes that require conditions below pH 9. For chrome tanning, the aim is to create pH 2.7–2.8 for the beginning of tanning with 33% basic chromium(III) sulfate. The acidic pH is associated with swelling of the protein by an osmotic mechanism due to the binding of acid onto the protein. Swelling can be avoided by the presence of neutral electrolyte according to the Donnan principle, to equalise the osmotic pressure inside and outside the pelt. The adverse environmental impact of neutral electrolyte has led to options to avoid it in this process step, including avoiding the process step itself. An established alternative is to use the so-called non-swelling acids: these are syntans, of varying reactivity, in acidic form. The more astringent their tanning power, the more effectively acid swelling is suppressed.