| Context |
The classical method of vegetable tannage, especially where flatness of the leather (and particularly of the grain surface) is of great importance, as with bookbinding leather, is pit tannage-usually accomplished by means of suspension in a rocker vat. Traditionally the skins are limed and unhaired, and delimed to a pH of 4.0 to 5.0, bated (and sometimes drenched with bran or acetic acid). The stock is then suspended as smoothly and flatly as possible in rocker vats containing used or new tanning liquor of a relatively low BARKOMETER reading. The strength of the tan liquor is gradually increased (the stock being transferred into pits containing progressively stronger liquor) until the tanning has just penetrated through the entire thickness of the skin, i.e., it has struck through. Splitting, if required and if not done previously, finishing , etc., are then carried out after drying.
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