Bookbinding and the Conservation of Books
A Dictionary of Descriptive Terminology

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vegetable tanning

The process of converting the protein (collagen and its related proteins) of a raw hide or skin into LEATHER by means of vegetable materials. Vegetable tanning produces a relatively dense leather, one that is firm and solid and yields a high weight of leather per unit of raw stock. It also produces a leather that is pale brown in color, and which tends to darken upon exposure to natural light. Depending upon the finishing treatment employed, the tanning material washes out of the leather very slowly.

Vegetable tannages are used to produce bookbinding leather not only because of tradition, but because they produce leathers having a soft drape and handle (in addition to their firmness), which retain applied grain patterns particularly well. Unless specifically treated, however, vegetable tanned leathers have but little water resistance.

The classical method of vegetable tanning, 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.

The traditional method of vegetable tanning was slow and expensive and, furthermore, did not always give the characteristics desired in the leather. Not only was there sometimes too much firmness to the leather, but frequently the color of the leather was not as pale or as uniform as it can be made by the use of more modern methods of tanning.

The use of stronger liquors, mechanical action, pH control, and control of the contents of acids and salts have enabled the tanner to produce satisfactory leather in a much shorter time, and efforts are constantly being made to reduce the time of tannage even further. Increased mechanical action, such as drumming, accelerates the speed of tannin penetration but also results in pebbling of the grain, unless some form of pre-tannage is employed. Some tanners in both Europe and the United States employ such systems, in which a comparatively short rocker pit tannage is used to assure sufficient tannage to protect at least the outer quarter or third of the stock, with the stock then being drummed with strong liquors so as to rapidly tan the interior of the corium. There are many variations of this method, ranging from stock struck through in the rocker pits, and then being tumbled in liquors of 100° Bk. or more for as long as 48 hours, to stock only partially penetrated and then being hung on sticks in slowly rotating rotors, being drawn through a comparatively small volume of tanning liquor of gradually increasing strength.

Leather to be used for bookbinding should be tanned to give a pale, uniform, biscuit shade-one which can be readily dyed and finished in a variety of colors. The tannins in the leather should be well fixed and not easily removed by wetting the leather, otherwise they may cause stains when the leather is paste-washed or otherwise moistened.

Light leathers, e.g., goatskins, calfskins, etc., in the finished state, are usually sold on the basis of area (price per square foot); therefore, there is not the temptation to fix an unduly high proportion of tan on the fibers, as is sometimes done in the case of leathers sold by weight. Overly prolonged vegetable tannage is to be avoided as it may cause difficulties in dyeing and finishing. See also: BATING ;DRENCHING ;FATLIQUORING ;LIMING ;SOAKING (1) ;SPLITTING (1) ;UNHAIRING . Cf: CHROME TANNING .

(248 , 291 , 306 , 351 , 363 )




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