Bookbinding and the Conservation of Books
A Dictionary of Descriptive Terminology

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unhairing

The beamhouse process in leather manufacture of removing the hair (or wool) and other epidermal structures from hides and skins, following liming (or any other preliminary unhairing process), but before tannage. Unhairing is generally considered to be the second of the three major beamhouse operations, the others being LIMING and BATING . The term "unhairing" is perhaps limiting in scope, because not only does the liming-unhairing process loosen the hair and other epidermal structures, it also cleans, loosens, and expands the dermal fiber network and alters the swelling characteristics of the skin.

Unhairing may be accomplished by: 1) treatment with lime (calcium hydroxide), caustic soda (sodium hydroxide), barium hydroxide, sodium carbonate, or ammonium hydroxide, etc.; 2) treatment with lime, as in treatment 1, sharpened by the addition of sodium sulfide, sodium bisulfide, antimony sulfides, or other appropriate chemicals; 3) treatment with lime, as in treatment 1, sharpened by the addition of amines, such as methylamine, or degradation products themselves from the skins being processed; 4) treatment with aqueous buffer systems containing inorganic salts, e.g., ammonium sulfate, sodium chloride plus enzymes with proteolytic and/or carbohydrase activity, usually derived from molds, bacteria, fermenting plant materials or animal tissues; 5) treatment with aqueous solutions of inorganic salts, such as potassium perchlorate, potassium thiocyanate, sodium chloride or sodium silicofluoride; 6) treatment with hot water, i.e., scalding; 7) treatment with strong aqueous solutions of sodium hydr oxide, sodium sulfide, etc., i.e., pulping; 8) action of bacteria, or SWEATING (1) ; 9) treatment with ammonia; and 10) alternating cycles of freezing and thawing in water. The last two methods are not in commercial use; scalding is used only for certain skins, such as peccary and pigskin; and treatment with strong aqueous solutions of strong alkalis, e.g., sodium hydroxide, is useful only when the tanner has no market for the hair. Treatment by sweating is virtually obsolete.

Conventional unhairing involves loosening the hair by means of one of the lime solutions given above, followed by mechanical unhairing by machine. The unhairing machine is similar to the fleshing machine, except that the blades of the cylinder are smooth instead of sharpened, and the machine runs at a slower speed so that the spiral blades scrape or rub the hair off without damage to the grain surface of the hide or skin. The hair is generally gathered, washed, dried, baled, and weighed for shipment. (248 , 291 , 306 , 363 )




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