Bookbinding and the Conservation of Books
A Dictionary of Descriptive Terminology

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liming

One of the beamhouse operations employed in leather manufacture. Its purpose is to degrade, and thereby loosen, the epidermal structure of hide or skin, including the hair, epidermis, sweat glands, etc., so that they may be removed. Methods of liming vary both in the chemicals used and in procedures. Unhairing and liming can be carried out simultaneously by immersing the skins in the lime and water mixture, often with the addition of other chemicals known as sharpeners, e.g., sodium sulfide.

Lime, which is calcium oxide (CaO), reacts violently with water to form hydrated lime (calcium hydroxide (Ca(OH) 2 )), which can be used with safety in liming and unhairing because it will not damage the collagen fibers of the skin, assuming it is properly used. This is because calcium hydroxide is not very soluble in water, and, in fact, a saturated lime solution contains only approximately 1/8 part lime per 100 parts water. Even this limited solubility, however, is sufficient to produce a solution with a pH in the range of 12.4 or 12.5, and, under such very alkaline conditions, some of the young keratin protein decomposes to produce sulfur compounds in the lime liquor. These compounds, in conjunction with the lime, accentuate the further breakdown of keratin. The lime therefore promotes unhairing, and the more keratin breakdown impurities it contains, the more rapidly it unhairs. Liming, however, must be carried out with extreme care, as the alkali also modifies and will eventually degradate t he collagen fibers of the skin. Skins limed for an overly prolonged time produce thin, loose, and weak leather.

Liming also causes the hide or skin to swell. In order to promote gradual and uniform swelling, so as to avoid distortion or buckling of the stock, the skins are left in the lime solution for 1 to 2 days, following which a sharpener is usually added to promote the process. The addition of sodium hydrosulfide (NaSH), sodium cyanide (NaCN), sodium hydroxide (NaOH), sodium carbonate (Na2(CO)3), dimethylamines, etc., quickens the process in two ways: 1) by attacking the keratin, resulting in faster loosening of the hair; and 2) by increasing the alkalinity, and, therefore, the rate of swelling. If too much sharpener is added too quickly, however, rapid unhairing results, accompanied by excessive swelling of the surfaces of the skin, while the interior remains unswollen. This results in buckling, which makes subsequent unhairing, fleshing (if not done before liming), or splitting difficult. It may also result in permanent distortion and weakness of the grain, and, in addition, any naturally occurring wrinkles or "growth marks" may become accentuated. After liming, the skins are then ready forUNHAIRING . (248 , 298 , 363 )




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