Bookbinding and the Conservation of Books
A Dictionary of Descriptive Terminology

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sun damage

Damage that occurs to a hide or skin when it becomes heated above a certain critical temperature while it is still moist and is laid out in the sun to dry (cure). A completely dry skin can become very hot and still not be affected under normal conditions of cure, but prior to this a skin is very susceptible to damage. Also, the drying of a salted skin proceeds so uniformly throughout its entire thickness that the cooling effect produced by evaporation off its surfaces maintains the temperature of the entire skin below the danger point until it is uniformly dry. Without salt, however, a skin tends to dry unevenly when laid out in the usual manner, which is flesh side up. This can happen because the flesh side can dry out completely and begin generating heat, while the underside remains moist, and, being shielded from evaporation, also begins to generate heat. This is especially the case when pieces of flesh, generally of a fatty nature, are present on the flesh side, and further retard the drying of areas beneath them. The result of overheating moist skins is to "cook," and therefore gelatinize, the skin fibers, so that they are no longer leather-forming collagen. The damage, however, is not visible in the dried skin, nor can it be detected even after the skin is soaked to return it to the pre-cured condition. It is only after liming that the damaged areas tend to disintegrate, leaving holes in the skin, that the damage becomes apparent. In many cases the damage is largely limited to the grain surface, which becomes deeply corroded, giving rise to the trade term "blister" for this particular type of damage. See also: PUTREFACTIVE DAMAGE .

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