A stable RH in the range of 45% to 55% and a temperature of 18 oC to 20 oC are the recommended environmental conditions. If an object consists of leather that is stitched or tacked to another more or less responsive material, fluctuations may lead to buckling or splitting of the leather. It may also cause the vegetable tans to migrate, resulting in the leathers surface becoming darkened and brittle. RH fluctuations may produce a progressive hardening of vegetable tanned leather. Very dry conditions (less than 30% RH) will cause loss of moisture and embrittlement high humidity (over 65% RH) encourages mould growth and hydrolytic degradation of the leather fibers. Leather should be protected from extremes and fluctuations of relative humidity (RH). Chrome tanned leathers can sometimes be identified by a blue-green line on a cut edge. Since the turn of the century, strong leathers with a high resistance to moisture have been produced by tanning with mineral salts, such as chrome and zirconium salts. Alum tawed leathers are easily damaged by water or moisture. For centuries, this type of tannage has produced fine, flexible, light-coloured leather, and is still used today to produce a small number of glove leathers and some furs. Alum Tanned or Tawed SkinĪ mixture of potash alum, sodium chloride, egg yolk, flour, and water is worked into softened skins to produce alum tawed leather. Depending on the type of tannin in the vegetable matter used, the leather may be prone to a condition known as red rot, a degradation of the leather that eventually produces a red, powdery surface. Once the tanning process is complete, oils or fats are often applied and worked into the leather to provide the necessary flexibility for use or for further manufacturing processes. Vegetable tanned leather is produced by immersing prepared skins in a series of tanning liquors, which are made of tannin-rich vegetable matter (bark, leaves, twigs) in water. Proper storage environments and supports are most important to leather collections, therefore preservation efforts should concentrate on improving these. Current museum practice is to minimize the application of dressings, lubricants, and cleaners because of their negative long-term effects on the leather. Until the last several years, leather objects in museums had been cleaned and softened to the same degree as when they were in use. With disuse, these objects also tend to become hard and inflexible. ( PDF Version, 1.63 KB) CCI Note 8/2 is part of CCI Notes Series 8 (Leather, Skin, and Fur) IntroductionĪlum, vegetable and mineral tanned leather items, such as boots, bags, and horse collars, are frequently found in museums in a stained and soiled condition.
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