Parchment, made from the skins of calves, goats, lambs and sheep, differs from leather in that it is not tanned but prepared by a special drying process on a frame. All, or nearly all, grease is removed in the process giving a surface suitable for writing but in sheep the natural high grease content may lead to a partial oil tan. The finest parchment is made from the skins of very young animals, usually calves, and hence is called vellum (though definitions of parchment and vellum may vary from country to country). In a typical sequence of preparation, little changed in two thousand years, a well-washed skin is left in a bath of 30% slaked lime for 8-16 days depending on temperature, de-haired, limed again then washed and stretched. The quality depends on careful control of the drying process on the frame. The skin is dried at about 20 °C, washed with cold water, partly dried and washed again to give a smooth gluelike surface. At the same time it is scraped and shaved thin. This process gives an arrangement of collagen in lamellae more or less parallel to the surface in contrast to the interwoven structure of leather. After scraping the skin is rubbed smooth with pumice or other abrasive material and allowed to dry in its stretched state.
If subsequently wetted, the dermal network may revert to a random arrangement with the skin becoming hard, horny, transparent, cockled and shrunken. Components of the skin will hydrolyse and dissolve in hot water and advantage is taken of this in the preparation of parchment size for gilding. In furniture parchment has been used as a constituent material of elaborate trimmings or passementerie (see Figure 16.14) and occasionally for seat coverings and panelling, as seen in the Bugatti armchair in Figure 3.5.
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