Another early writing early writing medium was parchment, made from the skins of animals, usually sheep and lambs. Naturally this was scarce and hard to get. Even more valuable was vellum, a parchment made from the skin of calves. In fact, so costly was parchment that the custom arose in the Middle Ages of washing the ink off old parchment manuscripts and using them over again. Such manuscripts are called palimpsests (palin, again; psao, rub smooth). In some instances, after the passage of years, the original writing of a palimpsest reappeared beneath the later treatment. Some interesting restorations have been made in this manner.