Vellum

A fine type of parchment. To make it, the skins of sheep, goats, calves or antelopes were scraped and stretched into something like paper. Many biblical books were originally written on, or copied onto, vellum.

Sometimes, to save money, a scribe would scrape off the original writing on a piece of vellum and then reuse it. Manuscripts with two layers of writing like this are called palimpsests. Often, the writing underneath is more important to scholars than the later writing on top. Nowadays, the half-erased under-writing can be read using ultra-violet photography.