Rare book conservation differs from that of most museum items in that the book has to be used afterwards. For this reason bookbinders are inclined to use traditional materials known to have survived with time: wheat starch paste, unbleached linen fabric and sewing thread, and hemp cords. Oiling of books, so long considered a good thing, is now felt to do more harm than good, largely because unskilled workers put on too much, and fail to distinguish tanned leather from parchment, alum-tawed, and cloth, which should not be treated. Vegetable tanned goatskin is currently the preferred covering material at the Library of Congress, with an increased use of vellum and alum-tawed skins, considered to be more durable than many tanned leathers.