Use of Metal
Hand written manuscripts were typically written on vellum (thinly scraped animal hide). In humid climates, vellum would expand, becoming thicker over time. For this reason, books frequently had closures to limit the extent to which a book could expand. These closures may have been leather thongs, metal clasps, or latches made from a combination of metal and leather.
Early printed books quickly moved from the use of vellum to paper, which has a more even consistency and does not expand. Even though no longer necessary, the use of clasps continued throughout the fifteenth and into the sixteenth century.
The design of clasps became yet another form of art associated with bookbinding - the shape, decoration, and style of closures differ from bookbinder to binder. Many of these clasps have not survived over time, frequently because the leather part of the clasp wore and broke. Even when part of a clasp is missing, the book retains evidence that it once had a clasp, eithe rin the form of the metal tab the clasp would connect to, or in the presence of the nails once used to attach that metal tab to the book.
In addition to metal clasps, some early bookbinders utilized metal studs, called bosses, on the front and back covers to protect the covers from damage due to careless use. Bosses offered early artists further opportunity for creativity, in the form of shape, size, and detailed engraving.