In the case of valuable books which are, for one reason or another, seldom referred to, or unique or presentation copies, it is a good practice to make slight essential repairs without disturbing the binding and to order, from an experienced binder, a book-shaped slip-case in which the volume may be preserved in its original covers without being subject to further wear or to injury from dust.

A fairly valuable book which must be rebound, should never be bound in calf or sheep, as these leathers, even when of the best quality, are very perishable. Sheep bindings, sometimes three hundred years old, may still be occasionally met with in remarkably solid condition. But the secret of such leather tanning seems to have been lost, and the modern sheep or calf binding cannot be counted on, even under the most favorable conditions, for more than one-tenth that length of time.

In certain climates, parchment or vellum makes a durable binding which, with age, acquires a beautiful, ivory-like surface tone; but these skins will warp the boards unless the book is kept closely set in on the shelf.

Turkey morocco is durable when well tanned, as it usually is. The best leather, for appearance and endurance, and also the most expensive, is red levant morocco. For efficiency and richness, although this is a matter on which tastes vary, it is best left "uncrushed" or, at least, only lightly pressed.

The best moroccos are those tanned entirely "acid free," or as nearly so as possible. "Niger" morocco, native tanned on the banks of the Niger River in Africa, and imported into England, is an acid-free leather used for expensive bindings. This leather is rather hard to secure, but its desirability is indicated by the fact that it is the only leather on which the severe tests described in the Report of the Committee on Leather for Bookbindings, elsewhere mentioned, had no effect.

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