Flexible glue, which does not become too hard either before or after use, is made by mixing glycerine with good glue. It has no special advantages for use by binders, but can be used in the library itself for some of the repairs which assistants may have to make.
PASTE
(For paste used in mending, see [p. 167]).
The best paste is that made in the ordinary way with flour and water. The paste should be fresh, for if it is not a certain fermentation takes place which is injurious to the book, especially to leather.
BOARDS
Not the least important part of the make-up of a finished book are the boards forming the sides. There are practically four grades of boards which are known as strawboards; binders, cloth or millboards; semi-tar, and tar boards. The cheapest grade is the strawboard, for which there is no place in library work.
Mill or cloth boards, used for practically all library work, are generally made from waste papers of all kinds, though some may have an admixture of rags. Clay is also mixed with the waste paper pulp and the whole subjected to heavy pressure in the final stages of manufacture. They are made in two sizes, 20 × 30 and 23 × 29 inches, and are put up in 50-pound bundles, the number of sheets in the bundle indicating the thickness of the board. The thickest is a No. 12 which has 12 sheets in a 50-pound bundle. They may have as many as 70 sheets in a bundle, which makes a very thin board. A 20 or 25 board should be used for fiction and juvenile books and a 16 or 18 for heavier books such as magazines. Prices of good cloth boards run from $40 to $50 a ton.
Semi-tar boards, made from better grade stock than mill board, should have in their composition a certain amount of rags and hemp or naval cordage. They come in the same sizes and weights, costing about $70 a ton.
Tar boards are the best boards obtainable. Made of naval cordage, they are very tough and hard to work. Many of them are allowed to season, in much the same way as lumber seasons, in order to prevent warping. They are much too expensive for library use, sometimes costing as much as $110 a ton.