THREAD
At one time there was no question but that the best thread to be used in binding was linen. In the light of present knowledge it may be doubted if this is true under all circumstances. All commercially bound books, without exception, are sewed with cotton thread. Even such large books as the Century Dictionary, Webster's Dictionary and the United States Catalog of 1912 are sewed with cotton thread. Since it was unquestionably the aim of the publishers of all these books to issue them in the strongest possible bindings, it is reasonable to suppose that linen would have been used if it were really the best for the purpose. The main reason linen thread has been advised for so long is because its tensile strength is much greater than that of cotton. On the other hand, it is just as true that what we may call the fraying resistance of a cotton thread is greater than that of linen. Take two threads of the same size, one cotton and the other linen, make a loop of each thread with one inside the other, saw back and forth, and the cotton will always cut the linen thread. Furthermore, linen thread deteriorates in quality unless very carefully stored. The publishers soon discovered these facts, and since tensile strength is not an important attribute in a thread that is being used in a book sewing-machine, they use a cotton thread with thoroughly satisfactory results.
In the case of library binding, however, the books are always sewed by hand and the sewer, as she draws the thread tight, is obliged to use some strength; for this reason tensile strength is an important attribute. Hayes' Irish linen thread is generally specified. The Barbour thread is referred to by Mr. Dana and by others, but the writer is not familiar with its use.
Other materials used in binding are gold leaf, paper for end papers, tape, headbands, cord and various concoctions used in finishing the books. Gold leaf should be 18 to 22 karat, and neither aluminum nor silver leaf should be used. Paper for end papers will vary somewhat with the size of the book, but should never be lighter in weight than a 60-pound manila. Large libraries may find it desirable to have an end paper specially made for their own use. Tapes should be linen and not over three-eighths of an inch wide. Cord for use in sewing in the regular way (not overcasting) is a soft twine which can be obtained from all dealers in bookbinding supplies. For all books less than twelve inches in height use four-ply cord; for larger books, five-ply.