Whether the thread is of one (knotted) continuous length, as in hand sewing through the folds, or consists of a number of individual threads, as in the usual machine sewing, it is a fundamental part of the make-up of a book. When consideration is given to the strain imposed on the sewing in the course of the normal handling of a book, especially a large volume, it is obvious that the thread used in sewing a book must be both strong and durable.
Sewing thread, whether for a publisher's binding or a book sewn by hand, is selected with regard to the thickness (and softness) of the paper, as well as the number of sections to be sewn. A thread that is too thin will not give sufficient swelling in the spine to enable the book to be properly rounded and backed, while thread that is too thick will produce a book that has too much swell, and that leads to difficulties in nipping and rounding and backing, as well as a tendency to produce creases in the inner margin.
Cotton thread is a fine continuous strand produced by plying two or more lengths of cotton strands with a tight twist and smooth finish. t is also made in varying thicknesses for various types of sewing. Although it is usually bleached, and bleaching is detrimental to the durability of thread, it is, and has been for more than 100 years, the most widely used thread for book sewing, especially in edition and library binding. The breaking strength of cotton thread commonly used in bookbinding is as follows:
Thread Size Number Breaking Strength (in pounds) 50 2.5 36 3.4 24 4.4 12 8.5 10 11.0 10 14.0
One of the disadvantages of silk and terylene thread is their tendency to unravel and "catch-up" during sewing.
This can be overcome by soaking the thread in polyvinyl acetate, diluted to about three times its original volume with water, followed by drying of the thread in air under normal conditions. This cements the individual filaments together slightly, and also insures that the tension is shared equally among all of the filaments and not thrown onto only one, as might otherwise occur if one filament in a loose bundle is slightly shorter than the others. The use of PVA does not appear to have any adverse effect when the accelerated aging test is applied to the thread, although it naturally reduces the softness and flexibility of the thread to some degree. For the most part, these threads are used only in machine sewing.
Linen thread is produced from straw of the flax plant. See: LINEN .
It is usually unbleached and is superior to cotton thread in both strength and durability. Thicknesses of threads used vary from Number 12, which is relatively thick, to Number 30, which is very fine and suitable for most books. At one time linen thread was used extensively in library binding, but it has largely been replaced by cotton, synthetics (principally nylon), and combinations of cotton and synthetic sewing threads.
Nylon thread maintains a smooth, knot-free surface and is stronger than thread made from either cotton or linen. It has the advantage of being relatively inexpensive and can be manufactured in finer grades so that books sewn with it can be more easily nipped, rounded, and backed, as the swell caused by the sewing is not excessive. Its principal disadvantages seem to be that it may cut the paper if the diameter of the thread is very small, and it has a tendency to contract upon release of tension, so that when a group of sewn books is cut apart the nylon thread tends to contract and loosen the end sections. (81 , 92 , 198 , 259 , 335 , 339 )