Bookbinding - the difference in thickness between the spine edge of a sewn book block and the fore edge. In a new book, the swell is made up of two elements: the thread and the fold. A book for rebinding will also have some swell from the previous backing operation.
- The thread used to sew the book block is the largest component of the swell. A single thread may seem too thin to cause a noticeable effect, but there are as many layers of thread as there are signatures in the book. It adds up. The thread is the only controllable element of swell. If the spine edge is too thick, a thinner thread should be considered. If it is too thin, use thicker thread.
- The folds in the pages of a book are another contributor to swell. No matter how firmly you rub the crease with the bone folder, the paper fibres will still retain some elasticity. This is actually a good thing; if all the fibres were to fold absolutely flat, they would break more easily, and your book's pages would tear out all the time.
- Re-bound books have an additional element of swell from the previous backing operation. The outermost signatures will be bent over, and the backs of most of the signatures will be significantly flattened and rounded. Unlike the other sources of swell, this is not a good thing, because it weakens each individual signature. As part of the tearing down process, any residual swell should be beaten out with a soft mallet.
The swell from the thread and the folding of the signatures is eliminated in the rounding stage of bookbinding.