Fig. 37.
Backing is perhaps the most difficult and important operation in forwarding. The sewing threads in the back cause that part to be thicker than the rest of the book. Thus in a book with twenty sections there will be in the back, in addition to the thickness of the paper, twenty thicknesses of thread.
If the boards were laced on to the book without rounding or backing, and the book were pressed, the additional thickness of the back, having to go somewhere, would cause it to go either convex or concave, or else perhaps to crease up (see [fig. 37]). The object of rounding is to control the distribution of this swelling, and to make the back take an even and permanently convex form.
Fig. 38.
If the boards were merely laced on after rounding, there would be a gap between the square ends of the board and the edge of the back (see [fig. 38]), though the convexity and even curve of the back would be to some extent assured. What is done in backing is to make a groove, into which the edges of the board will fit neatly, and to hammer the backs of the sections over one another from the centre outwards on both sides to form the “groove,” to ensure that the back shall return to the same form after the book has been opened.