Accurate folding is impossible if the "register" of the printing is bad, that is to say, if the print on the back of a leaf does not lie exactly over that on the front.
Crooked plates should usually be made straight by judicious trimming of the margins. It is better to leave a plate short at tail or fore-edge than to leave it out of square.
The old "joints" must be knocked out of the sections of books that have been previously backed. To do this, one or two sections at a time are held firmly in the left hand, and well hammered on the knocking-down iron fixed into the lying press. It is important that the hammer face should fall exactly squarely upon the paper, or it may cut pieces out. The knocking-down iron should be covered with a piece of paper, and the hammer face must be perfectly clean, or the sheets may be soiled.
Guarding--Throwing Out--Paring Paper--Soaking off India Proofs--Mounting very Thin Paper--Splitting Paper--Inlaying--Flattening Vellum
Guards are slips of thin paper or linen used for strengthening the fold of leaves that are damaged, or for attaching plates or single leaves.
Guards should be of good thin paper. That known as Whatman's Banknote paper answers very well. An easy way to cut guards is shown in fig. 8. Two or three pieces of paper of the height of the required guards are folded and pinned to the board by the right-hand corners. A series of points are marked at the head and tail with dividers set to the width desired for the guards, and with a knife guided by a straight-edge, cuts joining the points are made right through the paper, but not extending quite to either end. On a transverse cut being made near the bottom, the guards are left attached by one end only (see fig. 9), and can be torn off as wanted. This method prevents the paper from slipping while it is being cut.
A mount cutter's knife (fig. 10) will be found to be a convenient form of knife to use for cutting guards.
In using the knife and straight-edge a good deal of pressure should be put on the straight-edge, and comparatively little on the knife.
[Illustration: FIG. 10.--Mount Cutter's Knife]
To mend the torn back of a pair of leaves, a guard should be selected a little longer than the height of the pages and well pasted with white paste (see page 288). If the pair of leaves are not quite separated, the pasted guard held by its extremities may be simply laid along the weak place and rubbed down through blotting-paper. If the leaves are quite apart, it is better to lay the pasted guard on a piece of glass and put the edges of first one and then the other leaf on to it and rub down.