[Illustration: FIG. 16.--Section of colour-block. A. Colour mass. B. Depression. C. Surface of Plank.]
When the register marks corresponding to these colour forms have also been cut, and the paper washed off the blocks, the clear spaces may be used for pasting down new key impressions for the smaller colour patches and their corresponding register marks. In this way one side of a colour plank may contain several different colour forms and sets of register marks. As a rule the different colour patches would be printed separately, though in some cases two colours may be printed at one impression if they are small and have the same register marks.
When the blocks have been cut and cleared it is advisable to smooth with sand-paper the edge of the depression where it meets the uncut surface of the wood, otherwise this edge, if at all sharp, will mark the print.
For any particulars about which one may be in doubt, the sets of blocks at South Kensington Museum or in the Print Room at the British Museum are available for examination. In one of the sets at the British Museum it is interesting to see the temporary corrections that have been made in the register marks during printing by means of little wooden plugs stuck into the register notches.
In nearly all cases the Japanese blocks were made of cherry wood, but planks of box are said to have been occasionally used for very fine work.
However exactly the register marks may be cut in a new set of blocks, very puzzling errors occasionally arise while printing, especially if the planks are of thin wood.
Some of the blocks are necessarily printed drier than others. For instance, the key-block is printed with a very small amount of ink and paste. Other blocks may be even drier, such as the blocks which print small forms or details in a design. The blocks, however, which are used for large masses of colour, or for gradated tones, are moistened over the whole or a large part of the surface of the block, and if the wood is thin, and not well mounted across the ends, the block soon expands sufficiently to throw the register out. If the block is not mounted across the ends there will also be a tendency to warp, and this will add to the errors of register. But if the blocks are of fairly thick wood, and well mounted, the register will remain very exact indeed.