Colour blocking, which has gained such unexpected popularity, gives the finisher no little trouble. As blind blocking is done by the press, so also may colour blocking be done if the blocks are coloured with the brilliant colours specially prepared for printing. A colour roller, made by moulding English roller composition on a wooden pin, is used for colouring the blocks. The worker himself may cast these rollers in a well-greased mould. The roller composition is dissolved in water, and after being poured into the mould is allowed to stand a few hours to cool. The colour blocker keeps about three rollers ready, of which one is used for light colours, another for middle tones, and the third solely for black. The rollers are either fixed in a fork with two handles or an axle goes through the wooden pin which is bound in leather at each end wherein the axle revolves in the hollow of the hand.
The rollers spread the colour upon a stone of suitable size. The colour should be laid on the rollers—not on the stone—not too thickly, and then evenly distributed on the stone by bringing the roller over it backwards and forwards. Both stone and roller must always be cleaned after use; perfect cleanliness must be carefully observed throughout. The stone is cleaned with petroleum, the remaining colour is removed from the roller by passing it several times over a sheet of glass, rubbing it down with petroleum before again using it so as to remove any particle of dust adhering.
In order that the colours may appear as bright as possible, a little varnish is added to them, which in every case must be of a kind to suit the particular colour. Dammar varnish is suitable for light colours, and for darker colours amber varnish. Where great brilliancy is required, Venetian turpentine is added; but this lengthens the time required for drying if a little too much is added.
In every case, however, a little siccative must be added to every colour; a few drops materially quicken the drying.
Colour blocking can only be done when the blocks have been secured to the plate, as this must be opened each time it is coloured.
The simplest colour-printing is where one colour appears between the impression in gold, and in this case it is generally done by first making the impression in gold and then in colour.
In more complex work several colours are to be used; at the present time the work is frequently produced requiring colours side by side and superimposed, together with gold, bronze, and aluminium.
The process is in each case as follows: From the block supplied by the engraver an impression is made which forms a matrix. It is best to select a piece of board exactly the size of the covers to be printed and at the two places to be taken by the guide points pieces of vellum are firmly pasted, being already pierced for the guide points, these vellum additions being further secured by pasting over them a strip of paper or cloth. For every block to be used a similar board with its vellum additions is prepared. Upon the ground of this first matrix the other auxiliary matrices are likewise first impressed with the block—all of them black, of course, so that it would be possible at any time to fit in the blocks.
Every new block for colour printing is pressed into the ground of the auxiliary matrix belonging to it; also for the so-called first-impression block.