The Printed Book
by Harry G. Aldis, M.A. Cambridge at the University Press 1916
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Book illustrations Part 9
Several methods of producing colored prints were brought out in the eighteenth century. Of these, colored aquatint, that is, pictures produced by aquatint engraving and afterwards colored by hand, is by far the most important so far as book illustration is concerned. For the first thirty years of the nineteenth century it was the chief process employed in this country, and most of the colour books published by R. Ackermann during that period were illustrated by colored aquatint plates. Chromo-lithography, which succeeded aquatint as the principal method of printing in colors, came into practical use soon after 1840, and in its early stages was associated with the names of Day and Son and Owen Jones. Previous to this its parent process, lithography, had passed through various stages, including hand-colouring and lithotint, the latter familiar in illustrated books of about 1840, in which the whole picture, except the high lights, is covered with a yellow tint. The cheap children's books, which were printed in great numbers from the latter part of the eighteenth century, were mostly colored by hand. For the last sixty years or so wood-block printing in colour has been much used for this class of book. The books of Walter Crane, Kate Greenaway, and Randolph Caldecott were produced by this method, and owe not a little of their artistic success to the skill of their printer, Edmund Evans, who excelled in this art.
The Three-colour process, now so popular for the reproduction of pictures or objects in colour for book illustration, is based on the half-tone process, and, like it, is a purely photo-mechanical method. The success of the older methods of colour printing was dependent in large measure upon the artistic skill of the craftsman, or artist as he might in some cases be called; but, except in the operation of actual printing, the three-colour process affords little scope for artistic craftsmanship. In printing three-colour work three half-tone blocks, representing the primary colors, are employed. The negatives for these blocks are taken through filters of colored glass or glass cells containing colored liquid, in addition to the ruled screen. Each of these light filters allows only certain colors to pass through to the negative and stops the passage of all others.
The colors of the original are thus automatically dissected and grouped in three categories representing approximately the yellows, the reds, and the blues, each of which is contained on a separate negative. Of the three process-blocks made from these negatives that representing the yellow tones is printed first in yellow ink, over this impression the' red block is next printed, and finally the blue. The various colors and tints of the resultant picture are formed by the combination of these three colors printed over each other and varying in proportion according to the density of the printing surface of the respective blocks. The use of the right amount of colour and degree of pressure in printing are important factors in the success of the operation. It is also essential that the register should be absolutely accurate, that is to say the three impressions must follow each other in exactly the same place on the paper, or the result will be the blurred effect occasionally seen in cheap prints.