Chromoxylography was a colour woodblock printing process, popular from the mid-19th to the early-20th century, commonly used to produce illustrations in children's books, serial pulp magazines, and cover art for yellow-back and penny dreadfuls. In the 19th century the art of relief engraving and chromoxylography was perfected by engravers and printers, most notably in England by Victorian engraver and printer Edmund Evans. A wide range of hues and tones were produced by the color mixing of the chromoxylography process — a technique that was complicated, requiring intricate engraving and printing for good results. Less expensive products, such as covers for pulp magazines, were coloured with as few colours as possible, often only two or three, whereas more intricate and expensive books and reproductions of paintings used as many as a dozen or more colors. For each colour a separate woodblock had to be carved.
Full-colour printing in the 19th century relied on the relief process and colour wood engraving. Bamber Gascoigne explains that the "vast majority of colour wood engravings are reproductive work of the second half of the nineteenth century, at which time they were often referred to as chromoxylographs—meaning colour from wood, just as chromolithograph means colour from stone." In England in the 1830s, George Baxter repopularized colour relief printing, then called chromoxylography, by using a "background detail plate printed in aquatint intaglio, followed by colours printed in oil inks from relief plates—usually wood blocks". Relief prints were made by printing with engraved and coloured wood blocks. In the mid-19th century, colour printing processes for dime novels, penny dreadfuls, and children's book illustrations were rendered simply and often ineffectively in primary colours, using one colour per wood block, making chromoxylography a popular technique to colour illustrations of inexpensive books, such as chidren's toy books and serialized stories into the early-20th century. Edmund Evans, London's premier engraver and printer of children's books perfected the process, often using as many as 10 blocks, mixing and combining primary colours to create a broad palette of colours and hues.
 Methods and uses
To make a relief print, the printer began with a wood block, engraved the image on the block by carving out areas that were not to be printed (or inked). A wood block was carved for each primary colours, with the coloured ink coating the areas left in relief. Gascoigne explains that the process required a "master craftsman [to sit] with an original painting in front of him and work out which areas of the image should be printed in which of the available colours to achieve the desired effect." The printer engraved the image to the finer end grain of the woodblock. For more complicated work the carver worked on the end grain of the wood, and with the use of fine hatchings to the wood that were inked separately achieved the look of blended colours. For the children's book market, which had lower profit margins, the printer would use fewer ink colours, which could be optimised by mixing colours such as blue and yellow to create green.
Variations in tone were achieved with skillful carving to create the appearance of stipple. Areas that were intended to be printed in a solid colour were marked; hatching lines in the wood allowed colours to be overprinted, creating a variety of hues and tones. To create a blend of colours, blocks were hatched horizontally and diagonally to allow applications of multiple colours that resulted in browns, greens and greys. Gascoigne explains that a "blockmaker would know whether to engrave thin white lines (for an almost solid tone), medium white lines (a mid-tone) or crosshatchings (leaving larger or smaller lozenges of colour to achieve sometimes little more than a faint tint when seen at a normal viewing distance)." Overlapping diagonal lines were carved to create dot-like shapes on the surface that took less ink and resulted in paler tones.
Thicker hatchings were less expensive and easier to produce, but the result was not as attractive. Books that were to be produced as inexpensively as possible showed less engraving work and colour separation. The chief problem was to maintain correct register, achieved by placing small holes in precise positions on each block to which the paper was pinned. If done correctly, the register of colours would match, although sometimes ink squash is visible along the edges of an illustration. An electroplate was produced for each engraved wood block, inked according the block, thereby tinting the paper in specified areas. The colour was applied as a solid, or in stripes of various of thickness, allowing for changes in hue.
According to Gascoigne, 19th century chromoxylographs are identified by embossing on the back of the paper, distinctly delineated outlines, created by pressing the paper against the engraved blocks, and the presence of crosshatchings. He writes that "an impossibly and perfect and delicate area of crosshatching will suggest at first that the graver could not possibly have scooped out such small and regular interstices, but on closer inspection the lines in the two directions will be found to be of slightly different colours."
Because the process was inexpensive, and commonly used to illustrate covers of inexpensive books such as mysteries and romances ("yellow backs" or "dime store novels") or magazines, and in rare instances, newspaper covers, complicated colour combinations generally were not necessary. Most often the printer only used primary colors and black. The inexpensive technique of chromoxylography allowed publishers and printers to design covers as an attraction to purchase the book.
The process was also used to produce higher quality children's books and toy books. Evans considered full colour printing a technique well-suited to the simple illustrations in children's books. Evans reacted against crudely coloured children's book illustrations, which he believed could be beautiful and inexpensive if the print run was large enough to maintain the costs. In doing so, Evans collaborated with Walter Crane, Kate Greenaway and Randolph Caldecott. Books illustrated by Kate Greenaway, Walter Crane and Randolph Caldecott, engraved and printed by Evans, became popular and remain as classic examples of illustrations for children's literature.
Chromoxylography was additionally used to illustrate natural history books and to reproduce paintings. To achieve realistic reproductions an engraver often used 12 or more colour blocks.
 Examples of chromoxylography
Cover art of Edward Vile's penny dreadful Black Bess or the Knight of the Road showing few hatchings to create variations in hue and tone.
 Detail of a chromoxylograph
Image from Randolph Caldecott's "The House that Jack Built"
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