black, along with one or two colours and a flesh tone for faces and hands. In some cases, Evans may have used as few as four colour blocks: likely black, flesh and two primary colours; the addition of yellow allowed him a greater range. Each colour was printed from a separately engraved block; there were often between five or ten blocks. 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 match, although sometimes ink squash is visible along the edges of an illustration. Often the artist drew the illustration in reverse, and directly onto a block; in other cases the printer copied the illustration from a drawing. After the 1860s, images could be photographically projected onto the blocks, although it was more difficult for the printer to carve the reliefs without leaving the distinctive lines of the illustration. Books printed by Evans have been reproduced using some of the original blocks which have "remained in continuous use for over a century". Children's books Critics regard Evans' most important work to be his prints of children's books with from the latter part of the century with Walter Crane, Kate Greenaway, and Randolph Caldecott which revolutionized children's publishing. Early in the century children's book were often hand colored, and the chromoxylography processes Evans perfected "brought an immense improvement in coloured picture books for children in the last quarter of the century". In 1865, Evans agreed with publishing house Routledge and Warne to provide toy books—paperbound books of six pages, to be sold for sixpence each. They "revolutionized the field of children's books" and lent Evans his association with children's book illustrators. The market for toy books became so great that he began to self-publish and commission the artists for illustrations. When demand swelled beyond his capacity, he employed other engraving firms to fill the orders. The concept of a picture book for children, with the art dominating the text rather than illustrations supplementing the text, was an invention of the mid-19th century. According to Judith Saltman of the University of British Columbia, Evans' work as a printer of children's picture-books is particularly notable; she believes he printed the "most memorable body of illustrated books for children" in the Victorian era, and the three illustrators, whose works he printed, can