Conventional etching depends on the design being etched into the copperplate (intaglio). Blake, however, decided to reverse this method by etching in relief, that is to say, he etched away the background and left the image and text standing up in relief, as in a woodcut. To do this he used an acid-resistant liquid and drew and wrote, composing directly onto the copperplate (with brushes and a quill pen). When the acid-resistant liquid dried and hardened, the plate would be exposed to acid which etched away the uncovered parts of the copper. Because printing always reverses the design, the text had to be written backwards, and slanted appropriately, in order to appear the right way round when printed. Writing in this way may appear to be a daunting prospect but, as an experienced reproductive engraver, Blake was well trained in the skill. The plates were then inked with one or more colours and printed under light pressure in an etching press. Not only were the raised parts of the copper plate inked, the shallow (etched-away) areas could also be dabbed with colour and printed simultaneously. The sheets were subsequently hand-coloured with watercolour washes by Blake or his wife Catherine. The colouring of Blake's early printed books, such as the Songs of Innocence exhibited here, is characterized by the use of subdued colours and a limited colour range. However, the late colour prints, such as plate 51 from Jerusalem displayed nearby, were usually printed in strong, warm tones and richly hand-coloured. The evolution in Blake's approach to illuminated printing that accompanied this change has been described as a shift from the idea of 'print-as-page' to 'print-as-painting'.