Timeline of Technique
The three processes of printmaking Intaglio processes
During the sixteenth to nineteenth centuries, three principal processes were used in book illustration: relief, intaglio, and planographic. In relief processes the background of the design is cut away, leaving the design on the surface of the block or plate. When the block is inked, only the surface receives the ink, which is transferred onto the paper by rubbing or pressure from a printing press. Indentations from the block can usually be felt on the reverse side of the sheet.
In intaglio processes, the design is made below the surface of the plate through a series of grooves. In order to affect transfer from the plate to the paper, a special press, capable of exerting considerably more pressure than a regular printing press, is employed. Illustrations created through intaglio methods usually leave visible indentations of the outer edges of the plate from which they were produced.
Planographic processes, based on the mutual rejection of grease and water, differ from relief and intaglio, in that the design is drawn directly onto the surface of the plate with a grease pencil, which is fixed to the plate with a solution of gum arabic and acid. The plate is then dampened before printing ink is applied. Only those parts that are greasy accept the ink. Transfer from plate to paper is made by the pressure of a special press. Since the design is on the surface of the plate, no indentations are made on the paper.
The oldest of the three methods, relief, predates Gutenberg's invention of printing in Europe in the 1450s by several hundred years. In the first fifty years after the invention of printing book illustration almost exclusively employed the relief process in the form of woodcuts.
Normally the block was made the same height as the type, and could therefore be locked into the press along with the type and impressed together . Woodcut illustrations continue to appear in books throughout the sixteenth century, gradually being superseded by copper-plate engraving, which was deemed to be a superior method for illustration. Woodcuts continued to be used as illustrations in books at the cheaper end of the market up to the nineteenth century.
The relief process enjoyed a revival at the close of the eighteenth century through wood-engraving. Despite its name, a wood-engraving is prepared in essentially the same way as a woodcut, but differs in several respects. Firstly the design is cut on the end grain of the block, which is stronger and less liable to splintering than the side grain used in woodcuts. Secondly, a sharp tool called a burin is employed to produce much finer lines than the knives and gouges used in the woodcut. Finally, when the block is inked and impressed, the lines below the surface print white, thus producing an image consisting of white on black, in contrast to the black on white of the woodcut
Although woodcuts and wood-engravings constitute the vast majority of relief prints, other materials were used, such as linoleum, and metal. Although generally rare, there are examples of etched metal relief plates in the collection .
Wood blocks were not usually signed by the artist, though the cutter's monograms are occasionally found incorporated into the design .
Until the introduction of colour-printed illustrations in the eighteenth-century, colouring was a manual process. One notable exception to this in early medical illustration was the chiaroscuro woodcut of the brain found in Gaspare Aselli's De lactibus (Milan 1627). [The Fisher Library possesses no copy of this edition; it is illustrated in Christies' catalogue of the Haskell Norman sale, #251]. It made use of multiple wood blocks, the background being printed black on which washes in two shades of brown and red were superimposed. Non-inked areas printed white, while additional hatched shading produced a three-dimensional effect. But the chiaroscuro woodcut was not really suited to show the fine detail necessary for anatomical illustration, and was rarely used in book illustration.
In the nineteenth-century coloured wood engravings are frequently encountered .
In intaglio processes the design was usually made on a burnished copper plate. There are two principal methods used in intaglio printmaking. In the first method, engraving, the design is incised directly into the surface of the plate with a burin or other sharp tool, that removes a thin sliver of metal shaving from the copper plate. In the second, and much more common method, etching, the design is produced through the action of acid or other mordant. The design is drawn with a needle that penetrates the acid-resistant wax or resin coating previously applied to the plate. Then the plate is immersed in a bath of acid, which bites into those parts exposed by the needle. Deeper lines are produced by re-immersion in the acid, after lines which are not to be deepened have been stopped out with acid-resistant varnish.
The earliest copper plate illustrations in books appeared in the 1470s, though only in isolated instances. In the sixteenth century they were increasingly common, and became the principal process of book illustration by the mid-century and after . In the early nineteenth century, intaglio processes were overshadowed by the revival of wood-engraving, but by mid-century made a brief comeback through steel engraving, which had the advantage over copper by virtue of its harder surface from which more impressions could be taken.
Without the use of a microscope it is very difficult to distinguish engraving from etching. In any case, most illustrators employed a combination of the two processes .
Another kind of intaglio process used in medical illustration, though rarely, was the mezzotint - a method of engraving in tone, invented in the second half of the seventeenth century. In this process the surface of a copper plate is roughened with a serrated tool called a "rocker", which raised a uniform burr. The burrs are either left, or flattened to varying depths with a scraper, to produce gradations of tone ranging from deep black to greys. The burrs left untouched and the spaces between them hold the ink, and produce a solidly black, velvety texture. Further flattening and polishing with burnishers and scrapers produce highlights which print white. In contrast to other methods of engraving, in which the artist darkens a white surface with lines, hatchings, and dots, the mezzotint artist works back from dark to light, creating continuous tonal transitions from dark to light, and delicate half-tones. Under the microscope the roughened surface of the plate can be clearly visible and has the appearance of threadbare cloth. Most often employed to reproduce and to popularize the paintings of great masters, especially portraits, mezzotints were rarely used in scientific illustration. The effect of mezzotint was often stunning, though a major disadvantage was that the delicate burrs of the plate became quickly worn through the frequent wiping off of excess ink, and the pressure of the press. Although the images on this site include no examples of monochrome mezzotints, there are several examples of hand-coloured mezzotints .
Colour printing on a wide scale did not develop until the eighteenth-century. It was not until the publication of Sir Isaac Newton's Opticks in 1704, that the development of colour printing began to make significant progress. Newton's treatise provided a sound scientific foundation for colour theory, and it was not long before experimentation in colour printing was attempted. On the Continent the leading experimenter in colour printing was Jacques-Christophe Le Blon (1667-1741), who developed a three-colour method for printing illustrations. By using plates inked in the primary colours of yellow, blue, and red, Le Blon found they could be combined in various ways to produce any desired hue. Each plate was prepared using a mezzotint technique, which provided gradations of shading. Le Blon had acquired a privilege to publish a book of anatomical illustrations, but he died before the project could begin. One of Le Blon's pupils, Gautier d'Agoty, appropriated his master's method, adding a fourth plate in black, which provided additional shades of grey and black . Gautier d'Agoty claimed the invention of four-colour printing as his own, for which he obtained a copyright.
Yet another intaglio process, popular in the eighteenth century, was stipple engraving. This method is invariably connected with the name of Francesco Bartolozzi, an Italian artist who worked in Britain. This method combined the techniques of engraving and etching. Firstly, the outline of the design was etched onto the plate. After the ground had been laid, the design was transferred to the plate by the pressure of the press. The stipple engraver worked on the outline in a series of dots or specks made with an etching needle, or toothed roulette. Dots varied in size: strong shadows and darker portions were applied with coarser point, and were distributed comparatively far apart. Lighter and more delicate parts, such as flesh, were composed of finer and closer dots, varying in texture and grouping. The dots were intended to imitate the marks produced by crayon or chalk on paper. The plate was then etched in the usual way, stopping-out before each biting. Coarser or closer combinations of dots often merged into each other, producing a velvety richness. The plate was then cleaned off, and finished with the burin before inking and impressing. Under magnification the dots can be clearly seen. Coloured stipple engravings are often encountered, either hand-coloured  or colour printed .
Aquatint, another intaglio process, popular in the late eighteenth and early nineteenth centuries, was employed mainly to reproduce landscape paintings and watercolours.
Aquatints are created by etching a copper plate through a fine granular acid-resistant ground, consisting of resin attached to the plate by heat. The hardened particles resist acid, while the tiny crevices between the particles are attacked by the acid. The crevices hold the ink and print as a tone. Tonal values are controlled by the density of the resinous particles, and the depth of the etch. In areas where lightest tones are required, stopping out with varnish is used. Each successive bite produces darker tone. Tones may also be achieved through "feathering" - applying acid with a brush while lighter tones may be attained through scraping or burnishing. Aquatints were rarely used for anatomical illustration. Fine examples of hand-coloured aquatints may be found in Vicq d'Azyr's Traité d'anatomie et de physiologie … les divers organs de l'homme et des animaux (Paris, 1786), of which there is no copy at the Fisher Library [illustrated in Roberts & Tomlinson, facing p. 532]. The illustration of the anatomical theatre at Cambridge is also a hand-coloured aquatint.
Intaglio plates were very frequently signed by the artist and the engraver. The artist's name appears in the white space beneath the image on the left hand side, followed by "del." (=delineavit); the engraver's or etcher's name appearing on the right side, followed by "fecit", "sc." (=sculpsit) or other such term .
The principal planographic process employed in book illustration is lithography , invented by Alois Senefelder in 1798. As the name implies, lithographs were originally produced on a stone, though later zinc plates were used. Coloured or tinted inks were used from around 1830 to produce tinted lithographs which were then finished with hand-colouring . Chromolithography, patented in 1837, employed multiple stones, one for each colour, requiring considerable skill in alignment . By the 1830s lithography became the most common form of medical illustration, and remained so for the remainder of the century and beyond.
Plates were signed in a similar way to intaglio plates, with the artist's name to the right, the person responsible for transferring the image to the plate on the left, and often the name of the lithographic printer added in the centre. 
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