Engraving is a form of intaglio printing, meaning an image is printed by inking an incised (engraved) surface. The engraver uses a tool called a burin to incise lines into the plate. A burin is a small steel rod with a sharpened point, which leaves a distinctive V-shaped groove in the metal. To print an image the plate is inked and the surface wiped clean so that only the grooves retain any ink. To ensure that all the ink is transferred to the paper they must both be placed in a press which can apply great pressure. To facilitate this transferral the paper is dampened. This process means that the paper is left with an indentation called a plate mark from where the edges of the metal plate were pressed into the sheet. It is a much more physically demanding technique than etching because it requires great skill to incise an even line. Engraving is often combined with etching, as in some of Hogarth’s prints.
Etching is also an intaglio technique, but the recesses in the plate are achieved chemically rather than manually. The plate is coated with an acid-resistant ground. The artist then draws with an etching needle, which easily scrapes through the ground leaving lines of exposed metal. The plate is then immersed in acid, which bites (corrodes) into the copper plate where it has been exposed. If the artist wants some lines to appear deeper than others so that they will print more heavily, these lines can be exposed for a second immersion whilst protecting the other lines with some kind of acid-resistant varnish. When the ground has been cleaned off, the plate is then ready for printing.
Aquatint is based on the same principle as etching, using acid to bite into the plate. Aquatint will create areas of tone, rather than lines. This is achieved by allowing the acid to penetrate a resin ground to form patterns of small pits in the metal plate’s surface. Aquatint is also commonly used with etching, which delineates the areas of tone.
Lithography is a planographic rather than intaglio printing process, so-called because an image is printed from a smooth surface. It is based on the natural repulsion of water and oil. The artist draws on the lithographic stone (traditionally a limestone block) with greasy pen or chalk. The stone is washed with water and then printing ink is applied with a roller. This ink will only affix to the drawn lines, and not the damp parts of the stone. The image can then be printed off on a sheet of paper. Because no pressure is applied at the edges of the stone, there will be no plate mark. Lithography is capable of much longer print runs because it does not suffer the same surface deterioration that occurs with intaglio methods.
Hand colouring is very distinctive, identifiable through clues including that of uneven application, where an area of colour does not quite reach or overlaps a printed line, and that of uneven colouration due to a thicker layer taking longer to dry. The use of colour in etched caricatures became increasingly common, although they were sold as plain sheets as well for a lower price.