Metal plates are normally used for intaglio printing. These may he copper, zinc, steel, magnesium or other metal, but cardboard, wood or plastic have recently also been employed. Various lines, dots, or other indentations are etched, scratched or engraved into the plates to create a design. Printing ink is forced into the indentations. Superfluous ink is first removed with gauze and then usually the palm of the hand, so that only a thin film of ink, or no ink at all, is left on the surface of the plate. The printing plate is then placed upon a solid steel plate: the bed of the etching press. A sheet of paper which has been moistened to make it malleable is laid upon the etching plate and then several woollen or felt blankets are placed over the paper. The solid steel bed, the etching plate, the paper and the blankets are run under high pressure between the two large cylinders of the etching press forcing the paper into the indentations where it picks up the ink. Thus the design is transferred from the plate onto the paper. The composition on the plate will print as a mirror image, so the design must be drawn in reverse of the desired final result.

The plate can be worked in different ways:

(a) using only mechanical means (e.g. engraving, mezzotint, drypoint), or

(b) using etching processes where mordants like ferrochlorides, nitric or other acids are used to create indentations in the plates. (e.g. line etching, relief etching, aquatint, softground, photo etching etc.) In most modern intaglio prints, a combination of these and other techniques will be found.

1) Engraving. The engraving is the oldest form of intaglio printing. It was developed in the early 15th century and was strongly influenced by and connected with the craft of the gold and silversmiths.

The instrument used to create the image is called a burin or graver. It is a steel rod about 10 cm long; its point is diamond shaped and the tip sharpened to an angle of 45 degrees. The graver is pushed over the plate at a low angle so that its tip bites sharply into the metal and a shaving is lifted clear. Most lines start as fine cuts, grow in thickness and taper to a fine point. This is a typical feature of the engraved line and is quite different from etched or drypoint lines.

2) Drypoint. As in engraving, lines are incised into a metal plate by purely mechanical action. Unlike the graver, which removes the unwanted metal as a fine shaving, the drypoint needle pushes the metal to the side creating a furrow. Burrs are formed on either side of the drypoint line. Ink will adhere to the fragile edges of the burr so that the lines in the print will appear slightly blurred and have the rich texture that is typical of the drypoint. The earliest drypoints were done around the middle of the 15th century by anonymous masters - one of them was the 'Master of the House-book'.

3) Mezzotint. For the preparation of a mezzotint plate a rocker is used. The rocker is a steel tool with a slightly rounded cutting edge which is serrated with fine teeth. The surface of the plate is roughened all over with this tool so that a print made from it would appear uniformly black. The image is worked up from the rough surface with a scraper and a burnisher to create areas that will hold progressively less ink. Those areas which are slightly scraped or polished will retain some ink for printing. The more highly polished parts of the plate will hold little or no ink at all. Thus a whole variety of half tones can he achieved, from a velvety black, through the most subtle grey tone to a brilliant white where the paper is left unmarked. This technique was developed by Ludwig von Siegen, and was most popular in the late 17th and 18th centuries as a means of reproducing paintings.

4) Line Etching. The metal plate is covered with an acid resistant ground. Etching grounds may consist of wax. pitch, mastic and asphalt. Lines are drawn into the hard ground with a steel needle exposing but not incising the metal beneath. When the plate is placed into an acid bath (e.g. diluted HNO3), shallow or deep lines will be etched. depending on the concentration and temperature of the mordant and the time that the plate is left in the acid. Lines that are etched sufficiently are stopped out with varnish. The plate may then he returned to the acid and those lines which are still open are etched more deeply. This process can be repeated many times. Then the ground is removed and the plate is inked and printed. The shallow lines will appear as fine grey lines on the paper, whereas the deep-etched lines will stand out in solid black. The etching process was first used on iron, mainly for decorating weapons and armour in the 15th century. Attempts to 'offset' or 'print' these decorative patterns can be seen as the forerunners of etching as a graphic art. Dürer etched on iron plates which were prone to rusting; soon copper became the most popular metal, continuing to be the most widely used to date.

5) Relief or 'Open Bite' Etching. Some areas of the plate are covered with ground and other areas are left exposed. The plate is placed into the acid bath and the mordant bites the open areas. Without removing the first drawn forms, new ones can be added and etched, creating the impressions of layers on the print. Ink will remain in the deeper parts of the plate, accumulating towards the edges of the bitten areas. This emphasises the relief character of the print.

6) Aquatint. The aquatint was developed to reproduce various values of grey similar to the tones in wash drawings and water colour. The invention of this technique is attributed to Jean Baptiste Le Prince, whose early works date from 1768.

Varnish is used to cover all areas that are not to be etched. The whole plate is then covered with a fine layer of asphalt or bitumen dust and heated so that the particles melt and cling to the metal. The plate is subsequently placed into an acid bath - or the acid is applied directly with a brush. The longer the acid is allowed to bite into the metal, around the molten drops of asphalt the more ink the plate will eventually hold and the darker the areas will appear in the print. Additional gradations can be obtained by also burnishing the aquatint as one would work on a mezzotint plate.

7) Softground. In this etching technique a sticky and soft ground ('vernis mou') is used instead of hard ground. When a sheet of paper is placed on the plate and a pencil drawing is made on the paper, the softground will adhere to the back of the paper wherever pressure is applied. It will lift from the plate when the paper is carefully removed. The plate is then etched in the usual manner. The character of the printed line will be similar to the line drawn on the paper.

By using the softground it is also possible to reproduce a variety of textures. Textiles, leaves, grass and such like can he pressed into the softground where they will leave an impression which can he etched and printed.

8) Photoetching. The plate used for this process is covered with a light sensitive and acid resistant ground. A high contrast black and white film positive, or a pencil drawing on drafting film, is placed on the coated metal plate and then exposed to strong UV light. In the plate developer, those parts of the coating which were beneath the black areas of the film dissolve. However, the areas which were exposed to the UV light harden and remain as an acid resistant ground on the plate. The etching proceeds as normal: mordant bites the areas that are no longer covered with the ground (i.e. the (lark areas of the original). The light areas of the original are protected by the ground and will eventually print light.

9) Colour Etching. From a large number of colour etching processes, only one is described here. A variety of colour tones can be achieved by using differently etched plates of the same size, inked in yellow, red and blue. The first plate (in this case yellow) is carefully registered and printed, but the paper is left in the press with the rim of the paper still under the top roller of the press, caught between the blankets and the bedplate. The yellow plate is removed, the second plate (red) is placed in exactly the same position as the first plate - and printed. The paper again remains in the press, the second plate is replaced by the third and last colour.

The first colour etchings were done by Jacob Christof Le Blon around 1725. He had studied Newton's colour theory and tried to apply it to printmaking with only limited success, but the principle of colour separation is basically the same as that employed today.

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