The usual material for the etching plate is copper, although the earliest German etchings, at the beginning of the 16th century, were made on iron plates. Since the third quarter of the 19th century zinc has often been used, being cheaper and more sympathetic to coarser effects. Brass, aluminium and even composite metals, such as tinned or chromed steel, have also been employed. To prepare the plate, it is highly polished, then coated with an acid-resistant ground, dry to the touch but soft and flexible enough to be drawn through with a metal point without flaking.
The usual ground is formed from wax, asphalt and a resin such as mastic. A harder ground in which a drying oil is substituted for wax was developed in the second quarter of the 17th century for etchings produced in imitation of engraving. In the 18th century a softer, sticky ground was invented, which included tallow or, more recently, petroleum jelly. This so-called ‘soft ground’ is used under a membrane such as thin paper on which the drawing is executed with a pencil or stylus. Thus instead of being scraped from the metal plate by the point itself, the ground is lifted from the plate where the pressure of the point has adhered the ground to the reverse of the paper; the resulting line reproduces the texture of the overlaying paper. In the 20th century other substances, such as fabric, string etc have been used to pick up the ground, imprinting their individual silhouettes and textures.
The conventional wax-based ground is traditionally formed into a small ball wrapped in lint-free silk or other cloth and applied by rubbing the ball over the surface of the metal plate, which has been warmed. The melted ground is smoothed and thinned with a dabber, a pad for dabbing ink on plates or blocks, which is also wrapped in lint-free cloth or leather. Melted etching ground may also be spread with a roller made of hard rubber, linoleum or leather, or dissolved in a rapidly drying solvent and brushed or poured over the tilted plate.
After the ground is laid, it is usually coated with soot from a smoking flame to increase the contrast between the uncoated ground, which is luminous brown, and the metal exposed by drawing. It is thought by some that the smoking action enables the ground to resist the action of acid better. Alternatively, a white pigment may be incorporated into the ground, to simulate on copper the effect of a line drawn in red on white paper.
Preliminary drawings may be transferred to the grounded plate by means of carbon paper (or some comparable intermediary pigment layer), or a drawing in loose pigment (such as graphite or chalk) may be laid face down on the grounded plate and run through the roller press. This latter process obviates the need for the composition to be designed in reverse. The artist draws on the grounded plate with an etching needle or metal point sharp enough to penetrate the ground but polished so that it does not catch on the plate. Points may be of various thickness, but the width of the etched line is usually determined by the process of biting rather than drawing.
Alternatively, in a process possibly invented by the 17th-century Dutch etcher Hercules Segers and greatly developed in the 19th century, the artist may execute his drawing on the ungrounded plate in water-soluble ink, usually containing a high proportion of sugar syrup, then covering it with a water-permeable ground. The plate is then immersed in water so that the areas of ground above the ink lift away as the ink dissolves. If the drawn lines are of substantial width, they may be covered with an Aquatint ground, which ensures better retention of printing ink. Once the design has been executed on the grounded plate, the plate is exposed to the action of acid, so that all areas of bare metal are corroded. This acid, known as the mordant, may be nitric acid, usually diluted to half strength or less, or a combination of chemicals, such as hydrochloric acid and potassium chlorate (so-called Dutch mordant), ferric chlorate, potassium dichlorate, dilute sulphuric acid, hydrochloric acid (for aluminium) etc. It is the action of the mordant that gives printed etched lines their characteristic coarseness compared to engraved lines of equal strength; bitten grooves always retain a degree of roughness compared to the sharp incisions made by a burin or engraving tool, and this characteristic is transmitted to the ink lines formed in the grooves.
The various mordants each have restrictions as well as advantages; the by-products of such chemicals include the release of gas. Nitric acid, depending on its concentration, can bite very rapidly, forming gas bubbles along the lines. These must be continually brushed away for the bite to continue evenly, although their presence is a useful guide to where and how rapidly biting is progressing, so that foul biting or lack of reaction may be detected early on. In the case of ferric chloride, a gentle and predictable mordant, the action of biting forms sediment in the grooves, impeding its continuation. To forestall this, the plate is usually bitten while suspended upside down in the acid bath, so that the sediment falls away from the grooves. Thus the benefit of a dependable mordant is somewhat offset by the inability to monitor its action. Dutch mordant, another easily controlled agent, tends to bite more directly downward rather than also eating away at the sides (a characteristic of nitric acid). It is therefore preferred especially in areas of close hatching where individual lines are at risk of merging into a broader pit. However, as Dutch mordant does not release gas as it bites, its action cannot so easily be followed except by periodic probing of the lines to determine their depth. In sum, mordants are selected depending upon the type of metal plates, the sequence of intended biting procedures and the ultimate technical and visual effect desired.
Acid may be applied in various ways. Traditionally, a grounded plate was edged with a wax lip forming a shallow pan into which the acid was poured. Another method was to lay the plate, its edges and underside coated with an acid-resistant varnish, into a wooden or earthenware tray set at an angle over a receptacle: acid was repeatedly poured over the plate, which was rotated so that the acid affected all lines equally.
From the 19th century it became more common for the plate, its edges and back protected, to be immersed in an acid bath. Acid may also be applied directly to local areas of the plate with a brush or feather, the plate ungrounded in the lavis process (in which the plate is worked to create tonal areas imitative of ink and wash drawing): or it may be mixed into a paste which is spread on to the plate. These techniques permit local areas to be bitten with varying degrees of strength. Using acid in tandem with a viscous liquid (such as saliva), the action of the acid and the area affected may also be modified. After the lines have been bitten to the desired width and depth (determined by probing the grooves with a needle as they form or by timing the bath in a mordant of known strength), the plate is rinsed to halt the action of the acid and the ground removed, traditionally by polishing with charcoal or by the use of a suitable solvent.
Plates are commonly executed in a sequence of bitings in order to obtain tonal gradation through variation of line width. After the first biting, those lines that have been shallowly bitten and that are to remain the lightest in the final composition are covered with an acid-resistant substance, such as grease, fluid ground or varnish, in a procedure called stopping out, so that those lines which are to appear darker are further exposed to acid. Alternatively, additional lines are drawn on the grounded plate, so that lines already bitten will bite more deeply in a second bath compared to the new lines, whose exposure to the mordant action is shorter; or the plate may be re-covered completely with a transparent ground on which new lines are drawn to be bitten in a completely separate operation. A combination of these sequences of biting, stopping-out, redrawing and regrounding may also be undertaken. Each acid bath exposes the plate to additional risk of foul biting, and the more complex the process the greater the chance of technical miscalculation.
Once the plate is bitten and the ground removed, it is printed in the usual manner of all intaglio plates, by wiping ink into the lines, wiping clean the surface of the plate and passing the inked plate together with dampened paper through a roller press under concentrated pressure. Similarly, corrections to the plate are prepared in the same manner as in other intaglio processes: the faulty area is burnished or scraped down, the plate being pounded level from behind if a substantial amount of metal has been removed; its surface is polished before regrounding and re-etching. There is no technical obstacle to combining etching with other intaglio processes on the same plate. The final printed image may therefore contain etching, engraving, drypoint, aquatint and mezzotint in any combination, although different artists and schools of printmaking have held vastly differing ideas about the value of purity of technique.