1. Materials and techniques
Source: Oxford University Press
After stopping-out (with varnish) areas that are to remain white, the image is formed by applying the aquatint ground of resin (or a substitute of asphalt, bitumen or pitch) using one of two methods. The first is to allow the resin to settle on the plate as a dry dust, usually by inserting the plate at the bottom of a box in which the dust has previously been shaken. The plate is then heated so that each separate grain of the dust-ground melts and adheres to the metal. The second method is to dissolve the resin or asphalt in alcohol (or equivalent distillate); this spirit-ground is then poured over the plate. The alcohol evaporates, leaving a thin film of resin which cracks in the final stages of drying.
The plate is then immersed in acid, which etches the metal in the gaps around the grains of resin, thus forming a very fine crazed pattern of etched lines. Like an ordinary line Etching, the ground is then cleaned off the plate, to which ink is applied. The ink penetrates the etched depressions, and when the plate is printed, it creates a very fine network of lines that gives the effect of a soft grain or wash. The process produces areas of only a single tone, but the density of the tone varies depending on how finely the dust was ground and how thickly it covered the plate. Because it cannot produce lines, the process is often combined with etching.
Gradations of tone for modelling forms can be achieved by using different densities of ground on the same plate, by burnishing the aquatint after it has been etched or by etching some areas of the plate longer than others to give a darker tone (either by tilting the plate out of the acid or by successively stopping-out areas with varnish). Another method of drawing an aquatint, more suited to making an artist’s print, is called lift-ground Etching (or sugar-lift etching). In this the artist draws directly on the plate with a brush using a sugar solution, which swells when immersed in warm water and lifts the varnish to leave the brushed design as exposed metal; an aquatint ground is then applied in the usual way (or it can be applied to the whole plate before the sugar solution is brushed on), but it will affect the plate only in the exposed areas. When etched and printed, this gives the effect of a brush drawing.
Aquatints can be printed in colour in two ways: different coloured inks can be applied to the plate à la poupée (i.e. with a rag stump or ‘dolly’), or a separate plate can be made for each colour and the several plates aligned for printing by a system of registration pins. Colour can also be added to aquatints by hand with considerable success due to the affinity between aquatint tone and watercolour washes.
From Grove Art Online