Aquatint is a technique used to create areas of continuous tone on an etching plate, rather than tones created by hatched lines.
First, a negative of the image of the area where you want tone is painted on the plate with an acid resistant ground. If the plate was etched at this point the exposed area would become an open bite that will only print a light uneven tone. To achieve an even tone you need to apply an aquatint to the plate before it is etched. Inside an aquatint box, powdered resin is blown into the air to create a cloud of particles. The printing plate is then placed in the bottom of the box, so the powder can slowly and evenly settle on it. The plate is carefully removed from the box and heated up from below. The resin particles melt into little drops and bond with the plate. The plate is then exposed to acid creating tiny indentations in the plate’s surface around the resin drops and little plateaus where they have covered the plate. When the plate is printed, the ink sits in the indentations, but is wiped clean from the plateaus. In the final print the aquatint will appear as areas of even, flat colour.