Aquatint is a method of achieving gradations of tone through a very fine network of lines or dots on the plate. However, the lines and dots in the aquatint are formed through a different technique than is used with an etching, in which the image is formed by scratching through the ground.
The artist making an aquatint forms the image through the application of the ground itself, instead of applying a ground over the entire plate and removing the portions that are to be acid-etched.
To start, the artist selects a material for the ground which is capable of being reduced to fine particles, of being well-fixed to the polished-copper plate, and of resisting acid. The most commonly used grounds for aquatint have been resin (such as from pine trees) and asphalt. The artist scatters this material on portions of the plate as a ground-up dust and then heats it, causing the material to melt and stick to the plate. Alternatively, the material might be dissolved in alcohol and poured onto the plate, whereupon the alcohol evaporates, leaving behind the dissolved ground, but typically with a pattern of cracks like dried mud on a floor. In either case, the ground which remains adhered to the plate leaves a pattern of dots and cracks. It is in those areas, where the acid will etch the exposed plate, creating the etching which will then be inked and pressed on the paper.
Aquatint got its name because it was developed in the last quarter of the Eighteenth Century, when watercolors were enormously popular, because this technique allowed the etcher to imitate the effect of a watercolor wash.