A gravure printing technique similar to etching or mezzotint. Unlike etching, where image texture is simulated using lines or hatching, aquatints use rosin and acid to produce color tones on the plate. The traditional method for achieving this effect is to first dust the plate with powdered rosin, then dip it in an acid bath to remove parts of the plate not protected by the rosin. To make the rosin adhere, the plate must be heated; this can occur either before or after the rosin is applied. Of course, mixing powder and heat can be dangerous. Breathing large amounts of rosin isn't healthy, either.
The result is a rough surface covered with tiny rings that will hold ink in the same way an etched line does. The texture, which can range from a light, subtle gray to nearly black, depends on the amount of time the plate is left in the acid and the density of the rosin particles.
To create a full image, the printer uses a protective stop-out varnish on the areas he wishes to remain light (i.e., non-printed). This is followed by an application of the rosin and the acid bath. The printer then applies more varnish as a "stop" over finished areas. This process is repeated until the plate is done.
One of the masters of this technique was Francisco Goya.