Aquatint is a ground, a material that resists acid, but it is not a solid coating on the plate. Aquatint is used for making tones, and is composed of fine particles. The acid bites around the particles, creating a tooth in the plate to hold ink. The metal plate is first dusted with fine particles of rosin. Elaine de Kooning dusted the rosin on by hand in her Torchlight Cave Drawing #1, and you can clearly see the individual grains.
If the artist wants an even tone, the printer creates a rosin dust storm in an aquatint box so that the particles will sift onto the plate evenly.
After the rosin particles are on the plate, they must then be heated to adhere them. After that, the artist paints varnish or asphaltum (tar) on the parts of the plate where he or she does not want an image, and the plate is submerged in acid. The acid bites around each tiny grain of rosin. The deeper the bite, the more ink is held in the plate and the darker the tones will be in the print.
Alex Katz’s Reclining Figure was entirely done with aquatint. The tones are slightly grainy, but they are even because the rosin was applied in a box.
If the artist bites the plate unevenly by painting the acid over the prepared aquatint surface, the print is called a spit bite aquatint. Spit biting gives an effect similar to watercolor washes, as seen in John Zurier’s Pale Spring.
If the artist paints a liquid soap mixture on the plate instead of varnish to protect parts of the plate from the acid, the print is called a soap ground or white ground aquatint. The soap breaks down in the acid, creating irregular tones.
You can see Pia Fries using a tool to draw into the soap ground on a plate. Since the soap is a resist to the acid, the image is white on a dark background when printed.
To make a sugar lift aquatint, the artist draws on a plate fluidly with a brush dipped in a sugar and water solution. Unlike soap ground, which is acid-resistant, the sugar solution must be removed, or lifted, before the plate goes in the acid. Aquatint is laid on the plate, and it bites in the portions drawn by the artist.