Aguatint is a traditional printmaking method that resemples watercolor, and is a variant of the etching, making a design onto a metal plate by the use of acid.
Aquatint uses powdered rosin to create a tonal effect. The rosin is applied in an enclosed "dust box". Air is rotated in the box and then turned off, letting the rosin dust settle evenly onto the plate.
The dust is an acid resistant and adherred to the plate by placing the zinc plate on a plate heater.
This resistence to the acid is what is creating the bit of texture.
Just like the etching; marks are made onto the plate that can hold the ink and then run through a printmaking press with a sheet of paper, reversing the image onto the paper.
At first, areas wanted to be kept white are covered with a ground, that blocks the acid. Then the plate is etched by leaving it in the acid bath for a short amount of time. After this, more and more areas are covered up, subsequently leaving covered areas "stopped" at the tone it was etched to, and uncovered areas to keep going darker in tonality.
The longer time exposed areas are left in the acid bath, the deeper the acid "eats" into the plate, and the more ink those areas can hold. The more ink; the darker tonal color they will leave on the printing paper.