Aquatint

Aquatint is used to achieve tones and textures. The copper plate is covered with a porous ground (using small dots of acid-resist). This allows the plate to etch except where you prevent it by stopping out with varnish. By stopping out at various stages during the biting, a variety of tones can be achieved from the lightest of greys through to the deepest black.
Source: Etching, Alan Smith
The best method to use in class, (as we experimented with an older airbrush, but did not achieve best results) is to use the spray paint method:

• place the plate at an angle in the acid room
• shake the aerosol well
• work with the spray can between 2in to 6in from the plate, moving in straight lines from left to right
• an even covering, allowing about 50% of the metal plate to show in between the dots works well (the more you cover the lighter the tone, the less you cover more copper will be exposed and then the copper will print darker).
• You can spray, etch, block out more objects with hard ground, etch again, etc.
• The areas that are blocked out the earliest will print the lightest, and the more you put the plate in the copper, the darker those areas will etch.