Another way to achieve tonal areas in etching bedsides doing an aquatint, is by doing a soft ground.  Soft ground is different than hard ground in that the mixture of asphaltum (tar-like substance) is thinner.
The benefit of a soft ground is that it can be easier to move around the ground, take impressions of fabric or remove it from your plate in various ways. (It is also easier to see the tones you are etching with a soft ground over an aquatint).
The technique most frequently used with soft grounds is to use a thin textured fabric like silk to create tones. Once the thin layer of soft ground is dried, a thin texturized fabric is placed flat over the plate, a piece of matboard on top and all brought through the press.  The texture has now been transfered and can be permanently etched in ferric chloride.
Another way to use soft ground is to paint a thin layer on with a sponge brush, place a piece of wax paper on top, and draw with any type of textured tool (even graphite and crayon textures work). The textured lines can now be etched in ferric. This process makes for easy transfer of drawings.