A method of etching introduced in France in the 17th.Century enabling the printmaker to create a more subtle range of tones than was possible with traditional techniques.
Paul Sandby(English, circa 1730-1809) developed the process further so that he could recreate the appearance of a watercolour and his ability with this technique made his work very influential.
Powdered resin is applied to a metal plate which is then heated causing the resin to adhere to the plate. The plate is then immersed in an acid bath and the exposed metal between the drops of resin is bitten by the acid, creating a pitted, grainy surface. When ink is applied to the plate the bitten areas hold the ink which, when printed, create areas of tone.
The longer the plate is immersed in the bath, the deeper the acid bites and the darker the resulting area will print. Thus, a plate may be immersed several times in order to create a range of tonal areas. Further, an acid-proof "stop" can be applied to areas of the plate thereby extending the tonal possibilities.