Etching is an intaglio method of printmaking in which the image is incised into the surface of a metal plate using an acid. The acid eats the metal, leaving behind roughened areas, or if the surface exposed to the acid is very narrow, burning a line into the plate. Etching is also used in the manufacturing of printed circuit boards and semiconductor devices.
There are many ways for the printmaker to control the acid's effects. Most typically, the surface of the plate is covered in a hard, waxy ground that resists acid. The printmaker then scratches through the ground with a sharp point, exposing lines of metal that are attacked by the acid. Once the drawing in the ground is finished, the plate is submerged in acid for a period of time; longer submersion means that deeper lines are etched. The ground may be removed, or the artist may continue drawing in it, etching it again in acid to deepen existing lines while adding new lines. The ground may also be reapplied to protect existing lines while adding new ones. The ground may be removed and the plate printed to see its current printing state, to be followed by more work (or other techniques, such as aquatint, drypoint, or engraving); thus, many of Rembrandt's etchings exist in several distinct forms, with the final, "accepted" version apparently preceded by many artist's proofs.