A printmaking method in which the design is bitten into the plate with acid; the term is also applied to the print so produced. The design is drawn with a steel etching needle on a metal (usually copper) plate that has been coated with a waxy, acid-resistant substance. Where the waxy coating (or ground) is scratched through with the needle, the bare metal is exposed, and when the plate is placed in an acid bath, the acid bites only the lines so exposed. The depth to which the lines are bitten (and hence the darkness with which they will print) depends on how long the plate is immersed in the acid, and it is possible to achieve subtle variations of tone by ‘stopping out’ part of the design (covering it with a protective varnish) while other parts are bitten more deeply. This process of graduated biting may be repeated any number of times. In spite of this potential for reworking, etching is, in essence, a much more spontaneous technique than line engraving, as the artist can draw on the waxy ground with virtually the same fluency as with pen or pencil (it is even possible to carry a prepared plate to be used like a sketchbook, as occasion demands). Etching is thought to have originated in the use of acid to decorate armour. Prints were first made using the technique in the early 16th century (the first dated example, of 1513, is by Urs Graf), and it reached exalted heights in the 17th century in the hands of Rembrandt, the greatest of all etchers. Rembrandt often touched up his plates with drypoint, and etching has also been combined with other techniques as well as developing offshoots such as crayon manner. Several major artists of the 18th century made memorable use of etching, including Canaletto, Piranesi, and Goya (who usually combined it with aquatint), but by the early 19th century the technique was used mainly for commercial illustration. From the 1860s to the First World War, however, there was a great renewal of interest in it as a medium for original expression, especially in Britain. Whistler and Sickert were leading lights of this movement, which is called the Etching Revival. Etching is still a popular technique, Hockney being a leading contemporary exponent.