Etching isa method of making prints from a plate, usually copper, into which the design has been cut by acid. The copper plate is first coated with an acid-resistant etching ground, using a roller or dabber, through which the design is drawn with a sharp tool called an etching needle. The ground is usually a mix of beeswax, bitumen, and resin. The plate is then exposed to nitric acid which eats away those areas of the plate unprotected by the ground, forming a pattern of recessed lines. The depth of the line, and thus its darkness when printed, is determined by the length of time the plate remains in the bath, and also by the strength of the acid. When the plate is applied to moist paper, the design transfers to the paper, making a finished print. There are certain restrictions in using this technique. The artist cannot expose an area larger than a thick line, nor can he draw lines too close together, because the acid will bite under the intervening 'ground' and the acid will attack the plate indiscriminately. Print makers sometimes combined both etching and aquatint processes in the same print. During the late 18th and early 19th centuries, a technique called soft ground etching became popular. This involves drawing with a pencil on a sheet of paper placed over the copper plate coated with an extremely soft, sticky ground. The ground adheres to the paper wherever the pencil passes, leaving the metal exposed in broad, soft lines. The plate is exposed to acid as in normal etching, and when printed, gives results similar to pencil or pastel drawings.