ETCHING is an intaglio process in which the image is etched into a metal plate. The polished plate is covered with an acid-resistant layer, which is usually composed of resin, wax, and asphalt. The image is drawn into that layer with an etching needle. For the drawing to be more distinct, the plate is first covered with soot. Then it is immersed in an acid (the zinc plate — in nitric or hydrochloric acid; the copper, steel, or iron plate — in ferric chloride), which etches the image into the surface of the plate. After removing the acid-resistant layer, the plate is covered with printer's ink. As the plate is rubbed all over, the ink remains only in the etched lines. The image is then printed from the plate onto paper by means of a printing press. An etching plate can be bitten many times, and the method that will be chosen should be considered already in the phase of drawing the image. If biting only once, needles of various thickness are used: the background is drawn with a thinner needle; the foreground, with a thicker one. This results in a surface with thinner or thicker lines, respectively. In repeated etching, only one needle can be used, since the thickness of lines is determined by the time of etching the plate. The etching plate is more delicate than that of the copper engraving, hence it yields only 100 to 200 prints.