When a half-tone negative hag been made the film is stripped from the glass plate and reversed, as in the zinc-etching process, though some half-tone engravers use a mirror box or prism by which the picture is so disposed on the negative that it does not need stripping and reversing. A perfectly flat, clean, and highly polished copper plate, generally large enough to accommodate several such films, is then coated with a sensitive film according to one of several formulas, all based on the fact that gelatin or some similar body, if sensitized with certain chromic salts, becomes hardened and insoluble in water on exposure to light. This plate is then placed in the printing frame in contact, under pressure, with the glass negative plate and is exposed to light in the usual manner. The copper plate is then removed from the frame in the dark room and made ready for etching.
For etching half-tone plates on copper a saturated solution of perchloride of iron is used instead of the solution of nitric acid used for zinc etching. The time of etching ranges from about 5 to 15 minutes, according to the strength of the solution. One etching is generally sufficient, but it may be necessary to give the plate another "biting" if it has not been etched deep enough, or to re-etch it in order to strengthen contrasts. If, for instance, the sky in a half-tone plate shows too dark or is uneven in tint it can be made lighter or more even by re-etching. On the other hand, if certain features on a plate are too light they can be darkened by burnishing--rubbing the surface with a highly polished steel burnisher under just sufficient pressure to flatten slightly the fine points that form the printing surface of the plate. When the plate leaves the hands of the etcher it is turned over to the finisher, who with a graver removes spots or any other imperfections that may appear on it. Sometimes a roulette is used to lighten parts, and other tools are used for special purposes.
After a plate that shows two or more pictures has been etched and finished it is divided by sawing them apart. Each one is then put into a beveling machine, where its edges are trimmed and the usual border is made, if it is desired. The separate plates are then ready to be proved and mounted on blocks of wood which make them type high, ready for printing.