To make a half-tone plate of a photograph or other subject, it is necessary to break the negative up into lines and dots. It is for this purpose that the half-tone "screen" is used. The screen consists of two thin pieces of plate-glass, on the surface of which a series of very delicate parallel black lines have been ruled running diagonally across the glass. When these pieces of glass are placed together, face to face, the parallel lines ruled on them intersect each other at right angles, giving a very fine "mosquito-netting" effect. The method of making the negative is very similar to that described in making line negatives, excepting that in making a half-tone negative the screen is placed in the plate-holder directly in front of the negative. The subject is then photographed, and the result is a negative completely covered with a mass of fine transparent lines and dots.
Copper is generally used instead of zinc in making half-tone plates. In making a print on copper the light shines through the transparent lines and dots of the negative and hardens the sensitized surface of the plate. The black parts of the negative between the transparent lines and dots protect the sensitized surface. When the plate, after printing, is placed under a water tap, the parts of the sensitized surface that have not been acted upon by light wash away, leaving a print that becomes acid proof after being subjected to an intense heat.
The method of etching a copper plate is similar to that already described for etching zinc plates, excepting that sesquichloride of iron is used instead of nitric acid. In a half-tone the dots and lines are so close together that great depth is neither desirable nor possible, and no steps are taken to prevent undercutting.
The half-tone plate, after it has been carried as far as possible by mechanical processes, is capable of great improvement in the hands of skilful engravers. The plate as it comes from the etching bath may be termed a mechanical product. Though great skill is necessary in making the negative, the print, and the etching, the hand-finishing gives the plate many of its artistic qualities. The unfinished plate is apt to be more or less "flat" in appearance; the high lights may not be light enough, while the dark portions of the plate are apt, in cases, to be too light. The most common methods of finishing are reëtching and burnishing. The finisher dips a camel's-hair brush in acid and applies it to the high-light portions of the plate, or other places that are too dark, and allows it to act on the metal until these parts of the plate are lightened sufficiently. The parts of the plate that are too light are made darker by rubbing down the surface of the plate with a tool called the burnisher. The skilful, artistic finisher has other methods at his command of making the plate reproduce as accurately and as artistically as possible the original drawing or photograph. High lights are sometimes cut out entirely, or a fine engraver's tool may be "run" between the lines; while a "wood-engraved" finish is produced by cutting, in certain portions of the plate, lines similar to those used in wood engraving.