SENSITISING THE METAL.
After sufficient washing, the plate is allowed to dry, or is dried by a gentle heat if wanted quickly. if intensified with copper, it will require varnishing first, or the copper will chemically affect the bichromated film used in printing. Varnishing is done by flowing a weak solution of gum arabic over the plate while wet. Printing is now generally executed by the enamel or fish-glue pro cess. The copper or zinc plate is thoroughly polished with a paste of whiting, to which a few drops of ammonia is added, applied with a tuft of cotton wool. It is then coated with a bichro mated solution of fish glue. There are various formuhe for this, but the follow ing is well recommended:— Le Page's fish glue (clarified) ... 2 ez.s.
Water Ammonium hichremate ... 50 grs.
The bich•omate is first dissolved in the water, and then added to the glue, and is stirred vigorously till perfectly mixed. The mixture is then filtered through a piece of thick felt. A whirler is
commonly employed for coating. This is a device for rapidly and evenly distribut ing the sensitising mixture over the plate, and drying it quickly. Some patterns arc arranged to hold the plate face downward over a gas burner (see Fig. 907) others take it face upward as shown by Fig. 908. This is a very good plan, especially for large plates, but is more liable to collect dust on the wet film. A simple hand pattern of whirler is illus trated by Fig. 389, p. 271. The solution is poured in a pool on the centre of the plate, and spread over about three-quar ters of the surface, leax ing a little margin uncovered. The plate is then whirled by turning the handle of the whirler at a moderate speed, neither too fast nor too slow ; about four revolutions of the plate per second will probably prove correct.