ETCHING ZINC PLATES.
The plate is now cleaned by passing it for a few seconds through a very weak acid bath (nitric acid, 1 in 100) just suffi ciently long to dull the surface. It is then rinsed under the tap, the adherent Scum being removed with cotton-wool, and dried at a gentle heat or by an air current. Any retouching, spotting, or ruling is next done with lithographic writing ink applied with a fine sable brush or a ruling pen, as the case may be. Scratchers, etching needles, and other tools, are used for clearing away black spots, making white lines, etc. Parts re quired to be white are scraped away, while those which have to be black are drawn or painted over. The back and margins of the plate are now varnished with a resist, generally consisting of shellac in spirit. When this is dry, the plate may be placed in the etching bath, which should preferably be a 1-per-cent.
solution of nitric acid, although some workers recommend a stronger bath. With the strength mentioned, etching may take as long as thirty or forty minutes, during which time the bath must be regularly rocked. The disadvantage of the stronger bath is that it may possibly soften the film or cause ragged dots. It is said to be a good plan to immerse the plate in a weak solution of chromic acid before etching, with the object of remov ing any granularity caused by the burn ing-in ; this also expedites the etching process. Troughs of the kind shown by Fig. 909, with sloping, covered ends to prevent splashing, are very convenient ; although in more elaborate arrangements the rocking may be done by motive power, a lever being provided for stopping the motion when required.