Corrections on and other imperfections will now probably make their appear ance, which had before been unsuspected, brought to light by the burning in of the enamel. The plate should, therefore, be examined under the magnifying glass and these corrected with a fine engraver. They will appear most injuriously in the high lights, and the remedy is to trim away the film between and so relieve the dots. This is rather a delicate operation and takes careful handling.
The Etching etching fluid for copper is a solution of perchloride of iron. It is advisable to use it at full strength at first, gradually diluting. It is a curious fact that the strength of the solution is to a certain extent increased by the addition of water. Make up sufficient to cover the plate well but not so deeply as to prevent seeing its action in etching. A serious ob jection to using the fluid full strength is, that it is so opaque as to prevent the process of etching being observed and watched.
The the solution of chloride of iron into a shallow dish and insert the cop per plate. The action of the iron will be at once seen in a change of color, resulting from the accumulation of sediment from corrosion. Brush this off with a soft camel's hair brush, and do so with a light circular motion rather than broad sweeps over the plate. Do so every half minute or so, when it is seen to accumulate over the plate. Occasionally blow the liquid from some part of the surface and keep a keen watch whether any of the dots show signs of etching away, when it must be at once stopped. Should the enamel give way too readily under the etching it was probably too acid, or the exposure under the printing frame had been too short, or the burning in not carried far enough. It may etch deep enough in five minutes to print well, but it is better, for giving greater ease in making ready at press and better printing results, to give a deeper etch, and if the enamel does not show signs of giving way, fifteen minutes may not he too long to give it. The progress and depth may be examined by the finger-nail upon the margin and stopped when it is judged far enough ; then taken out and brushed well with the camel's hair brush under the tap, then dried by gentle heat over the stove.
Enameling the zinc be the metal chosen to work with, nitric acid is employed in etching. Zinc plates,, readily polished and prepared for engraving purposes, may be obtained,
and it is greatly preferable to so purchase them than undertake the labor and trouble of polish ing. It must, however, get a final polish with engravers' charcoal before flowing with the enamel solution ; and to eradicate any greasiness on the surface it may be placed for a few minutes in the lye used for cleaning the glass plates, and then given the final polish. The process of flowing the plate with the enamel solution is exactly similar to that described above for copper; but in burning in, the zinc, being a much softer metal, will not stand the amount of heat that copper does, and so the enamel cannot be burned to so deep a color, and, therefore, greater caution is respired to burn to a degree sufficiently hard and yet not bring the zinc to the melting point.
Etching on Zinc —A very weak solution of nitric acid is all that is necessary in etching half tone work on zinc. A solution of i drachm acid to io ounces of water, which will to taste be about the strength of vinegar, will be found right. It is usual to use a rocking tub, in which the plate is placed and the solution flawed over from end to end, while the oxidized zinc is gently brushed away ; it may be done quite as well, however, in a small porcelain or other flat dish as with the copper plate, and the same watchfulness of the film under the action of the acid must be exercised.
The Etching construction of the rocking tub for zinc etching will be readily understood by the following illustration of the latest and most improved form : All that than remains is to mount the plate on wood to the height of type and take an impression from it on the printing press.
PROOF SPIRIT.—This term was originally employed to denote alcohol just strong enough to ignite gunpowder when poured over it and fired. A mixture of about equal parts of distilled water and absolute alcohol. It is defined by the Act 58 Geo. III., c. 28, to be " such as shall, at a temperature of 5I° of Fahrenheit's thermometer, weigh exactly 4-1 parts of an equal measure of distilled water." Its specific gravity = at 15°C., and it contains 491 per cent. by weight of absolute alcohol.