The most perfect work is said to be obtained by the latter method. There are many technical difficulties to be encountered in working the process, difficulties which for twenty years kept it at the experimental stage, and of little practical utility, as, for instance, after etching the plate for a short time the acid has a tendency to bite laterally as well as vertically, and so undermine and weaken the lines intended to be left standing. This is obviated by some photo etchers in using certain chemicals, and others by an ingenious manner in the working by etching in stages. This is done by re-inking and dusting with bitumen, heating the plate after each “etching,” which has the effect of melting and spreading the resinous or bituminous ink on the face, and slightly down the sides of the lines. Successful work is only attained by attention to the various delicate technical manipulations, which only experience can give.
Pen and Ink Drawing, reproduced by process block.
Reproduced by half-tone process block,
from a Monochrome Sketch,
reduced in size.
HALF-TONE PROCESS BLOCKS.
The subject, whether it is a drawing, engraving, painting, or a photo print, must in each case be photographed to obtain upon the negative the texture which is the means of producing the gradations of light and dark we find in the finished block. The theory is simple enough, but the practice is extremely difficult, and one requiring great skill and care. A glass screen having cross lines ruled upon it, with perfect mechanical regularity (about seventy lines to the inch for large work, up to two hundred lines to the inch for very fine half-tone blocks, as may be best suited to the subject in hand). The screen is fixed in the camera between the lens and the negative to be exposed. The fine black lines of the screen are reproduced upon the negative as white lines, breaking up the picture into a series of minute square dots, of varying size according to the light and shade of the original picture. A photo-transfer is then taken as previously explained. (In Hentschel’s process the secret of the prepared transfer paper is in the possession of Mr. Hentschel solely.) After being transferred to the zinc, and manipulated with certain chemicals, the plate is gradually etched until a sufficient depth has been obtained. It is then trimmed and mounted type height ready for the printer.
