With this "screen" placed nearly in contact with an ordinary photographic plate, film to film, we proceed to make a copy negative, in the camera, of the original picture. This, when developed, will give the picture and an image of the screen, which has been interposed, together on one plate.
If this is now clear to my readers, they will probably at once see what will result when such a plate is used on a sensitised sheet of zinc or copper, and the soluble parts of the film washed away as before suggested.
The parts of the film made insoluble from exposure to light will be reticulated all over by minute soluble dots or lines where the image of the screen has interrupted the light, and more so or less in proportion to the lightness or deeper shade of the original, and by this means what would have been unbroken blacks are sprinkled over, so to speak, with tiny white interstices, the ink when applied remaining on the alternating projections of undissolved film.
Suppose the film to have been of bitumen and spread upon a sheet of zinc, we should have a reticulated image in insoluble bitumen with interstices of plain zinc. This bitumen forms a protective coating, so that when immersed in a weak solution of nitric acid the acid only eats a way into the bare metal. Gradually, and by subsequent acid baths, the parts covered by the film are left in strong relief, and in a fit condition to print from. The film which has thus resisted the acid is then washed away, leaving the zinc relief.
To carry out the above process many details, which I have not thought it the office of this book to enter into, will be required. Thus the solvent used for developing, or, in other words, washing away the soluble portions of a bitumen film, is turpentine; but water is used in the case of bichromatised albumen. An acid resisting preparation is finally applied to the plain zinc relief, and the whole block re-etched or "re-bitten," so as to strengthen the image; certain precautions, moreover, are taken to prevent the acid "under etching" the image—and a great deal more which, of course, would have to be clearly described were it intended to teach the process of block-making.
In the variety of half-tone blocks, known as Typogravure, a different method of breaking up the surface is adopted; no intervening screen is used, but the surface of the metal has imparted to it a preliminary roughness or grain, and the image is printed and etched on this rough surface. These blocks, when carefully printed from, yield exceedingly nice results, the grain having something of an "aquatint" character, which appears to be more discriminating than that derived through the use of the ruled screen. The softness of outline and freedom from anything like a mechanical texture is well seen in blocks made by this method. The remarkable difference obtained from the same block by different printers will at once suggest that a very great deal depends upon the printing quite irrespective of the quality of the block itself. Many letterpress printers make a specialty of block-printing, the chief art being in the "making-ready" and "underlaying," by which terms is understood the careful adjusting of the block, so that its surface be at exactly the proper elevation to secure the proper amount of pressure, neither more nor less, when on the printing machine. Some further remarks on this subject will be found in Chapter X.
UNTOUCHED HALF-TONE FROM PHOTOGRAM.