For preparing larger pictures on stone or metal plates, in which the grain must be coarser than with small pictures, the fully-developed and still damp pigment image is dusted with powdered resin and the film allowed to dry with the powder adhering to it. The pigment image thus dusted is treated, as previously described, with alum and glycerine, exposed to the action of the sandblast, and then etched with phosphoric or nitric acid.
As the soft pigment film receives a grain by the dusting, this is reproduced of the same size by the action of the sandblast on the film of varnish on the stone or plate.
The principal points of this process of preparing photo-lithographs or phototypes in half-tone, with a grain, are as follows:—
1. From the photographic negative a pigment image is taken {79} and transferred to a litho stone or zinc plate, which has been previously coated with varnish.
2. The pigment image is then exposed to the action of the sandblast; and
3. Then the pigment image is removed, and the image obtained on the film of varnish is etched.
In the K.K. Hof und Staatsdruckerei photo-lithographs as well as phototypes have been prepared with excellent results by this process. The former in result are similar to the productions of the Orell and Füssli process, and the latter were noticeable for good gradation. On the other hand, it cannot be denied that this process, which is patented, has been already surpassed by autotypy, which is far simpler and easier to work, and is founded on a scientific basis.
(D.) LITHO-HELIOGRAVURE.
This process, invented by Chas. Eckstein, of Hague, the general director of the Topographical Bureau of the Royal Netherland General Staff, also belongs to those processes of photo-lithography by means of which any picture in half-tone can be reproduced. The breaking up of the half-tones is effected, as with the autotypic methods, by the use of a lineature, but instead of this being done in the photographic exposure, it is effected direct on the stone before the transfer of the photographic image. In this particular process the stone itself is not coated with a light sensitive film and printed on direct, but the ruling is made on the stone, and then a negative pigment image—a diapositive—is transferred to it exactly the same as with photogravure on copper. A further feature is that the result is not an ordinary transfer stone result, but an intaglio etched stone, which is printed from the depressions.
For this original process the basis is a ruled pattern stone, prepared with great care and accuracy. For preparing this a grey stone of the best quality should be taken, and one without any chalky spots, crystals, or holes. This should be polished with a coarse cloth and oxalic acid solution till it has a very high polish. On the highly-polished stone surface is spread a very thin but even film of asphalt.