2. PHOTO-LITHOGRAPHIC PROCESSES IN WHICH THE GRAIN IS NOT FORMED DURING THE EXPOSURE.
Of the many existing processes by which the breaking-up of the half-tones into printable lines or grain is effected, not in the negative, but subsequently by mechanical and chemical action on the stone or plate or the transfer film, I will only mention those {75} which appear to me to be the most important, the new methods which have already found firm footing in practical work or which appear likely to do so. All the older processes, which were either never actually successful or have been superseded on account of difficulty of working, we may properly pass over.
(A.) THE PATENT ASPHALT PROCESS OF ORELL, FÜSSLI, AND CO. USED FOR THE PREPARATION OF COLOUR PRINTS.
I have already mentioned that the use of light sensitive asphalt for the preparation of printing plates for the various branches is by no means new.
The fundamental principles of the well-known processes we find also used in the process of Orell, Füssli, and Co.
The grained stone is coated with a thin film of light-sensitive asphalt, the composition of which we have previously learnt, and exposed under a reversed or stripped negative. The image after correct exposure is developed, and then, according to the construction of the asphalt, in combination with the graining of the stone and the developer, there is formed a finer or coarser grain, or a closed or open image.
The development is effected with petroleum, to which one-fifth or one-sixth of its volume of benzine is added, or with turpentine alone. Development takes place rather rapidly, and after a short time the image appears on the stone. The print is then treated as was suggested in asphalt printing; the stone is first gummed, and then by rolling up with ink and etching rendered suitable for printing. The pictures produced by this have a very fine grain and are in appearance very much like good collotypes; still two or more tones are necessary to obtain an absolutely closed and vigorous action.
Very similar to Orel and Füssli’s process is the following, also patented.
(B.) THE COLOUR PRINTING PROCESS OF WEZEL AND NAUMANN OF REUDNITZ, LEIPZIG.
In this process also all the colour stones of a chromo picture are produced from a single stripped and reversed negative which is taken of the original. For the preparation of any colour stone there are three distinct stages. The first stage is the exposure, the second the development, and the third the development continued still further, and any necessary small help by the lithographer. In each stage a special mode of treatment is required, according to whether the stone in question is to be used for a light, a more vigorous, or full colour. The stones for grey and the light blue or light red inks—thus those for many surfaces, but with few details—are treated differently to those for yellow, the second red {76} or blue; and these, again, differently to those for brown or quite dark colours.