The image appears a bright brown on the golden yellow ground of the plate. The development is effected in an aqueous solution of methyl violet, when the image assumes a beautiful blue violet colour. The developed image is hardened with alcohol. The results by this method are very good, and although this is recommended for half-tone etching, it is also suitable for photo-lithography.
Excellent results also attend the sensitive “Marine glue solution,” prepared by A. Hebensperger, of Munich, to which a second solution called “Chlorogutt-losung” is added. The sensitiveness to light is very high, and the treatment about the same as with the first described substance. The “marine glue solution” consists of albumen, bichromate of soda and ammonia. The dye solution of Victoria blue dissolved in benzine.
The process published by Waterhouse consists of coating a prepared zinc plate with a thin chromated solution, and then drying in a few minutes with a gentle heat—
| (A.) Arrowroot | 2 | parts |
| Potassium bichromate | 9 | parts |
| Water | 700 | parts |
| (B.) Alum | 1 | part |
| Water | 1 | part |
For use mix—
| Solution A | 40 | parts |
| Potassium bichromate | 5 | parts |
| Solution B | 15 | parts |
This is printed under a reversed negative for about six minutes in the sun. The plate is then laid for about half an hour in cold water in order to remove the chromium salts, and then cleansed from all soluble parts with a pad, and after well washing set up to dry. It is then inked up with transfer ink, and after about fifteen minutes washed with turpentine. This process, as, indeed, all direct printing processes, offers many advantages for preparing colour plates from a coloured object with one negative. The register of the separate plates is very exact, which is not always the case with a transfer process. In reproducing line drawings the unnecessary parts on the stone or plate can be covered.
CHAPTER V. THE HALF-TONE PROCESS FOR PHOTO-LITHOGRAPHY.
THE half-tone process is used in photo-lithography to reproduce originals which do not contain distinct lines or points, but closed tones, which are executed either in one or several colours. With this process any coloured picture, oil, water, pastel, indian-ink or sepia drawing, as well as any photographs from nature of living, moving scenes, or still life objects can be prepared for reproduction on the printing press.
I will first describe briefly the theory of the process, and then come to the practical part.