2 Other Processes
Besides the asphalt other mixtures of other light-sensitive substances may be used for direct printing, and these are mostly organic substances in combination with a chromium salt, and possess also a greater sensitiveness to light than asphalt.
The solutions which have been specially used in our establishment consist of—
| (A). Albumen | 5 | g. |
| Ammonium bichromate | 3 | g. |
| Distilled water | 500 | g. |
| Ammonia | 15–20 | drops |
| (B). Chloroform | 250 | g. |
| Benzine | 50 | g. |
| Alcohol | 20 | g. |
| Mastic | 2 | g. |
| Aniline red | 2 | g. |
Solution A is strongly sensitive to light, and is used for coating the stone or zinc plate; with solution B the stone is coated before developing. The details of the process are as follows:—
(A.) Coating the Plates
After the stone has been carefully polished and well dusted it should be first flowed over with distilled water, in order to increase the possibility of spreading the sensitive solution. When the water has run off, the stone whilst still damp should be flowed over with solution A, and care should be taken that the same is evenly distributed and that no bubbles form. The stone should be placed immediately on the whirler and rotated till the solution is dry. Obviously a thick film must be avoided, as in such a case the print would not adhere to the stone.
If a zinc plate be used it must be well prepared and be rendered free from grease. To prove this the plate is flowed over with distilled water, and when this spreads equally all over and is not repelled from any part, the solution can be poured on. If, however, the plate does not willingly take water everywhere, it must be again cleaned or polished. The other operations are the same as for the stone.
(B.) the Printing
When the film is quite dry, printing may be proceeded with. The duration of printing depends obviously also here again on two important factors—the quality of the negative and the strength of the light. Under a good negative with absolute bare glass in the lines of the drawing I print to about 10° Vogel, for example:—
| In direct sunlight (middle of April, 11 a.m.) | 2 | min. |
| In the shade (middle of April, 11 a.m.) | 5 | min. |
| In electric arc light (3000 c.p., at a distance of 50 cm. = 20 ins. without reflector) | 25 | min. |
The picture is faintly visible. If the negative was too thin, or if it be over-printed, the image develops badly or not at all. With under-printing the film washes right away.
Immediately after printing the stone or plate is coated with solution B, which is distributed as evenly as possible by rocking backwards and forwards. It is not necessary, however, to obtain even distribution with a whirler. The whole of the coated surface becomes deep violet.
(C.) The Development
When solution B has become completely dry, the stone is placed in a suitable vessel with pure water. After about five minutes the places not affected by light dissolve if the stone is rocked, and it is only necessary to go over the drawing with a pad of cotton wool to clean the same completely. The drawing now appears a deep violet on a bright ground. The development is continued without rubbing strongly with the pad—which might rub up the drawing—till all the details of the drawing appear clear and distinct. This especially applies to the details in the deep shadows.
(D.) Further Manipulations
When the stone or plate is correctly developed, which can be easily told, it should be well rinsed and again exposed to make the film adhere firmly. It should then be gummed and treated as previously described in the asphalt process. The “rubbing up” of the stone or plate is not necessary; it is better to roll up the drawing with an ink roller. Zinc plates for deep etching can be lightly etched, then dusted with fine asphalt of resin powder, heated and then further etched.
R. J. Sachers, of Toronto, Canada, published a sensitive solution which has the following composition:—
| Distilled water | 150 | parts |
| Fish glue | 50 | parts |
| Ammonium bichromate | 2 | parts |
| Ammonia | 2 | parts |
| Chromic acid | 1 | part |
This solution is brown, not very transparent, with an olive green tinge, and must stand and settle at least twelve hours before it can be decanted, filtered, and used. The time of exposure is about—
| Direct sunlight | 2–5 | minutes |
| Diffused light | 12–15 | minutes |
| Electric arc light (3000 c.p., 50 cm.) | 30 | minutes |
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.