The resulting absorption band is closed until E, reaching from violet to red (over C). This red pigment was examined by Eder, who obtained very good results, using ammonia in the solution.
The corresponding screen is a combination of malachite green with acridin yellow or acridin orange N. 0.
For the blue color plate the dye is made up as follows:
| By weight | |
|---|---|
| Stock solution, No. 3 | 0.5–1 part |
| Distilled water | 100 parts |
| Nitrate of silver (1.500) | 100 parts |
| Ammonia | 1–2 parts |
This dye yields a strong band, commencing at B, reaching to C 3/4 D; since the orange screen used herewith necessitates a long exposure, the action seems to extend into the infra-red (beyond A).
As a rule, cyanine is used instead of the tetraethyldiamidooxytriphenyl carbinol {550} (HCl salt), but the former is apt to produce fogged plates. Methyl violet or crystal violet has also been suggested.
Exposures should be made in direct sunlight or with artificial pure white light (acetylene); electric light is too variable.
The most suitable methods of reproduction are half-tone, and the prototype methods; also Turati’s Isotypie. The greatest difficulty in 3-color printing nowadays is presented by the want of accurate printing. We must use the proper paper and pure fast colors; the inking rollers should be smooth, not too soft, and free from pores or weals. The blocks must be firmly fixed typehigh, otherwise they take color irregularly. A good printing machine is, of course, most essential.
To supplement the above working directions: After having kept the plates for 2 or 3 minutes (constantly moving the dish) in the dyes, they are removed into a dish containing filtered alcohol, which extracts the superfluous pigment. Plates thus treated dry much more rapidly, develop cleaner, and show no fogging.
Most of the above dyes may be obtained from the “Berliner Actiengesellschaft für Anilinfabrikation,” the acridin only from the “Farbwerk Mühlheim, a/Main, vorm. A. Leonhard & Company.”