This paper, which is also darker coloured, does not allow the light rays to pass through, and is specially suitable for the transfer of autotypes to stone or zinc.
For sensitizing this paper bichromate of potassium, sodium, or ammonium is used, but the sodium salt has acted the best.
In winter a 1 to 20 bath is best; in summer, 1 : 22 or 1 : 24. The treatment of the paper, as well as of the print, is exactly the same as with other transfer papers. The bath will keep for fourteen days. Caustic ammonia ought not to be used.
(B.) PHOTO-LITHOGRAPHIC PAPER BY ADALBERT FRANZ.
A photo-lithographic paper, with which I have obtained very good results on stone as well as on zinc, with all kinds of drawings, is sold by A. Franz.
This is also a pure gelatine paper. But Franz adds to the gelatine solution manganese sulphate (this must not be confounded with manganese borate, siccative powder, which makes the transfer ink dry so quickly that a transfer to the stone or zinc is impossible).
Through the researches of MM. Lumière the manganese salts have gained considerably in importance, and they have been introduced into photo-mechanical processes. When an addition of about 10% of manganese sulphate is made to the acid bichromate baths for sensitizing photo-lithographic papers, the printed places assume a very much darker colour, which is an advantage in judging the print, in developing the finest lines, and points are retained much more easily than on paper sensitized with the addition of ammonia.
This paper possesses the advantage, which is also more important, that the—especially in summer—unpleasant ammonia bath need not be used. The paper is sensitized in a solution of—
| Potassium bichromate | 40–50 | g. |
| Manganese sulphate | 5 | g. |
| Water | 1 | litre |
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