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). Albumen5g.
Ammonium bichromate3g.
Distilled water500g.
Ammonia15–20drops
(B). Chloroform250g.
Benzine50g.
Alcohol20g.
Mastic2g.
Aniline red2g.

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 {65} 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.)2min.
In the shade (middle of April, 11 a.m.)5min.
In electric arc light (3000 c.p., at a distance of 50 cm. = 20 ins. without reflector)25min.