Ammonium bichromate ought not to be used. The bath would then become cloudy immediately, and both bath and paper made useless.
When sensitized the paper can be kept from eight to ten days without losing its good qualities.
With this a transfer ink is supplied in tubes, which must be mixed with an equal volume of turpentine and benzine, so that it becomes quite fluid. The print, when it is taken from the frame, is fastened on a board with drawing pins, and with a fine camel’s hair brush coated with this fluid ink as evenly as possible, and just so much that a light grey tone is formed on the print. This paper must not be inked up with a roller. As soon as the turpentine and benzine have evaporated the print is laid in cold water, and after about ten minutes developed with a clean tuft of cotton wool. The print does not show a strong relief; the development takes place quickly and regularly. The ink, in spite of its being so thin, is very greasy, so that only a very light skin is necessary for the subsequent transfer. The tint produced by a negative slightly too thin cannot be easily removed.
The other operations are the same as for the other photo-lithographic papers.
Franz’s photo-lithographic paper is characterized, as already mentioned, by giving extraordinarily sharp clean transfers, which, indeed, maybe due to the use of the very thin liquid greasy ink. It is, therefore, especially to be commended for very fine work.
(C.) ALBERT’S PHOTO-LITHOGRAPHIC PAPER.
August Albert’s is also a very good photo-lithographic paper. It can be obtained commercially in two qualities, viz., “Photo-lithographic paper” and “Autotypic high glaze paper.” This has been tested in the K.K. Lehr- und Versuchsanstalt für Photographie und Reproductionsverfahren in Vienna, and has received a certificate of excellence. The high glaze paper has a prepared film of several coatings; the developed prints when dry keep the high glaze.
The sensitizing bath consists of—
| Water | 16 | parts by weight |
| Potassium bichromate | 1 | part by weight |
| Alcohol (ordinary) | 4 | parts by weight |
and to this so much ammonia is added as will convert the reddish colour into a light yellow; a small excess of ammonia does no harm. The bichromate bath should not have a lower temperature than +15° R, or else the solution cannot penetrate the film sufficiently, which results in a defective development. The duration of sensitizing is from four to eight minutes, till the paper is quite soft and damped through. It is preferable to sensitize too long rather than too short. {57}