The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Gum-Bichromate Process
of an actinometer is often more convenient, especially for those who are accustomed to one. The exposed print is now soaked in cold water, and if the colour soon begins to wash off the paper on rocking the dish, under exposure is indicated; and in this case cold water alone may perhaps finish the development. According to the behaviour of the print, warmer and warmer water may be used, and to increase the detergent power of the water a stream may be poured from a height on the surface of the print. When the development is complete, the print may be soaked in an alum bath, after which it is rinsed and allowed to dry. Dr. Mallmann suggests that a special preparation of gelatine made non-setting may advantageously replace the gum ordinarily used in the direct carbon or pigment process. This change gives, according to Dr. Mallmann, images of finer texture, and the material does not so dissolve from the whites as to produce an appearance of scaliness; moreover, the image is much more firmly adherent to the paper. Hydrate of chloral is one of those substances which may be used to prevent the setting of gelatine, and this without interfering with its sen-sitiveness to light when bichromated; and the following is mentioned as a known receipt for preparing a strong gelatinous solution, which remains fluid on cooling : -
Water...... ............iooo parts.
Hydrate of chloral ... ......... 250 ,,
Gelatine ... ... ... ... ... 400 ..
Herr Watzek confirms Dr. Mallmann's results, and mentions that an addition of potash to the warm development water expedites the development when a long exposure has been given, while the use of sawdust (see Artigue's Process) brightens the high lights.
The Chief Recent Improvement: Multiple Printing. In its simple form and as described above, the gum-bichromate process is capable of giving good results, but it not unfrequently happens that the scale of tone is imperfect at one end or the other; indeed the obtaining of a full scale of tone involves the very complete realisation of all technical conditions. By re-sensitising the surface of the print first made, then once more printing and developing, defects in the gradation of the first print-ing can be remedied ; thus one print may give the light and