The Dictionary Of Photography

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

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Preservative Case                                       Pretsch Process
image tan be obtained in any colour, and almost any material, a reversed positive being used to produce a positive. It has been lately recommended in The Amateur Photographer for the pro-duction of lantern slides, and from experiments made by the author seems very suitable for the purpose. The following formulae are recommended for the preparation of the organic tacky body : -
Obernetter's Formula.
Dextrine
.. 60 grs.
White sugar ... .........
.. 75 h
Ammonium bichromate
• 30 M
Glycerine
2 to 8 mins
Distilled water ..........
3 ozs.
Woodbury's Formula.
Gum arabic.........
. 60 grs.
Glucose ... ......
• 45 11
Glycerine ... .........
. 10 mins.
Potassium bichromate ......
. 30 grs.
Distilled water ..........
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2 ozs.
A plate is coated with either of the above solutions, and dried at a gentle heat, and then exposed under a positive, reversed as regards left and right, for three or five minutes to sunlight, or ten or fifteen minutes to diffused light; on removal from the printing-frame a faint image is seen. The plate is then exposed to the air for a few minutes to allow it to absorb moisture, and fine plumbago, as used by electrotypers, is applied with a flat brush, when it adheres to those portions protected from light, and the lights and shades are represented more or less by a coating of graphite. When the image is fully developed, and there is no further adherence of the graphite, the superfluous powder is dusted off, the film is coated with collodion, and then well washed to remove the unacted-upon gum and bichromate ; the film may be detached from the plate, and used for enamels, ivory, opal, or any textile fabric. (For the application of this method to etching on glass, see Hyalography.)
Preservative Case (for paper, etc.). See Calcium Tube.
Pretsch Process.
See Galvanography.
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