File:The American annual of photography (1919) (14782475445).jpg

Identifier: americanannualof3334newy (find matches)
Title: The American annual of photography
Year: 1919 (1910s)
Authors:
Subjects: Photography
Publisher: New York : Tennant and Ward
Contributing Library: Harold B. Lee Library
Digitizing Sponsor: Brigham Young University

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r bromide, suchas hypo, ammonia or an excess of sulphite or carbonate mustbe present, when under certain conditions the dissolved silveris reduced to metallic silver in a very fine state of division, par-ticularly in the shadow portions of the negative, where nobromide is liberated during development. Fine grained emulsions in which the grains of silver halideare very small and therefore more readily soluble are most sus-ceptible to this form of fog, especially if the development isforced, but the fog may be prevented by the addition of alittle potassium iodide to the developer. The difference between the cause of sulphite fog anddichroic fog appears to be merely a question of the time in-terval between solution and reduction of the silver. The formation of the fog most generally occurs during fix-ing, especially if the fixing bath does not contain acid or if itis old and exhausted, when it contains an excess of dissolvedsilver and spent developer. It is possible to obtain dichroic 36
Text Appearing After Image:
NASTURTIUMS. G. W. HARTING. 37 fog with a fresh solution of plain hypo, in which case the silversalt dissolved by the hypo is reduced to metallic silver by thedeveloper carried over by the plate to the fixing bath. This isespecially true if the gelatine coating of the plate is ab-normally thick and if the developing and fixing solutions arewarm. In such a case the developer does not have time todiffuse out of the gelatine film before the hypo begins to dis-solve away the silver bromide. A fixing bath containing am-monium chloride (which is added to accelerate the rate of fix-ing) will give dichroic fog unless the bath is kept acid, in viewof the ammonia liberated by the action of the alkali in thedeveloper carried over by the plate. Dichroic fog never occurs in the fixing bath if a fresh acidbath is used and if the plate is rinsed before fixing. If dichroicfog still persists when using an acid fixing bath at a tempera-ture not above 70° F. the trouble lies in the developer. KALEVIMAG

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