The Dictionary Of Photography

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

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Gum-Bichromate Process
and, assuming that the latter be the commercial salt, this is equivalent to
Grain. See Weights and Measures.
Gramme. See Metric System ; also Weights and Measures.
Green Fog. See Fog.
Gronnd-Glass. See Focussing Screen.
Gnaiacum. See Gum Guaiacum.
Gnm Arabic (Ger., Gummi arabicum, Arabische Gmnmi; Fr., Gomme arabique; Ital., Gomma arabicd). A gummy exuda-tion from the stems of various species of acacia. It is of peculiar bland taste, odourless, insoluble in alcohol and ether, but entirely soluble in water, in which form it is used as a Mountant (q.v.). It is also used in the powder process, gum-bichromate and photo-lithography. Its adhesiveness is increased by addition of aluminium sulphate, less so by ordinary alum. It is decomposed at a temperature of 3000, and is converted into dextrine by the action of sulphuric acid.
Gnm-Bichromate Process. This method of printing, which has become so popular during the last five years, is really older than the transfer carbon process, and in addition it is somewhat simpler in execution. In its original form, gum-bichromate appears to be chiefly due to M. Poitevin, Mr. John Pouncy, Mr. Thomas Sutton, and Mr. J. C. Burnett, who worked between 1850 and i860; but its revival is largely due to M. Demachy and M. Rouille-Ladeveze. The method may be summarised as follows: - Ground pigments are mixed with a thin mucilage of gum-arabic, and a little soluble bichromate, this .mixture being applied to the paper as a thin wash. When dry, the paper is exposed under a negative, and-the picture is developed by soaking in water (cold, warm, or hot) until the unexposed portions of the pigmented gum are sufficiently washed away. The following working details are taken, in abstract, from an article in The Amateur Photo-grapher, March 6th, 1896, and a subsequent note: - "A rather rough paper is desirable, quite apart from any esthetic con-siderations, as the roughness breaks up the continuity of the mucilaginous film, and assists in enabling the water to penetrate freely; the various drawing papers, and Dutch hand-made papers,