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Nov., 1940] ELIMINATION OF HYPO FROM IMAGES 499
coal gas and in the atmosphere of industrial regions. This external fading is accelerated by the presence of acidic gases and by high temperature and high humidity.
The use of water-miscible (paste) adhesives for mounting also contributes to fading, since such adhesives are usually hygroscopic and the resulting moist condition of the print is favorable to more rapid chemical reaction.
Fading due to external agents may be minimized by (1) the use of a waterproofing lacquer over the print surface, (2) the use of Dry Mounting Tissue, and (3) bathing the print in a solution of a salt of a noble metal, such as gold chloride (with sodium thiocyanate), when the metal ion displaces the outer layer of the silver grains, usually with a negligible change in image color. A suitable formula consists of gold chloride, 0.1 gram; sodium thiocyanate, 10 grams; dissolved in 1 liter of water.
It is essential that the solution be prepared just before use and in the following manner: Add 10 cubic-centimeters of a 1-per cent gold solution to a 1 -liter vessel and dilute to approximately 700 cubic-centimeters. Dissolve the thiocyanate in a small volume of distilled water and add slowly to the gold solution with continuous agitation. Then make up to 1 liter with water.
Bathe either the freshly washed or dried prints for 8 minutes at 70° to 75°F with agitation or until a just perceptible change in tone occurs and then wash for 5 minutes before drying. The life of the bath if used immediately is approximately thirty 8-inch X 10-inch prints per gallon of solution.
The gold treatment also stabilizes the image against fading by hypo. It is not quite as effective, however, as the peroxide-ammonia treatment and has the disadvantage that the color of the image is changed slightly.
With respect to fading by hydrogen sulfide, lacquering the print surface is helpful but not as effective as the gold treatment. Dry mounting in combination with a lacquer is surprisingly effective, from which it is apparent that the hydrogen sulfide attacks the image appreciably from the rear through the paper stock as well as at the surface.
A combination of these procedures, namely, (1) use of the peroxideammonia hypo eliminator, (2) treatment with the gold solution, (3) the use of Dry Mounting Tissue, and (4) lacquering of the print surface, will insure maximum permanency of the gelatin silver print