February 16, 2008

Anodizing For Photographic Reproduction

Impregnation with solutions of light sensitive salts makes it possible to use anodized aluminum as a base for the reproduction of photographs. Film that are to be used for this purpose have to be white or colorless, and must have a certain thickness and porosity. Such films produced affects the tone of the photographic reproduction. The films formed in chromic acid are colorless but too thin to hold the necessary amount of light sensitive materials.


Films suitable for photocopying can be obtained by anodizing in a 15 percent solution of sulfuric acid at 23 ± 2 oC with an anodic current density of 25 A/sq.ft. for 45 minutes. In order to obtain a light sensitive materials are insoluble in water, they are deposited by successively immersing the plates in solutions of appropriate compounds which will react to form the light sensitive material. The sunplast method of obtaining reproduction on an anodized aluminum is by the blue printing process in which the plates are impregnated which light sensitive ferric salts in a solution having the composition 125 g. ferric ammonium citrate, 100 g. potassium ferricyanide dissolved in 1 litter water.


The impregnation is continued for 30-40 minutes after which the plates are washed in water, dried at room temperature and then exposed to light through a negative. The development and fixing of the image is done by washing with water. The finished reproduction has of course a blue color. If necessary it can be intensified by treatment in a 5 percent solution of hydrochloric acid, or weakened using a 5 percent solution of ammonia. The blue copies can by further chemical treatment be colored brown, green or black. Such reproduction are not very light-fast and are not resistant to heat or to the action of various reagents, all of which limit their usefulness.


Reproduction obtained with the said of heat-sensitive silver compounds are of much greater interest. The anodized plates are impregnated with a solution of potassium bromide, rinsed with water, and then impregnated with a 10 percent solution of silver nitrate. The amount of silver bromide deposited in the film depends on the thickness of the film and on the number of times the film is treated successively with the potassium bromide and silver nitrate solutions. Even with a film of adequate thickness, it is necessary to repeat the treatment 15 to 20 times, with intermediate rinsing of the plates in water. After sensitizing the plates are treated with a bleaching solution containing 50 g per liter potassium ferricyanide and 50 g per liter potassium bromide. This has the effect of improving the sharpness and clarity of the reproductions.

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