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CHAPTER IX
DESENSITIZING PLATES
Whilst this process can hardly be considered as strictly belonging to the domain of color photography, it has been considered justifiable to include the same, as so many methods of development depend upon somewhat critical examination of the image in its first appearance. Further, it may be of considerable practical value to the color worker, as it enables one to use a dark room illumination, which would otherwise be unsafe.
Many suggestions have been put forward with the idea of destroying the color sensitiveness of the emulsion before development. C. Simmen1 (p. 303) suggested that Autochrome plates should be immersed in a strong solution of sodium bisulfite, and used the acid amidol developer with the same idea. He used a 10 per cent solution of bisulfite for 2 minutes. Le Roy2 recommended a 10 per cent solution of sodium hydrosulfite, Na2S204. H. G. Brockman3 bathed his plates in 3 per cent solution of potassium metabisulfite for 30 seconds, washed in running water for 1 minute and developed in the normal way by the light of a No. 1 Bray gas burner, behind two thicknesses of canary medium. R. Krayn4 patented the use of a preliminary bath of sulfuric acid and development with amidol or ferrous oxalate.
E. Stenger5 tested the action of the acid amidol developer and found that there was no reduction of color sensitiveness of panchromatic plates, in fact rather the reverse. E. Valenta6 was the first to point out that many of the red sensitizers, such as wool black, dianil black, etc., were destroyed by an acid bath, and confirmed this for cyanin and the isocyanins, and with one of the latter the Autochrome plate is sensitized. Actually W. Weissenberger8 was the first to utilize this idea, as he bathed plates with an acidified solution of cyanin, which gave no red sensitiveness, but this latter appeared as the acid evaporated. C. E. K. Mees and J. K. Baker9 tried the effects of treating plates with metol-hydroquinon, ferrous oxalate and hydroquinon and found but slight reduction of the panchromatic properties, but in this case only the ferrous oxalate was acid.
W. J. Pope10 patented the use of a bath of potassium nitrite, acetic acid and water, or a dilute solution of sulfurous acid for desensitizing. N. Sulzberger11 patented the use of potassium ferrocyanide for reducing or destroying the sensitivity of silver bromide or chloride left in plates or papers after development, a more permanent image being thus obtained. In a later patent12 the use of palladium ammonio-chloride was claimed. Sir W. N. Hartley13 worked out a modified formula of the Balagny acid
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