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CHAPTER XIX
SCREEN-PLATE PRACTICE
Hyper-sensitizing. — The spectral sensitiveness of the Autochrome plate does not extend practically beyond wave-length 6500, which is actually almost on the border line of the orange, therefore, the reproduction of very deep reds and purples is by no means satisfactory. J. Thovert1 (p. 533), found that by bathing the plates in 1 :200,000 solution of pinachrom not only were better reds obtained, but that the plate was actually faster. The bath should be made alkaline, if there is any chance of the water being acid, but not more than about 3 drops of ammonia per liter should be used. Later, Thovert2 recommended the same dye, but 1 : 1,000,000 with bathing for 5 minutes, then rapidly drying without washing. The water might be rendered alkaline as before, but ordinary tap water was better.
It is somewhat difficult to determine the exact composition of the compensating filter for such plates ; but a predominating yellow tint in the pictures indicated too long an immersion in the sensitizing bath, or too high a temperature of the same. Predominating blue, on the other hand, was due to too short immersion; while a prevailing reddish-orange tint was caused by too long immersion or too strong a bath ; excess of greens pointed to too weak a bath. For flashlight work Thovert recommended a 1 : 1,000,000 solution of erythrosin with 2 minutes bathing, then briefly rinsing and drying.
C. Simmen3 also recommended the extra-sensitizing and found the best dyes were pinaverdol, pinachrom and pinacyanol; but the exact ratios could not be given. In some cases the best proportion of pinaverdol to pinacyanol is 8 : 1, in others 7:4. It is thus necessary to start with a mixture of pinaverdol 3 to pinacyanol 1 and to add more of the latter if the greens predominate, or of the former if the reds are too strong. He stated that working with an aqueous solution the plates do not keep so well and frequently stains were caused; the best bath was made with 30 per cent of alcohol. The solution was rendered distinctly alkaline with ammonia. The composition of the bath was :
Dye solution, 1 : 1,000 ale 2 ccs.
Ammonia, 22° Be 1 ccs.
Ethyl alcohol, 90 per cent 330 ccs.
Water 660 ccs.
The above quantity is sufficient for about 550 sq. ins. of plate surface. The plates should be bathed for 5 minutes, irrespective of temperature, and then dried without washing.
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