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526 History of Three-Color Photography
A. Pyrogallol 3 g.
Citric acid 3 g.
Water 1000 ccs.
B. Silver nitrate 5 g.
Water 100 ccs.
For use add 10 parts of B to 100 parts of A, and immediately apply to the plate, and rock the dish till the intensification is sufficient. The mixed solution rapidly turns brownish-yellow and becomes cloudy. In the latter case it should be poured away, and freshly mixed solution applied if necessary. As the intensification is due to the precipitation of metallic silver, a perfectly clean dish must be used, otherwise the silver will deposit everywhere.
The duration of intensification should not be more than 30 seconds, and may be complete in less, and this is easily judged by examination by transmitted light. If the intensification is not complete in this time, the solution should be poured away, the plate well washed and a clearing bath of:
Acid permanganate solution 20 ccs.
Water 1000 ccs.
Applied. This acid permanganate solution is merely the reversing solution. The plate should be again well washed and freshly intensified. Unfortunately the above solution of pyrogallol soon grows mouldy and then spoils. To remedy this, T. K. Grant90 recommended the addition of 0.05 per cent of salicylic acid as preservative.
G. E. Brown91 stated that the silver solution was apt to deposit metallic silver on standing, and that care should be taken not to disturb any precipitate, that might be in the bottle, or the small particles of silver might stick to the film.
Several substitutes were suggested for the above nascent silver bath; but with a little care there is very little more trouble with it than some of the other solutions recommended. F. Monpillard92 preferred Monckhoven's potassio-silver cyanide intensifier. This gives excellent results, but the action must be very carefully watched, in order that the action does not proceed too far, and produce actual reduction of the lower densities, which is characteristic of this solution. This is the more likely to occur in consequence of the small quantity of gelatin used, as all solutions act much more quickly than on an ordinary plate.
R. Namias93 suggested a variant of the cupric intensifier, as follows:
Cupric sulfate 20 g.
Potassium bromide 20 g.
Hydrochloric acid 5 ccs.
Water 1000 ccs.
As soon as the plate has completely bleached, it should be rapidly washed, then treated with a 5 per cent solution of silver nitrate, acidified with nitric acid, and when black enough it should be placed in a neutral permanganate