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Color-Sensitive Plates 229
suspended particles, therefore, it is better to make up a stock, more than is ordinarily required, and allow to stand to settle, a week not being too long. Then the upper part should be carefully decanted. Filtration of collodion is not an easily performed task.
Many formulas have been given, but we shall be content with two ; the one a pure bromide, the other a chloro-bromide, which is specially recommended for color work, as it gives better rendering than the pure bromide. The bromide emulsion is :
Pyroxylin 24-30 g.
Alcohol 300 ccs.
Ether 300 ccs.
The quantity of pyroxylin is determined by the viscosity. The above ingredients should be placed in a flask capable of holding at least 1500 ccs. A silver solution should be made by dissolving :
Silver nitrate 50 g.
Distilled water 50 ccs.
Ammonia q. s.
As soon as the silver has dissolved in the water, add strong ammonia till the precipitate first formed is redissolved, and a perfectly clear solution obtained, then add :
Absolute alcohol 25 ccs.
This solution should be cooled down to 18° C. and added to the collodion gradually, with constant shaking. Some of the pyroxylin may be thrown out, but this may be disregarded, as it will soon redissolve on vigorously shaking. Some of the silver may also separate out in fine crystals, which can also be ignored. To this silvered collodion should be added in the dark room, the following :
Ammonium bromide 32 g.
Distilled water 35 ccs.
Alcohol 50 ccs.
Warm till perfect solution is obtained, and whilst warm add to the collodion. The flask should be well shaken for 5 minutes, then distilled water added in small doses, with constant and vigorous shaking. Although it is some little trouble, it will be found to pay to add not more than 5 ccs. of water at a time, and the emulsion will be gradually precipitated as a fine sandy powder, when about 200 ccs. of water have been added. The exact amount of water that it is advisable to add in this way is easily determined, as the emulsion begins to flock out or no longer run smoothly on the sides of the flask. It should then be poured into 5 liters of warm water, about 40° C, with constant stirring, and allowed to settle to the bottom of the vessel. The supernatant liquid should be syphoned off, the vessel filled up again with water, the emulsion shreds being stirred up for about 5 minutes, again allowed to settle, and this operation repeated five times.