POSITIVE EMULSIONS Positive emulsions may again be subdivided into chloride and bromide emulsions, the former including chloro-bromide also, and such as are suitable for gaslight papers and plates.
Chloride Emulsion (Eder) A. Sodium chloride . z88 grs. 3o g.
or Ammonium chloride 264 ,, Hard gelatine . • 384 ,, Distilled water . 8 oz. 40o ccs.
Hydrochloric acid . 5 drops ro drops B. Silver nitrate . . 576 grs. 6o g.
Distilled water . 2 OZ. 100 ccs.
C. Hard gelatine . . 384 grs. 4o g.
Distilled water • to oz. 50o ccs.
Dissolve C by heating to 12o° F. (nearly 49° C.), and add B at the same temperature ; then add A, also at 120° F., and allow to stand at this temperature for ten minutes ; then rapidly cool and set. This emulsion yields a satisfactory gaslight paper or plate, which readily gives warm tones. If the hydrochloric acid is omitted, and 240 grs. or 25 g. of citric acid is added, very warm tones, from yellow to reddish brown, are more easily obtained. If the above emulsion is cooled down to 95° F. (35° C.) immediately after miring, and 6o minims or 6 ccs. of liquor ammonim (•88o), and 2 oz. or ma ccs. of distilled water are gradually added with constant stirring, a much more rapid emulsion is obtained, which gives black tones more readily.
The great trouble in mixing all chloride and chloro-bromide emulsions is the formation of a coarse grain, which is reduced by the developer without exposure to light. For this reason the beginner is advised to modify the above formula as follows : Add the whole of the gelatine to A and increase the quantity of water to 12 oz or 600 ccs., and add the silver nitrate dry, in small quantities at a time, with vigorous stirring. When all the silver is added continue stirring for fifteen minutes, and then add 8 oz. or 40o ccs. of distilled water. As an alternative the emul sion may be set and washed as usual, and then the extra quantity of water added. Either way, there is less chance of the occurrence of coarse grain, and the results are equally satisfactory.
There is but little difference in the final result whether the sodium or ammonium chloride be used; the former gives a little more contrast. Much harder working emulsions can be obtained by adding to the above quantity of chlorised gelatine 1•25 grs. or •o5 g. of pure copper chloride ; or greater contrasts may be obtained by increasing this still further.
For some gaslight papers an unwashed emul sion is used, but there is far greater liability to the formation of black spots.
Chloro-bromide Emulsions Emulsions containing both bromide and chloride of silver are more sensitive than pure chloride emulsions, and whilst giving warm tones with increased exposure, they give better blacks, there being less tendency to greenish tones. The ratio of bromide to chloride is a matter of taste, but the more chloride the less the gradation, and the more bromide the faster the emulsion.
Wratten's Formula Nelson's No. r gelatine 4o grs. zoo g.
rinse two or three times in water and add to— Distilled water . . 4 oz. 856 ccs.
Dissolve at 125° F. (nearly 52° C.) and add— Ammonium bromide (neutral) . . rro grs. 55 8 Sodium chloride. 3o „ „ Hydrochloric acid (10% solution) . . ro mins. 5 ccs.
Then add in a fine stream— Silver nitrate . . zoo grs. roo g.
Distilled water . . I oz. 220 CCS.
with constant stirring. Digest for ten minutes at 150° F. C.), and add— Hard gelatine . . 175 grs. 87.5 g.
which has been previously washed and soaked in water for half an hour and well drained. When the gelatine has melted, set, and wash for half an hour with six changes of water, drain well and melt, and make the total bulk up to 1,75o ccs. by adding water. Finally Tannin.. . . . . 1.03 g.
Wellington's Formula Potassium bromide . 384 grs. 4o g.
Sodium chloride . 192 „ 20 „ Citric acid . . 96o „ 100 Hard gelatine . • 1,344 „ 140 ,, Distilled water . . 20 oz. r,000 ccs.
Heat to 15o° F. (65.5° C.), and add— Silver nitrate . . 96o grs. roo g.