NEGATIVE; EMULSIONS The manufacture of negative emulsions is by no means such an easy matter as positive emulsion making, but with care slow emulsions of very satisfactory quality can be produced. The beginner is not advised to attempt very rapid emulsions, as they are extremely difficult. There are two principal methods for negative emulsion making—the acid or boiling process, and the ammonia method. The former will give, as a rule, the cleaner plate, but it is not possible to obtain so high a speed. With care equally clean plates may be obtained by the ammonia method and greater speed. The acid process will be treated first.
Slow Emulsion Potassium bromide . 720 grs. 75 g.
Potassium iodide . 15 „ 2'75 „ Nelson's No. 1 gelatine 317 „ 33 33 Hydrochloric acid . lo „ I cc.
Distilled water . . II oz. 55o ccs.
Heat to 12o° F. (nearly 49° C.), and add slowly with constant stirring Silver nitrate . . 96o grs. 100 g.
Distilled water . xi oz. 55o ccs.
also heated to 120° F. Digest in a water bath at boiling point for half an hour, and then add— Hard gelatine . . 1,200 grs. 125 g.
which should have been well washed in water, soaked for half an hour, and drained for half an hour. Cool the emulsion, and set. This should give an emulsion of about 25 H. and D.
An emulsion of about double the rapidity, and giving somewhat greater contrast can be obtained by cooling the above emulsion down to 95° F. (35° C.) and adding— Liquor ammonia (•88o) 72 mins. 7.5 ccs. Distilled water . . 1 oz. ,, and stirring well for about fifteen minutes, then setting and allowing to stand for twenty-four hours.
Rapid Emulsion Potassium bromide . 1,200 grs. 125 g.
Potassium iodide . 24 3, „ Hard gelatine . • lf 50 „ Distilled water . . 10 oz. 50o ccs.
Heat to 14o° P. (6o° C.) and add in a fine stream with constant stirring— Silver nitrate . . 96o grs. Ioo g.
Distilled water . . 7 oz. 35o ccs.
also heated to 140° P. Digest in a water bath at boiling point for forty-five minutes, and then add Hard gelatine . . 48o grs. 5o g.
Distilled water . . 8 oz. 40o ccs.
The gelatine should be well washed in two or three changes of water, drained, and then dis solved in the distilled water at I10° F. (43.3° C.). This should give plates of from 15o to 18o H. and D., which are rather soft working but clean. Greater contrasts can be obtained by adding ammonia as suggested for the slow emulsion.
Slow Ammonia Emulsion Ammonium bromide . 816 grs. 85 g.
Potassium iodide . 29 „ 3 'I Hard gelatine . 1,392 „ 3) Distilled water . . 20 oz. 1,000 ccs.
Heat to iio° P. (43.3° C.), and add, with con stant stirring— Silver nitrate . 96o grs. I00 g.
Liquor ammonia: (.88o) q.s. q.s.
Distilled water . 6 oz. 300 ccs.
at a temperature of about 70° P. (21° C.). The silver should be thoroughly dissolved and enough ammonia added to redissolve the pre cipitate first formed. The exact quantity will, of course, depend upon the strength of the ammonia, but about 65 or 70 ccs. can be added at first, and then further additions made very cautiously, stirring well, till quite a clear solution is formed. The temperature rises to about 9o° F. (32.2° C.), so that it is advisable to cool this silver solution down by standing the vessel in cold water for a short time. As soon as the emulsion is mixed, the vessel should be placed in cold water, running water for preference, and the emulsion well stirred till quite thick and then put away in cold water to set. If allowed to stand for about sixteen hours before washing, this should give a clean working plate of about 3o to 5o H. & D., which will give great contrasts and wide latitude of exposure.
Medium Rapidity Ammonia Emulsion Ammonium bromide . 864 grs. 90 g.
Potassium iodide • ,, ,, Soft gelatine . • 50 „ Hard gelatine . . 480 „ 50 „ Distilled water . . 20 oz. 1,000 ccs.
Heat to szo° P. (nearly 49° C.), and add, with constant stirring— Silver nitrate . . 96o grs. soo g.
Liquor ammonia? (•880) q.s. q.s.