Pure bromide and chloro-bromide collodion emulsions were much used for transparency making, but of recent years the gelatine lantern plates have completely ousted them from prac tical use. They are, however, now employed for negative work, and the most satisfactory formulm are those given by Von Hiibl.
Emulsion Silver nitrate . . . so g.
Distilled water . . . 5o ccs.
Dissolve, and add liq. ammonia? (.88o) in sufficient quantity to form a perfectly clear solution ; allow to cool, and add the silver solution to— Collodion (4 %) • • . 600 ccs.
This should be in a large bottle, preferably one that will contain about three times the total volume, and if any of the cotton or silver settles out no notice need be taken of it. To this silver collodion add the following, whilst still warm : Ammonium bromide . . 32 g.
Hot distilled water . . 35 ccs.
Alcohol . . . . so „ Shake the emulsion for about five minutes, and then add in small quantities with vigorous agita tion about one-fourth of its volume of dis tilled water. This causes the emulsion to pre cipitate. It should then be poured into about ten times its bulk of water and well stirred, the water drained off, and the washing repeated three or four times. The shreds of emulsion are finally collected on a linen filter, and gently pressed ; then shaken up with alcohol, and again pressed out. The emulsion shreds should now be shaken up with 81 oz. or 25o ccs. of absolute
alcohol, and allowed to stand for twenty-four hours ; at the end of this time 5/ oz. or 15o ccs. of the liquid should be poured off and replaced by 5i oz or 15o ccs. of absolute alcohol in which 7f grs. or o•5 g. of narcotise have been dis solved, and 8- oz. or 25o ccs. of ether added, well shaken, and allowed to stand three days and then— Absolute alcohol . . . 25o ccs.
Ether . . . . . 250 „ added and the emulsion filtered.
Emulsion This can be made in precisely the same way as described above by reducing the ammonium bromide to 4:6 grs. or 27 g. and adding 23 grs. or 1.5 g. of pure anhydrous lithium chloride. Both these emulsions are very suitable for posi tive work and also for sensitising with eosine and other dyes for colour negatives.
Von Hiibl recommends a glycin developer, but hydroquinone is the general favourite. Collodion positives and negatives can be intensified, re duced, or toned like any other silver images.
In process work, collodion emulsion has been revived of late years because of its suitability for colour work, owing to the emulsion being susceptible to colour sensitising with aniline dyes. A chloro-bromide emulsion is used for this purpose, and it is always exposed in the moist state. The sensitising dyes are sometimes added to the emulsion, and in other cases flowed over. Excellent emulsions are on the market.