To oz. of pyrogallic solution, add 2 minims of b, and three of c. Should the image come out slowly, and be wantiog in detail, add 2-3 min. of ammonia carbonate. The alkali will also add to the vigour of the negative when the image has heen fully developed.
Collodion Emulsion Process.—Collodion emulsion is a mixture of iodized or bromo-iodized collodion and silver nitrate.
Washed Collodion Emulsion.—Collodiou made as already described with the iodizer added should be carded to the dark room, and mixed with 1 oz. collodion, 2 min. acetic acid, 1 dr. glycerine, 4 dr. alcohol. Shake up, and set aside in the dark to settle for two days. Add 2 min. of hydrochloric acid, shake up, and again allow to digest for 21 hours. Now pour the emulsion into a shallow porcelain tray to set. When thoroughly set, cover with distilled water for one or two hours. Pour off the water, and flood with 5 gr. tannin, 2 gr. gallic acid, 2 dr. acetic acid, and 1 oz. water.
The emulsion should remain submerged for three hours. It may then be placed in flowing water until no trace of acid remaios. It should afterwards be consigned to folds of clean linen, and worked about until the water has been expressed. It may now be dried at a low temperature, and preserved in a light-proof bottle.
Pellicle so made may be dissolved for use in a mixture of 1 oz. ether and 1 oz. alcohol. Plates coated with this eroulsion, and dried in a warm dark room, are ready for use, and should be developed with alkaline pyrogallic acid as in the tannin process, and fixed with soda hyposulphite. Colludioo-emulsion plates are more sensitive than dry collodion plates that have beeu washed and coated with a preservative. Intensification may be effected as with tbe wet process.
Gelatine Emulsion Process.—The manufacture of gelatine emulsion is one of the most recent advances in photog,raphy, and marks new era in its history. Its introduction has rendered it possible to obtain portraits and views in the fraction of a second. Pedestrians in the streets, trains at express speed, birds on the wing, may be caught on the gelatine dry plate.
In making gelatine emulsioo, the reagents required are the same as those employed in the collodion process, with this difference, that gelatine takes the place of collodion.
The manufacture of this emulsion is hedged round with difficulties, arising out of the instability and impurity of gelatine, and the rapidity with which it is decomposed by atmospheric conditions over which the chemist has no control.
It is imperative that great care be observed in selecting a suitable gelatine. Cognet's gelatine is in many respects best fitted for the purpose, but it has one great defect, small particles of grease are locked up in its flakes. These make their presence known in opaque spots in the finished
plates. Nelson's "No. 1 photographic gelatine" is, on the whole, the safest to employ. Having fixed upon this gelatine, the next operatioo is working out the combining proportions of the haloid salts and silver nitrate, so as to leave the bromo-iodide in excess. Free silver nitrate is fatal to the process. The following is a reliable formula :—(a) 600 gr. gelatine, 330 gr. ammonium bromide, 10 gr. ammonium iodide, 7 oz. water; (b) 600 gr. silver nitrate, 7 oz. distilled water.
Of the above gelatine, take 500 gr. only, add to this the bromo-iodide salts and 7 oz. of water. Allow the gelatine to avvell for 15 minutes, after which, place the jar containing the gelatine aud salts in a hot-water bath at 71° (160° F.). Dissolve the silver nitrate in its allotted water, and TaiSe to 82° (180° F.). Add the silver drop by drop to the gelatine solution, stirring vigorously until the last drop is taken up. Place the emulsion thus formed with its porcelatn jar in a light proof pan (an ordinary tin saucepan) half full of water, and boil for 20 minutes. Allow to cool down to about 38° (100° F.), and after swelling the remainiog gelatine, add it to the boiled emulsion. Place the whole in a cool place to set. When thoroughly set, the jelly may be turned out of the jar on to a square of strong netting (preferably silk), having meshes about I, in. in diameter. Spread a piece of flne white muslin over a wide basin filled with water. Gather up the netting, and force the emulsion through its folds into the water, beneath which is the sheet of muslin. This will part the jelly into thin shreds, and aid washing. The object of washing the emulsion is to get quit of the free salts in its composition, which would injure the process were they left in. It is best to employ a wooden wasbing-trough, Fig. 1093 : a represents the trough ; b, a light-proof lid ; c, a funnel, with gas-pipe worm beneath to prevent light entering the tank ; d, water-tap to keep up a coustant stream of water ; e, a second outflow-tap to carry off the soluble salts which fall to the buttom of the water in the tank; f , a light frame, with muslin stretched across, in which the shreds of jelly rest while washiog. Six or eight hours' flow of water is all that is required to fit the emulsion for use. During warm vieather, ice should be placed in the water, and the feeding-funnel should be packed with ice, so as to keep the temperature below 10° (50° F.). After washing, run off' the water through the lower tap, and allow the emulsion to drain for au hour or two.