Distilled water . . 6 oz. 30o ccs.
at a temperature of 8o° P. (nearly 27° C.). This solution should be made as before described. When mixed, the emulsion should be digested in a water bath at a temperature of 120° P. for half an hour, and then— Hard gelatine . . 48o grs. so g.
which has been well washed but not soaked, added. Cool down gradually and allow to stand for sixteen hours before washing. This should give plates of from so° to s2o H. & D.
Rapid Ammonia Emulsion Ammonium bromide . 1,152 grs. 120 g.
Potassium iodide . 24 „ ,, Soft gelatine . 50 „ Hard gelatine . . 240 ,, 25 „ Alcohol . . . 2 oz. zoo ccs.
Distilled water . . 18 „ 900 „ Heat to s30° P. C.), and add— Silver nitrate . . 96o grs. loo g.
Distilled water . . 6 oz. 300 „ Liquor ammonia? (•880) q.s. q.s.
at a temperature of 95° P. (nearly 24° C.). Digest in a water bath at 12o° F. (nearly 49° C.) for one hour, and then add— Hard gelatine (well washed only) . 1,68o grs. 75 g.
Cool the emulsion and pour out into flat dishes ; allow to stand for twenty hours. This should give plates of from 200 to 225 H. & D.
There are many little dodges which can be learnt only by experience and experiment, but the following hints may not be useless.
In making acid emulsions it is advisable always to run the silver into the bromised gelatine in a fine stream with continuous stirring. In the case of ammonia emulsions, it is not so important to add the silver in a fine stream, but vigorous stirring should be continued all the time.
If regularity of results is required it is important that the water bath should always be kept at a constant temperature, and, further, that the emulsion should be stirred about every five minutes, otherwise the silver bromide may settle to the bottom of the vessel and give coarse grained thin-working plates that are absolutely ;useless. It will be noted that distilled water
is advised in all the formulm given in this article ; this is important, as too often ordinary tap water is contaminated with iron and other impurities, which lead to fog or loss of sensitive ness. It is advisable even to use distilled water for washing the gelatine.
Greater rapidity can always be obtained by reducing the quantity of gelatine during the mixing, but there is great danger of the forma tion of coarse grain and fog. If the gelatine is much reduced it is always advisable to add about so per cent. of the total bulk of alcohol, which not only prevents this but also obviates the occurrence of dichroic fog with the ammonia method.
Directions have already been given for setting and washing emulsions. When the emulsion has been washed enough it should be removed from the water and left to drain for about an hour, so as to free it from the adherent water as much as possible. In fact, it is as well to use a clean dry linen cloth, and after the emulsion has drained, place it in this and, collecting the whole into the form of a bag, squeeze thoroughly. The emulsion is then ready for melting and coat ing.
A test plate should always be coated first. Melt the emulsion in a water bath at s zo° P. (nearly 49° C.), then take out a little and cool down to 95° P. (35° C.); coat a plate and put away to dry. The bulk of the emulsion can now be rapidly cooled down again and, when set, alcohol containing o•s per cent. of carbolic acid poured over the toy to the depth of about half an inch, the emulsion being then put away in a dark, cool place. If it is to be used soon the alcohol may be omitted, but this will keep a stock of emulsion in good condition for a week or so.