The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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minutes, and pour out to set. By digesting 10 minutes only a very fine-grain emulsion is obtained, which gives good lantern slides. The proportion of iodide solution may be increased from 3 to 8 parts with advantage when the plates are intended for landscape work ; but in this, as in every formula where the iodide is increased, the bromide must be correspondingly reduced, which may easily be done by referring to the tables given at the end of this work. Very rapid plates are not easy to make, but in order to complete this note we may briefly indicate how to prepare the same. The formula for the emulsion may be precisely the same as that already given ; but the iodide should be the full quantity of 8 parts, and the bromised gelatine be heated to 6oo F. (= 1400 F.), and the silver solution added to it, and after about 40 to 50 minutes' digestion, in the water bath, at a tem-perature of 400 C. (= 104o F.), a sensitiveness equal to about 230 W. will be easily obtained. When making such an emulsion, it is advisable to cool rapidly and wash rather quickly, as the digestion at such high temperatures has a tendency to give rise to frilling and blisters. Some manufacturers use chrome alum to prevent this frilling, and it is possibly due to this that some commercial plates require so long to develop and fix - not always a desirable feature. Chrome alum too is always faintly acid, and tends, therefore, to the lowering of the sensitiveness ; this, however, may be avoided, I think, by neutralising it by the cautious addition of liq. ammonia, and, so far as I have been able to determine, this in no way affects the hardening action of the alum on the gelatine. On the Other hand, if the chrome alum solution be rendered more than very faintly alkaline, there seems to be a greater tendency to fog*. Practically, an addition of 20 parts of a 2 per cent, solution of chrome alum to every 1000 parts of emulsion will not be amiss, and it is advisable, if it is desired that the plates should keep long, to make an addition of 15 to 20 parts of a 1 per cent, solution of potassium bromide to every 1000 parts of emulsion ; these additions must be made immediately before coating.
The Acid-Boiling Process. The disadvantage of not being able to obtain such a high sensitiveness, nor such great regularity as with the ammonia process, rather places this hors de combat, one well-known writer even going so far as to say that it is impossible to obtain good plates by this process. Possibly the