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Chloro-Bromide emulsions
Quite a bit has been stated here and elsewhere about Chloro-Bromide emulsions. I have refrained from commenting on this as I have none in my Formula Book yet. Since I am about to embark on some testing, I thought it might be appropriate to make some comments.
Here is the most important!
No Cl/Br crystal of a defined ratio of halides can be made by a single run. You must use a double run of Silver Nitrate and Halide both to form a mixed halide Cl/Br with a clearly defined ratio in every crystal.
Due to the solubilities of the two Silver Halides involved, if you add Silver Nitrate to a mixed kettle, you get an undefined mix. (A mess, maybe a useful mess, but a mess).
In fact, I have seen some of these "messes" used in production in B&W and color paper products from bygone eras. However, to get good, reproducible results you must use a double run make.
This is not to say that what you do make with a single run is not usable, it is merely undefined. To know what you have you will probably need X-Ray Diffraction and Electron Micrographs to examine the crystals for their structure, and even minor deviations from the norm will cause some fluctuations in the characteristics of the emulsion such as speed and contrast for starters.
In fact, the variability of these early emulsions due to this lack of understanding is what led to the variability in early B&W paper products, and led to the development of a whole arsenal of addenda meant to control speed and contrast of the emulsion. These addenda, BTW, often had bad effects on reciprocity and latent image keeping and therefore some emulsions had to be segregated into classes for use in products.
Ektacolor Type C paper and Ektachrome Type R paper, in their first incarnation, were prime examples in which speed was so poorly controlled that the paper came with different speed and color balance ratings for each batch. In addition, the "class" distinction here in reciprocity and latent image put the emulsion into the photofinishing class (adjusted in curve shape for short exposures of 1/2" or less and quick processing) to professional class (adjusted in curve shape for 10" and longer and any time between exposure and processing up to several days was ok).
More information for the avid emulsion makers out there.
PE
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From the film shooters will rise a well developed practice of the alternative processes that, in time, will be adopted in the age of the digital image to free it from the extreme boringness of pressing print.
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You see how interested in this topic people are! 
PE
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Hahaha, that's what I thought!
I just hate to see a thread go unreplied...
From the film shooters will rise a well developed practice of the alternative processes that, in time, will be adopted in the age of the digital image to free it from the extreme boringness of pressing print.
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 Originally Posted by holmburgers
bump
Bump?
I don't understand this terminology.
Where does it come from?
Does bump = nudge?
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[Chloro-Bromides < I ignore these and feel that they are too hard for starters]
In what sense?
[No Cl/Br crystal of a defined ratio of halides can be made by a single run. You must use a double run of Silver Nitrate and Halide both to form a mixed halide Cl/Br with a clearly defined ratio in every crystal.]
Do Cl/I or Br/I or Cl/Br/I differ in this respect?
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Yes, Cl/I and Br/I differ in this respect. Cl/Br/I are subject to the same problems as Cl/Br.
PE
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Ron:
Would this be solved by doing a run of AgCl in one vessel, AgBr in another and mixing the product? Or did I miss the boat at the dock?
Last edited by JOSarff; 03-12-2011 at 12:40 PM.
Reason: not enough caffeine in my system yet
There is no such thing as taking too much time, because your soul is in that picture. -Ruth Bernhard
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Originally Posted by JOSarff
Ron:
Would this be solved by doing a run of AgCl in one vessel, AgBr in another and mixing the product? Or did I miss the boat at the dock?
Joe;
This will not work properly. The halides will begin an exchange that might give you the same thing every time, but you would not know what you had exactly which is the basic problem with Cl/Br making. And, the explanation requires a lot of chemistry! That is a major reason to avoid this can of worms.
PE
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Is there an example of a chloro-bromide emulsion somewhere I can look at to see the level of complexity?
There is no such thing as taking too much time, because your soul is in that picture. -Ruth Bernhard
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