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