Changing the location of iodide distribution in the grain has a huge impact on all subsequent operations. It is especially true if you are single jetting and adding iodide to the kettle, or adding iodide through a separate jet during precipitation or growth.
When iodide is added, a crystalline lattice defect is introduced, and this will appear as dislocation lines. This factor has a huge impact on the response of the grain to ripening and growth stages of the crystals. In particular, if you add iodide to the kettle, the crystal will be smaller and they will be a lot slower to ripen or grow. You'll have to downregulate the jets during growth to avoid renucleation, perhaps in addition to ripeners (like a combination of thioether and ammonium sulfate) in case of bromide emulsions. Also, because crystals are smaller and there is greater total surface area, you'll need to upregulate sulfur sensitization and spectral sensitization to maintain optimal digestion. Here, it's very important to control grain size distribution because if there are large grains here and there, then those grains will be fogged way before smaller grains become fully sensitized. Thus you should aim least disperse emulsion, which also gives highest contrast. If you want lower contrast, as in the case of negative emulsions, you should blend 3 emulsions of different grain sizes. Here, it's important to design each emulsion so that they can develop together.
Crystals with dislocation lines introduced at the optimal location will have higher sensitivity than crystals free of such intentional defects, even after optimal S+Au digestion and spectral sensitization. This is one key technology Fuji scientists figured out around 1990.
People like Trivelli, Smith, Carroll, et al. did not know about this very important and intriguing fact, so their experimental data are all over the place and it's hard to make a systematic understanding of what they observed. Many studies varied iodide content and measured photographic speed, but no wonder, their results are mixed. So the emulsion making had been called a black art. But thanks to people like Urabe, Tani and others at Fuji, the mechanisms underlying this were largely figured out.
Most of recent emulsion related patents incorporate one or more steps to take advantage of this knowledge, although Fuji and Kodak patents usually use different steps.
Photo Engineer said:
The book "Silver Gelatin" by Reed and Jones is excellent on this topic. I highly recommend it to all interested in this.
I have coated canvas with my emulsion using a paint brush. It gets pretty limp during coating and processing. Kind of like working with a big paper towel, except it doesn't tear.
Thanks Ryuji for your comment. Your do very good work yourself and have made a lot of progress. I'm still working on my emulsions. They are no where near optimum, but I do have values for Sulfur, Dye, and Iodide content. Now it is a matter of combining them all at one time instead of doing one at a time or doing them in pairs. I have tried them in pairs but not all 3 in one experiment. That will probably be next week.
PE
