One thing that should be in the book is a good discussion on solubility products and how they are calculated and used.
I think a better understanding of that subject (solubility product, Ksp) would go a long way towards understanding the complexity of this subject (Cl-Br crystal formation and growth).
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Originally Posted by JOSarff
No one has addressed a "proper" or modern Cl/Br emulsion. All texts give plenty of examples of simple Cl/Br emulsions but they have absolutely no idea of what they are really making.
Regarding what Kirk says in his post, this is a matter of the solubility of each of the Silver salts and as such is very complex. I can just say simple that if you mix 10% Iodide with 90% Bromide you get a 10/90 Br/I and I can add that most all of that Iodide will be in the center unless it is double run or you use a silver halide solvent for digestion. But, with a Cl/Br there is little differentiation unless you use a double run, and even then the rules are rather strict!
I hate to get into that much chemistry in the book. If you follow an existing formula, you will get a pretty much repeatable result and reasonable curve shape and speed, but you will not know - really know, what you have made. I avoid them to avoid the complex chemistry. Kirk, a chemist, would love to see the chemistry! I know he might be the only person who would follow it though. Maybe 2 or three others.
I might add that Sulfur sensitization of Cl and Cl/Br emulsiions is difficult and until the mid 70s, Sulfur + Gold sensitization was almost impossible. So, to maximize speed and contrast this way, you have to do a lot of experimentation. And, once sensitized, the emulsions become quite sensitive to keeping.
Glafkides mentions in the text of the Brovira formulas in his book, that keeping is a problem with some of the Brovira family and has left out one step in the making of these that deserves mention. Some of the Cl/Br Agfa formulas use a stabilizing agent to prevent fog and loss of contrast during aging. That too is another grave omission on the part of that table.
Kirk, I thought of that but now think that is not what PE is thinking about.
The solubilities of AgCl and AgBr are closer to each other than than that of AgCl (or AgBr) and AgI.
This fact would seem to mean they would have a more uniform composition with a single run and that double runs would produce a greater similarity for all grain types...
I can imagine a few things, but it is not clear to me what is at the center of these comments. ClBr is my cup of tea. To me AgCl is the more mysterious....
Last edited by Ray Rogers; 03-12-2011 at 04:40 PM. Click to view previous post history.
Ray, AgCl and AgBr are easy compared to AgClBr! AgClI and AgBrI are easy compared to AgClBr. In the OP, I outline some of the problems.
I've found several formulas that take you through washing and setting, then end. Nothing about finals and finishing.
Looking at these formulas, Wratten, Valenta, et al, it looks to me like they would work better as a Run Salt, rather than ading the silver to the salt solution. Hmmm?
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Ron, that is interesting. For some reason I found the opposite to be true.
That's why I am so curious about what you are doing.
It will be nice when I can finally examine your procedures to see what we are doing differently... How about an advance toward the price of your book?
The best formulas might be for example 90/10 Cl/Br or 10/90 Cl/Br, but the worst might be 50/50. Now, how do we get there? We have to design the right starting conditions and then run Salt + Silver in the correct ratio and at the correct concentration to get either of the first two and avoid the last one. This requires a specific setup. If one starts from scratch, then one must do it virtually trial by error and you take what you get. I did it for a few runs and nearly went broke buying silver. Then I tried AgBr and it worked first try.
The above might answer you in part. I've said before that you can take a given published formula with all of the kinks worked out such as the Brovira formulas, and come up with a slow, low contrast result with poor keeping, but it will work and work repeatably. But, to start from scratch and get everything you need, well, that isn't easy.
Right now, the Azo type emulsion will keep about 1 - 2 years as is, and the coatings will keep over a year with no visible change at room temp. The Kodabromide type coatings have kept for a year and the emulsion for 6 months or more. The problem is that I keep running out before I can test the longevity the same way as the Azo type because it is harder to make. The AgCl/Br are even harder to make if you want them to keep as I noted above and as commented on by Glafkides. He reports that a grade 5 paper can become a grade 1 paper with high fog in just a few weeks if you don't make it right and then he leaves out the step. That step is the addition of a tailored amount of a keeping agent clearly stated in the BIOS and FIAT reports (which you appear to have). I don't have my notes here and have not looked it up for a while. The reason is that the compound crystallizes out as soon as I add it to the emulsion using the method given in those reports.
Another misprint? IDK, but I cannot stabilize a Brovira type emulsion yet. (AgCl/Br or AgCl/Br/I)
Based on reading, my perception is that Cl/Br emulsions are primarily print emulsions while Br/I emulsions are primarily used in film -- is that accurate?
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Sorry for barging in with such a simple question, but...
I think we should first define why we actually want to do ClBr emulsion, instead of pure Cl or pure Br emulsion if they are that much easier to do. Wouldn't a pure Br emulsion make a very good printing paper? What are the benefits in apparently harder to make ClBr emulsion?
Yes, I did those experiments when the price of silver was much much lower.
When you give keeping times for the emulsion, is that refridgerated or at RT?
IIRC G. points to the instability of the particular metal complex used and suggests that a double salt would be better. He said it fell to a grade 3 I think.
As far as the compound crystallizing I thought I had seen a possible reason / solution the last time we discussed it, but I haven't tested my hypothesis.
(I don't have the compound in question.)
I found that the strictly "pure" silver halides are more problematic.
There are many differences.
But emulsions are very plastic, and all in all,
the differences can be dealt with.
YMMV but for the most part,
Either emulsion engineers engineer emulsions efficiently
or easy-going enthusiasts enjoy evolving.
These things generally have multiple controls:
Crystal habit, curve shape, D-max, stability, tone, toning behaviour, development rate, fixing rate, environmental friendliness, etc.
As I mentioned above, I found the "pure" halides less than optimal.
I searched a long time for the reason, thought I had found it, but now can't remember what it was.
I would say yes, but there are noteable exceptions. A lot of papers used today are different and slower special purpose films might be ClBr.
Last edited by Ray Rogers; 03-13-2011 at 05:12 AM. Click to view previous post history.