PDA

View Full Version : Chloro-Bromide emulsions



Pages : [1] 2 3

Photo Engineer
12-25-2008, 07:47 PM
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

holmburgers
03-11-2011, 03:18 PM
bump

Photo Engineer
03-11-2011, 03:28 PM
You see how interested in this topic people are! ;)

PE

holmburgers
03-11-2011, 03:33 PM
Hahaha, that's what I thought!

I just hate to see a thread go unreplied...

Ray Rogers
03-12-2011, 03:17 AM
bump

Bump?

I don't understand this terminology.
Where does it come from?
Does bump = nudge?

Ray Rogers
03-12-2011, 04:18 AM
[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?

Photo Engineer
03-12-2011, 10:52 AM
Yes, Cl/I and Br/I differ in this respect. Cl/Br/I are subject to the same problems as Cl/Br.

PE

JOSarff
03-12-2011, 12:39 PM
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?

Photo Engineer
03-12-2011, 01:00 PM
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

JOSarff
03-12-2011, 03:13 PM
Is there an example of a chloro-bromide emulsion somewhere I can look at to see the level of complexity?

Kirk Keyes
03-12-2011, 04:13 PM
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).

Photo Engineer
03-12-2011, 04:30 PM
Is there an example of a chloro-bromide emulsion somewhere I can look at to see the level of complexity?

Joe;

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.

PE

Ray Rogers
03-12-2011, 04:35 PM
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....

Photo Engineer
03-12-2011, 04:40 PM
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.

PE

JOSarff
03-12-2011, 04:54 PM
Ron:

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?

Joe

Ray Rogers
03-12-2011, 04:58 PM
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?

Photo Engineer
03-12-2011, 06:26 PM
Joe;

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.

Ray;

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)

PE

Neanderman
03-12-2011, 11:12 PM
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?

Ed

hrst
03-13-2011, 01:29 AM
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?

Ray Rogers
03-13-2011, 05:04 AM
Ron;

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.)

In general...
I found that the strictly "pure" silver halides are more problematic.

hrst;

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.

Ed;

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.

Ray