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The key to the door of crystal types

Photo Engineer

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A number of years ago, Herb Wilgus at Kodak came up with this chart. It was used later by Bruce Kahn for his course at RIT. Bruce has given me his permission to use this chart.

I was one of the last people to 'interview' Herb for the emulsoin modeling and scaling software before he died of cancer in the 80s. He was a great guy and a great researcher.

Here it is, finally! Thanks to Herb and Bruce.

This chart taken with my other posts will enable anyone to predict the grain shape of any precipitation of AgBr. Of course there are charts for Chloride and Iodide, as well as mixed crystals. This is a key to the model we used and only works for double run (double jet) emulsions which are made under feedback control.

For those who ask about the other charts, I ain't tellin'!!!!!

For emulsions made using a single run of silver nitrate into salt, you get a mix of all possible shapes and these are K grains (klunkers, not Kodak).

PE
 

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Ron,

I do appreciate all of your help in this forum. I hope it'll stand for a long time. I fully intend to carry this art on into my generation. It amuses me greatly how you post hints to everything, but not the whole story. Sly person, making us reinvent the wheel!

...Can't say it's a bad thing though. At least for me, learning by a bit (or... a lot) of trial and error makes me remember. I hope I can still do all this when I'm 80!
 
Good Grief!! Part 2

PE,

When it rains, it pours! Another puzzle piece falls into place.

I am glad I took notes!!!

Thank you for sharing this with us...

(I wonder if the other charts can be derived somehow.....hmmmm)

Bob M.
 

PE,

I do not have a background in chemistry, but I'm trying to learn.

Is this chart showing the shape of the crystals when the film is manufactured, or after development? If it is after development, is the scale on the chart the PH level of the developer? Is this chart for bromide emulsions?

I apolagize for being so clueless about the technical aspects of all this and asking stupid questions.

Thanks.
 
This is the pAg of the emulsion when made, and has no relationship to pH or to developed silver. It is for bromide emulsions.

The only similarity between pAg and pH is this:

pH = negative log of the hydrogen ion concentration. The lower the number, the higher the amount of hydrogen ion and the more acidic.

pAg = negative log of the silver ion concentration. The lower the number, the higher the amount of silver ion in solution. This measure is no longer used at Kodak. We use a vAg scale which is the actual measured voltage.

PE
 

Thanks for the clarification. So why do the crystal shapes change as the pAg increases? Which of the shapes is ideal?
 
PhotoSmith said:
Thanks for the clarification. So why do the crystal shapes change as the pAg increases? Which of the shapes is ideal?
Click to expand...

The pAg is the determining factor for what type of crystal forms, or that is to say, the way the silver halide falls out like snowflakes as they condense from the liquid. So, just as you can get sleet, hail and flakes from water in the air depending on the humidity and temperature, you can get cubes, octahedra and t-grains from silver halides.

The grain shape is used to control speed, sharpness and grain. T-grains are very fast because they are flat and present a huge surface to the light. Cubes can be made very small and are used in papers and in print films for fine grain and sharpness. There is a use for every type of grain, even if it is to be a foundation to grow another type of crystal.

You could make a cube and then convert it to an octahedral shape by growing something over top of it. This would then become a core shell or converted emulsion.

PE
 
Very cool. I'm glad someone else has to worry about all of that, so I can use their products to make my images.

I find it very fascinating. I think the more we know about our materials and chemistry, the more control we have in mastering our art and craft.

Why are the T-grain films more responsive to variations in processing technique than the more traditional emulsions?
 
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PhotoSmith said:
Why are the T-grain films more responsive to variations in processing technique than the more traditional emulsions?
Click to expand...

I'm not aware that they really are. The film itself might be by design (see below).

T-grains do have more surface area than cubes or octahedra for equal mass, and that might be part of it.

OTOH, some conventional grain films are also more sensitive, I have heard. This might be due to the fact that almost all of the films today are made of blends of emulsion sizes and this causes them to be sensitive to some development conditions. Each component develops differently and some effort must be made in film design so that all of them react alike so as to make a smooth curve. Any error in processing can emphasize the differences in the blend.

PE
 
Thanks for taking the time to explain all this.

I assume that if we use high accutance developers, the developer doesn't change the shape of the original grain structure very much, but fine grain developers soften the edges of the grain crystals giving the appearance of finer grain. Is this correct?
 
Simply, the developer changes the form that the silver metal takes and therefore influences the structure of the image. This can be done for any shape or size of crystal.

It is more complex than this due to solvent developers, edge effects and etc. but that is it in a short form.

PE
 
Thanks PE. It was good to learn a little about silver grain structure in emulsions from you. I appreciate it!
 
Fascinating as usual, Ron. But I must admit that my first thought on seeing the title of the thread was that some new age types stumbling onto this through googling 'key', 'door' and 'crystal' were going to be in for a surprise.
 
Well, there is a double pyramid shape, but I doubt if it will focus any special energy for healing.

PE
 
Temperature Effects?

Reviewing this thread again with equations in hand, I had a new question. PE, I guess this is directed your way or maybe Kirk would know as well.

The crystal type chart in the OP gives pAg at 70C. But since the Ksp of bromide changes with temperature, pAg and vAg change with temperature too. If you are trying to make say, cubic grains, at a different temperature like 50C, do you aim for the same pAg, say 7, for a cubic grain? (Or t-grain, or octahedral or whatever you want to make)

-- Jason
 
To give na example, at 60C with 0.1 m/l NaBr, the vAg = -39.54 mv but at 50C it is 43.60 mv. At 50C it requires a sollution of 0.08 m/l to get to -38.16 mv or back to the original.

At 50C the pAg = 9.99 at 0.08 m/l
At 50 C the pAg = 10.08 at 0.1 m/l
At 60 C the pAg = 9.98 at .1 m/l

You can see vAg tracking pAg here but with a 5 mv difference in vAg but only about 0.1 units in pAg. Since I have not done the exact experiment you describe, I think you will find it best to try yourself. I predict little difference. The size may differ more than the crystal habit under your suggested changed conditions.

Is this good enough or do you want more information?

PE
 
So as long as the ion concentration doesn't move too far out of the range of your intended target, there will more likely be size differences but not great differnces in the habit due to temperature? Then it sounds like when you are designing, you would pick your target pAg such that you had some leeway for temperature and ion concentration drifts. You would want to be in the middle of the range, not at the ends, which would make sense.

Using equations from your book and RIT in a spreadsheet along with the chart you gave I have been reading through some papers from RIT and looking at what the authors did and plugging their numbers back into the model to see if they made sense along. Mostly, they do but some are iffy in that they are close to the edge of the range. Kind of a thought/paperwork exercise. It was raining the last couple of days and this was something interesting to play with.

I've got a silver electrode built but haven't gotten the reference made yet. But I have so many experiments planned it's insane - some of them may yet even get done!
 
Well, the RIT work is not as rigorous as that done at EK. After all, we had to control vAg all over the world.

PE
 
Yes, they are as well as that of the Debye Huckle equation. However, the experiments at RIT may not have been as exacting as those at Kodak. The EQG (Emulsion Quality Group) and CELS (Committee for Emulsion Lab Standards) made sure that a mv was a mv the world round.

PE
 
However, the experiments at RIT may not have been as exacting as those at Kodak.
Click to expand...

I wouldn't doubt that for a moment. On the other hand, many folks from or formerly from EK (ie Burt Carroll) and Polaroid are cited as assisting with or guiding some of this work, so how bad can it be? University master's thesis experiments are usually pretty low budget so they might be good for guidance of efforts that could be pulled off by individuals outside of the corporate environment.
 
There was much that Burt could not reveal to his students due to the NDA with Kodak. He could guide and teach, but only within certain limits. The programs at Chiba in Japan were far deeper than those at RIT in the same time period. I knew both Burt and Dr. Kubo from Chiba, and the description that I had was that the MS program at RIT was about similar to the BS program at Chiba. In fact, Chiba had a PhD program in photographic science and engineering.

PE
 
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