wetplate question for chemists

[smieglitz]

I took up wetplate collodion a few months ago and have recently run into some contrast problems with it. Part of these problems might be resolved by using potassium cyanide as the fixing agent but I am hesitant to use that chemical because of its toxicity and issues of storage. The historical alternative has been sodium thiosulphate in about a 20% solution.

I'm wondering if ammonium thiosulphate might be substituted for the regular hypo without ill effect. This question has been posed on other (wetplate) forums and the response by practitioners is generally: "Why take a chance? Use the traditional method..." I'm wondering if the rapid fixing agent might produce a brighter, higher contrast image more like the effect of the KCN. I can of course test the chemical fixing potential by running a plate through it, but I'm wondering about issues of longevity, stability, etc.

AFAIK, the wetplate emulsion is a combination of silver iodide and silver bromide salts formed by the reaction of a silver nitrate bath on cadmium, potassium, and ammonium iodides and bromides in the "iodized collodion." Anything here that might be problematic (w.r.t. color, archivalness, tonality, etc.,) if ammonium thiosulphate was substituted as the fixing bath?

Any insight appreciated.

Thanks,

Joe

[Aggie]

http://www.cwreenactors.com/phorum/ Ask here and they will know what you need.

[smieglitz]

Aggie,

Thanks for the pointer. I'm aware of that forum (and others) but I am seeking the insight of scientists rather than practitioners. I'd rather not pose this question on either wetplate forum for various reasons.

I'm hoping someone here with a background in chemistry might provide general information on what to expect if I substituted ammonium for sodium thiosulphate in the wetplate process.

Joe

[Ole]

All I can say is "give it a try".

I have seen with other old processes that ammonium thiosulfate tends to bleach the image a little, the finer-grained the image the more it bleaches. So with POP, VDB and lith prints I sometimes use a little of it if I want to clear the highlights.

Since cyanide has an even greater affinity for silver than thiosulfate does, I would expect it to have the same effect - only more.

There should be no problems with image stability or longevity when using ammonium thiosulfate instead of sodium; and it's much better than potassium cyanide with regards to photographer longevity.

You could try a "rapidized" sodium thiosulfate: Add a little ammonium chloride to the sodium thiosulfate. My OF-1 fixer is based on this, and works fine with most silver processes (and clears the highlights in POP and lith prints almost as much as TF-4).

[smieglitz]

Thanks for the info Ole. A slight bleaching effect may be just what I'm looking for to clear some veiling on ambrotypes.

Joe

[Kerik]

Joe,

You probably know this already, but I've read/heard that one of the risks of non-cyanide fixers is that there is a risk of the collodion emulsion shrinking and cracking over time. I have no idea weather this is true or one of those things that gets passed down without evidence. I'm using HCN for fixing and I think with a little care it can be used safely.

[Donald Qualls]

I'm using HCN for fixing and I think with a little care it can be used safely.

I think you're deeply confused -- possibly dangerously so, Kerik.

HCN is a gas at room temperature, or when dissolved in water forms Prussic acid; it's the gas generated when potassium cyanide or sodium cyanide contact acids, and is the active ingredient in the gas chambers used to execute criminals.

For fixing wet collodion emulsions, KCN, potassium cyanide, is your chemical -- it's still hazardous, in that it will evolve hydrogen cyanide if it contacts acids, but it's not automatically deadly like HCN is in quantities above the lowest concentration you can smell. I would note that in a demo video I've seen of wet collodion tintype making, the KCN fixing step was done outdoors, after development was completed and the plate had been washed to remove all traces of the (acidic, ferrous sulfate based) developer. Given the rapidity with which you must work in wet plate, I'd say cyanide fixer is more hazardous than it needs to be if thiosulfates will do the job.

[smieglitz]

All I can say is "give it a try".

And so I did today.

I tried Sprint ammonium thiosulphate-based fixer concentrate diluted 1+2 with water. I figure this is about a 15% concentration. (I called the copmpany but was told the formula was proprietary and they couldn't disclose the actual concentration-Kodak's MSDS for their Rapid Fixer part A listed ammonium thiosulphate content as between 40-50% so I'm basing my guess on that figure.)

Here's the test ambrotype on black glass. It took about 15-20 seconds to reverse the image and clear the plate which is much quicker than hypo. I also think the image appears a little bit warmer than when I use the sodium thiosulphate.

I was very pleased to see this result.

Joe

[JG Motamedi]

Joe,

There was a post somewhere on Bob Szabo's CW forum about using Ammonium Thiosulphate as a fixer. Unfortunately after changing software much of that forum's history is gone. As I recall, the gist (once it went beyond the "use the traditional method...") was that you could use ammonium thiosulphate with no major repercussions. Maybe once I work through the 25kgs of sodium thiosulphate I recently bought, I will give it a try too...

[smieglitz]

Joe,

There was a post somewhere on Bob Szabo's CW forum about using Ammonium Thiosulphate as a fixer. Unfortunately after changing software much of that forum's history is gone. As I recall, the gist (once it went beyond the "use the traditional method...") was that you could use ammonium thiosulphate with no major repercussions. Maybe once I work through the 25kgs of sodium thiosulphate I recently bought, I will give it a try too...

25kg!!! You can spike a lot of aquariums with that.

Ok. But hurry up. It is so lonely here in Farbyland. LOL.

What a great word - "farby."

Joe