Thanks for all the comprehensive and informative answers gentlemen!
Doremus
If you remove the thiosulfate, the ferricyanide bleaching action results in virtually colorless and invisible silver ferrocyanide complexes as highlighted above by @Raghu Kuvempunagar . So also without thiosulfate the action is immediately visible.
Arguably, bleaching and fixing separately gives more control because you can go back if you overdo the bleaching step, at least if you use a rehalogenating bleach.
This is factually incorrect as anyone can attest to who has done this. The resulting 'blix' remains active for quite some time and in practice more than long enough to work on a print or negative. While it is not long-term stable and it is indeed recommended to mix prior to use in the quantity needed for the session, the 'within minutes' qualification is overly pessimistic.
I didn't contest it did, Ralph. Perhaps you need to re-read my previous post; I think you may have misinterpreted it.Farmer's Reducer(ferri+fixer)will perfprm exactly as I said earlier.
Am I correct in assuming you're relaying AI (e.g. ChatGPT) output now? Just checking. Either way, I notice you wrote 'ferrocyanice'. Farmer's reducer and regular 'ferri' bleaches are based on ferricyanide, not ferrocyanide. I suppose it's technically possible to start with the latter, but AFAIK it's uncommon. All commercial and DIY bleaches using this chemical approach that I've come across start with ferricyanide, which is readily available and cheap.Here's what happens when fixer is added to the potassium ferrocyanide solution:
This bit of the AI output is a little confusing, IMO. While taken at face value, it seems plausible enough, at the detail level it's a bit odd. Firstly, there's the mention of fixer supposedly stopping the bleaching, which is a notion that AFAIK hasn't been put forth by anyone. So it's a little odd that the answer involves a response to that statement, which wasn't made. Secondly, there's the odd issue involving ferrocyanide vs. ferricyanide. I suspect that this may have originated in a typo in the prompt in the AI tool which might have erroneously involved 'ferrocyanide' instead of 'ferricyanide'. Perhaps you can confirm by going back to your prompt and checking for this typo. Thirdly, there's the somewhat vague phrase about "the fixer simultaneously [dissolving] these bleached silver compounds" - esp. the 'bleached silver compounds'. What are they, exactlty? For a good understanding of what's really going on from a chemical viewpoint, it makes a difference whether the fixer facilitates the removal/dissolution of a silver-ferrocyanide complex, or whether a silver-thiosulfate complex is dissolved. The practical implication is whether the bleach is being 'used up' in the reduction (chemically speaking: oxidation) process or not.The fixer doesn't stop the bleaching—instead, it creates a more efficient and controlled process. The potassium ferricyanide (formed when ferrocyanide is oxidized) continues to bleach the metallic silver by converting it to silver ferricyanide, but now the fixer simultaneously dissolves these bleached silver compounds.
This is what I was responding to specifically, and as also put forth by @Raghu Kuvempunagar above, the silver compounds that result from ferricyanide bleaching are not really visible on an emulsion on paper. They are visible (with some difficulty) on film, but usually only in those areas where bleaching is complete, as these subtle milky hues are readily overpowered by any remaining metallic silver image. I'm sure you have done a fair share of bleaching of film and paper, as have I, so you must be familiar with this effect. Again, this is what I was commenting on, and the nature of my comment was that Farmer's reducer is not unique in immediately showing the degree of bleaching. A non-fixing bleach shows the same. This is true not only for ferricyanide-based bleaches, but also for instance dichromate- and permanganate bleaches.Without fixer, the bleached silver compounds would remain in the emulsion as visible residue.
I'd like to see chemical evidence of this. The thiosulfate ion is not a buffer to the best of my knowledge. Secondly, what are these supposed acidic byproducts of the bleaching reaction? If you look at post #15 where I posted the brief explanation of the ferricyanide bleaching mechanism as described by Glafkides, there's as far as I can tell no direct influence on pH. What am I missing, can you pinpoint this?The fixer helps maintain a more stable pH environment, as the thiosulfate solution buffers the acidic byproducts of the bleaching reaction.
Interestingly, staining is mentioned here, but no explanation is provided in the preceding content. Perhaps you can ask your AI tool to expand on that part of the answer.This combination creates a reducer that works faster and more efficiently than ferricyanide alone, with reduced risk of staining and improved control over the final result. It's particularly effective for local dodging and burning corrections in traditional darkroom work.
I didn't contest it did, Ralph. Perhaps you need to re-read my previous post; I think you may have misinterpreted it.
Am I correct in assuming you're relaying AI (e.g. ChatGPT) output now? Just checking. Either way, I notice you wrote 'ferrocyanice'. Farmer's reducer and regular 'ferri' bleaches are based on ferricyanide, not ferrocyanide. I suppose it's technically possible to start with the latter, but AFAIK it's uncommon. All commercial and DIY bleaches using this chemical approach that I've come across start with ferricyanide, which is readily available and cheap.
This bit of the AI output is a little confusing, IMO. While taken at face value, it seems plausible enough, at the detail level it's a bit odd. Firstly, there's the mention of fixer supposedly stopping the bleaching, which is a notion that AFAIK hasn't been put forth by anyone. So it's a little odd that the answer involves a response to that statement, which wasn't made. Secondly, there's the odd issue involving ferrocyanide vs. ferricyanide. I suspect that this may have originated in a typo in the prompt in the AI tool which might have erroneously involved 'ferrocyanide' instead of 'ferricyanide'. Perhaps you can confirm by going back to your prompt and checking for this typo. Thirdly, there's the somewhat vague phrase about "the fixer simultaneously [dissolving] these bleached silver compounds" - esp. the 'bleached silver compounds'. What are they, exactlty? For a good understanding of what's really going on from a chemical viewpoint, it makes a difference whether the fixer facilitates the removal/dissolution of a silver-ferrocyanide complex, or whether a silver-thiosulfate complex is dissolved. The practical implication is whether the bleach is being 'used up' in the reduction (chemically speaking: oxidation) process or not.
This is what I was responding to specifically, and as also put forth by @Raghu Kuvempunagar above, the silver compounds that result from ferricyanide bleaching are not really visible on an emulsion on paper. They are visible (with some difficulty) on film, but usually only in those areas where bleaching is complete, as these subtle milky hues are readily overpowered by any remaining metallic silver image. I'm sure you have done a fair share of bleaching of film and paper, as have I, so you must be familiar with this effect. Again, this is what I was commenting on, and the nature of my comment was that Farmer's reducer is not unique in immediately showing the degree of bleaching. A non-fixing bleach shows the same. This is true not only for ferricyanide-based bleaches, but also for instance dichromate- and permanganate bleaches.
What I responded to was the 'better control', which is arguably not correct in my view. Farmer's reducer gives immediate feedback and when used judiciously, it gives good control. But arguably, better control would be awarded by a bleach that (1) is reversible so that any overbleaching can be corrected, and/or (2) a bleach that's more gentle/not so fast. As to (1), Farmer's reducer is at the disadvantage as it's irreversible. As to (2), this can of course be controlled by dilution, but practical experience (which I'm sure, again, you have as well as I do) shows that at the same concentration of ferricyanide, Farmer's reducer is considerably faster (and more rapidly spiraling out of control) than a plain ferricyanide bleach (without fixer).
I'd like to see chemical evidence of this. The thiosulfate ion is not a buffer to the best of my knowledge. Secondly, what are these supposed acidic byproducts of the bleaching reaction? If you look at post #15 where I posted the brief explanation of the ferricyanide bleaching mechanism as described by Glafkides, there's as far as I can tell no direct influence on pH. What am I missing, can you pinpoint this?
To be accurate, we may have to differentiate here between 'fixer' and 'thiosulfate'. The AI response appears to use them interchangeably, but this is not correct in this case - and I can see how things might change if we take 'fixer' to more literally mean just that: a fixer solution that contains thiosulfate, but possibly also other compounds. Most photographic fixers are indeed buffered (not by the thiosulfate itself; typically with an acetic acid/acetate buffer) to maintain a favorable pH. Adding such a fixer to a ferricyanide bleach will add a buffer action to the resulting reducer solution. But it's not due to the thiosulfate.
Interestingly, staining is mentioned here, but no explanation is provided in the preceding content. Perhaps you can ask your AI tool to expand on that part of the answer.
And to reiterate, I disagree with the 'improved control', partly because of the faster action that AI mentions itself, and partly due to the irreversible nature of the reducer vs. the simple bleach. Whether the reducer is more efficient, as AI states, is also an interesting detail that's not clear in the present form of the answer. It might be part of the first bit I highlighted.
Overall, I think the (assumed) AI response contains some interesting leads, but I'm also quite hesitant to take it at face value as you can see. Perhaps it's good enough if someone just wants to know if they can use Farmer's reducer on a print and they don't care how it works, whether there are alternatives and whether such alternatives might be suitable or even superior for their use case. But for those who do want to know what happens and who do want to make the best choice of their own work, I think the response is incomplete at best, and misleading or even incorrect at worst.
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