Silver iodide re-exposure and development

[Fragomeni]

I’ve seen a bit written about this but wanted to ask for clarity.

It’s known that iodine can be used for spot bleaching. That’s let a few to ask if it can be used for reversal processing and there are a few threads on this. This thread refers to a person who bleached an already exposed and fixed print and then redeveloped and got some level of redevelopment. This leads me to ask, if the print hadn’t been fixed yet, and had been exposed to light for a second exposure and then redeveloped, would a positive image result?

From what I’ve read, using iodine in solution (drug store iodine and colorless) bleaches a print to white. Water can be added to make it less aggressive. My understanding is that the developed silver is bleached and then neutralized so that it cannot be exposed / developed further but the remaining silver bromide in the paper remains unchanged. Apparently the same can be achieved with a solution of iodine potassium bromide but not potassium iodide (converts all silver including bromide rendering the print not able to be re-exposed). Much of this is explained in this thread which references an old German iodine-based reversal process based on what I’ve just described.

After reading all of this and then some other threads, I’m a bit confused. The old method described in German supposedly worked but in some responses in other threads, people have said that there can be issues with re-exposure. I’m just wondering if anyone has actually tried this in practice (based on the formulations described here — not alternatives — and also not just theorizing) and can clarify if bleaching in store bought iodine and then re-exposing will indeed create a positive image or is there something else going on that will cause some kind of fogging or other problem? Thanks for the help.

 

[grainyvision]

B/W Reversal bleach uses an oxidizer to basically turn silver metal into something that can either be directly soluble in water or fixed away in thiosulfate. The chemistry around this I understand to be that silver salts are more stable (ie, silver chloride etc) but that silver metal can be oxidized by these. Thus, you do first development, put it into the bleach and the end result is a relatively untouched undeveloped part of the emulsion, and a new part (that was silver). Iodine is an oxidizer, just as chlorine and bromine are, but iodine is easier to handle. Permanganate, dichromate, and peroxide are all proven possible to use for this type of bleaching and all are quite strong oxidizers. Ferricyanide is also an oxidizing agent, but seemingly can only be used to form either silver iodide, silver bromide, or silver ferrocyanide, all of which are developable. Copper sulfate is an especially weird bleaching since it can be used to form silver chloride, but also is a relatively weak oxidizer. Silver and iodine in your case should react to form silver iodide. Sensitization specs are also little bits of silver metal within a grain of silver halide, so it'd basically revert the emulsion to an unexposed state. Pure silver iodide emulsions would be very difficult to develop though, since the iodide released would also be a very potent restrainer. This could be how it can be used as a reversal bleach, use a weak developer that would be able to develop silver chloride/bromide but not do much with the silver iodide. The silver iodide is definitely still light sensitive, but due to the potential "cleaning" of the grains by the process it might be an extremely slow speed emulsion. Silver iodide is also extremely slow to fix out, so you'd likely have to deal with some bleaching of the silver image in order to completely clear the silver iodide left over.

Another fun way to do bleaching could be using hypochloric acid (available at hardware stores as muriatic acid). This would react with silver and form silver chloride. This could be used to "restore" ruined paper by fully developing them to silver, then bleaching it to make a (very slow) silver chloride emulsion that can be used like new. However, the contrast curve would be completely different, and it'd likely react with sulfurs etc which take silver chloride from a 0.001 ISO speed emulsion to a 1 ISO speed emulsion, so the end result would be pure silver chloride and with no impurities that increase speed. It'd be interesting if you could somehow add those impurities back in somehow, but I'm unsure thats possible

 

[Fragomeni]

My previous experiments with this ended up on hold and I missed this response earlier and am just seeing it now. This is great info and definitely worth exploring. Thank you for your reply and this info.