Iron bleaching for reversal B&W film, works, but how to improve

relistan said:

My suggestion would be to iterate on what you are already doing @Ausar . You have something that is headed in the right direction. Try using citric acid instead of acetic acid. In all the testing I did, I found that citric acid sequestered more silver than I was able to get acetic acid to do. It will stain, however, (forms silver citrate) and requires a clearing bath like the one you are already using. Additionally, having read a lot of patents, most target pH values are like I said, in the range of 2-4 for effective bleaches. I don't think I'd want either my film or my tanks in pH 0.2 soup, never mind my extremities.

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Ausar said:

Is there any alternative formula available？

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Ausar said:

ok, will do further test this weekend, including using EDTA-FeNa instead of using Fe2(SO4)3, let's see if it will make difference.

BTW, I heard that citric acid is prone to mold growth, any way to make it last longer?

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koraks said:

You could try sodium polysulfide / sodium sulfide. However, this breaks down to form hydrogen sulfide, which is smelly, toxic and tends to fog photographic materials nearby that are not packed airtight.

Note that thiourea is a common chemical building block, so the odds that you can somehow get hold of it in China are pretty good. There are thousands of tons of it in your country at any given moment, I'm pretty sure.

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lamerko said:

I have noticed that some chemicals in China have slightly different names or are less common than in Europe/USA.

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koraks said:

Yes, it may be easier to find under its alternative name "thiocarbamide". Or simply by CAS number: 62-56-6. According to Wikipedia, around 4000 tons are produced annually in China alone. Surely, some of it finds its way onto the local market, although most of it is likely used in the textile industry as a precursor of thiourea dioxide. The latter can also be used as a sepia toner btw, but I've never tried it myself. @Cor likes to use it on prints, but I don't think he ever tried it in a reversal process as we're discussing here.

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koraks said:

"Rehal" = rehalogenating.
The idea is that you develop the negative image in the usual way. Then you fog all remaining silver halide into silver sulfide using a thiourea toner (sodium sulfide / polysulfide toner will work, too). Afterwards, you bleach the original silver negative image using a rehalogenating bleach. This can be for instance a normal ferricyanide + bromide bleach. Finally, the silver halide negative image is fixed out.

The advantage is that this can be done with quite ordinary chemistry and the process is easy and robust. The drawback is that the silver sulfide image will be brown to yellow in color. It's not possible to obtain a neutral greyscale image this way. If you like sepia toned slides, this approach works great, however.

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koraks said:

Yes, I think it should work. The drawback is that you can't modify anything with bleaching etc. since silver sulfide is highly stable.

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