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I'm working on a 19th century photographic process called Anthotype. It basically involves using plant juices that fade when exposed to sunlight. For my light sensitive juice I've choosen concentrated alcoholic chlorophyll extract. After a little less than 20 minutes on full sunlight it fades completely! Where the normal exposure time for anthotypes is, say, 3 days.

But there's a problem: Of course I know that anthotypes cannot be fixed and I'm trying to do research on it. There's no reason why the untouched plant juices can't be altered to be made iresponsive to light. This is the case of chloropyll, and you can combine it with copper sulphate to fix it completely.

The point is: I don't have such chemical, and while I can get/ make it I have to store some prints I've made.

Is someone doing something similar? How can I prevent the photograph to overexposing after it has been done?

Some things I can try but don't think will work are: Mordants, increasing the pH (chlorophyll is more susceptible to degrade under acidic pH), protecting the print with wax so to get rid of oxygen radicals that may destroy it, seling it on vacuum inside a plastic ziplock bag...

Any help is really useful!

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    \$\begingroup\$ Store in complete darkness? \$\endgroup\$
    – Michael C
    Mar 4, 2019 at 20:17
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    \$\begingroup\$ Copper sulphate seems to be readily available on plain amazon..... so complaining about a lack of copper sulphate is rather a case of misplaced vitriol. Read the MSDS though. That stuff is unhealthy and an emetic, don't get it in you. If you can get spent circuit board etchant (sodium persulphate based), that should also contain a good amount of copper sulfate. Don't pour any of that sh..t down the sewer though! \$\endgroup\$
    – rackandboneman
    Mar 4, 2019 at 20:53
  • \$\begingroup\$ Decades ago, when I was chemistry-set age (maybe 11), I'd buy a can of the stuff off the shelf at the local pharmacy. I bought potassium permanganate (to go with glycerine), mercury, etc. too. The CuSO4 was great for electroplating things with copper. Life was so much simpler before those pesky MSDS things. \$\endgroup\$
    – Ray Butterworth
    May 10, 2021 at 0:21

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Perhaps try a weak borax and water solution. It works to fix turmeric- based anthrotypes. Also consider grass- stained clothing: the the heat or detergent seems to “fix” the green stains and make them hard to remove.

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