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You asked about safety. Sodium persulfate isn't that risky [1]; your main concern is respiratory, so keeping the temperature low is a good intuition. If you decide to work with higher heat to accelerate the etch, wear breathing protection.

Sodium persulfate is a strong oxidizer, which is what makes it an effective etchant, but that also makes it a fire risk. In storage, keep it cool and dry; silica gel moisture scavenger packets probably aren't a bad idea. And, of course, keep it away from anything flammable.

Finally, it's a good idea not to dispose of the used etchant solution down a drain. Check with your municipality to find a safe disposal method, rather than add it to the ecosystem where it will poison wildlife.

[1] https://www.fishersci.com/store/msds?partNumber=O61141&produ...

Thanks for this! Would it be okay to etch with the container lid on? That would take care of fumes even at slightly higher temperatures, but I don't know whether it'll cause some other problem.

I'd hesitate to try that. You'd be making a pressure vessel, and while this doesn't seem a tremendously energetic process, it will become more so with increased temperature. Having a small explosion in your flat probably isn't a good time, either!

If it were me, instead of a sealed container I'd just run the process under an open window with a fan propped in it to exhaust whatever fumes develop, and maybe under an improvised fume hood made of a cut-open garbage bag and masking tape. (If you do that, make sure to leave space at the bottom for intake air. The idea is to make a funnel that will pull room air in, mix it with process gas, and exhaust the lot outside instead of into your living space.)

The window-and-fan method worked well when using vinegar-based iron acetate stain, and later polyurethane sealant, on a quite large table I made back when I lived in a tiny apartment; for a low-volume process like this, it'll probably do just fine, I'd think.

That would be a good option in the summer, but with sub-zero temperatures in the winter, that would probably not be ideal :/

I might try a sealed container with some kind of valve or loose seal and see, thank you!

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