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.
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.
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...