I make my own lightweight concrete pots as containers for the candles I make. I use a non toxic resin based (manufacturer’s product statement) concrete floor sealer to seal same. I also use perlite as my lightweight aggregate, which acts as a heat insulator (as it is used in concrete insulation material in some outdoor ovens etc,). My pots don’t gets as hot as glass or metal containers. I have been wondering about the sealer I use and if it melts or not. I don’t think so. But to help further this discussion, I have poured a little sealer off into a small container and will heat test in a hot water bath in 7 days time (curing/hardening time). Will see if it melts in similar situation to melting wax using a pit and container with wax in it. Hopefully, this will give me an idea if the sealer I use decomposes at all under heat and if so at how hot a temperature it will. Not total science, but close enough for my purposes.
Honestly, I doubt even if the sealer does decompose a little under heat, in environments where candles burn, I’m thinking there should be enough clean oxygen in the air to dissipate any potential nasty chemicals, which would be minute in quantity it’s the he sealer I use, I’m thinking (hoping?). Who knows?
As to concrete being porous, sure, but if your pots are made with a tight enough matrix, seepage should not be an issue in the candle making application you are talking about IMO. For me, the secret is getting your concrete mix (your slump) wet enough to work the air spaces out of, but dry enough for it to set as strong as possible. With ‘heavyweight’ aggregates (like stone or sand), this should be achievable IMO. I seem to be able to achieve it with a lightweight aggregate (perlite). Not sure my pots will seep even without sealer, but I may be wrong. Might try that next batch!
As I said above, this is a good discussion. I’m always keen to research and learn and have my ideas challenged. Cheers.
