"by submerging the container into a larger one with boiling or near boiling water. " Hmmmm... in photography if you need to process a film at 20°C in a room at 10°C a good way is to is to put the film tank in a large can of water at at 20° to buffer the temperature (of course you chemical should be heaten to 20° too). I've always called it a bain-marie even if no boiling water is involved....