I trid a technique from a magazine, where you build up layers of UTEE (Ultra-Thick Embossing Enamel) and then stamp in it while it is hot. The magazine suggested not using a clear stamp because it could melt. Well the stamp material stamp worked fine, and didn't melt, I stamped so deeply it came off the block and I had to peel it out of the UTEE. The effect was a bit disappointing and won't show up in a photo very well.
I think the stamp material is pretty heat resistant so I decided to try it out with molten candle wax. Now if you search the internet for stamping or embossing on candles, you get info on how to attach bits of stamped tissue paper onto candles. Who knows what chemicals are in the smoke if it burns down to the paper?
What I did was to melt a flat piece of candle with my craft heat gun and press the stamp into it. The stamp again came off the block but was easily peeled off. I got a deep impression as the wax came up through the holes in the design. So you can get a customised design on a candle without adding anything that might affect its safety. You can do this with bought stamps of course but it might be hard to clean any wax out of the indentations.
My next step will be to try attaching the stamp onto the inside of a metal candle mould and make a candle in the mould. If the stamp pieces come off the mould as the candle shrinks, it would work, I think. The stamp material stamp will curve so I think it could be used in the inside of a soft drink can. Its possible to cut open the can mould to get the candle out, especially if you use a thin cheap one.