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I buy lots of candles, which means I end up with lots of candle holders. I want to be able to reuse these containers, in some cases for food storage since some of the candles are in jars. This means I need to completely eliminate all of the wax.
How can I ensure that the container is free of any wax residue?
I've done similar things after using food containers for candles. You may not actually need all these steps, but this is how to be very thorough.
What I'd do is put them in a cool oven, just hot enough to thoroughly melt the wax (100°C should be enough).
First, put them face down supported on wires over a drip tray. The vast majority of the wax will run out.
Then, still hot, and holding using oven gloves, wipe the insides thoroughly with a dry rag.
In terms of food safety, while I doubt you could ever reuse the containers commercially within the rules, I'd probably be happy at that point to use containers from unscented candles. After all, paraffin wax and beeswax are both used in food.
I'd still go a step further though, and definitely if anything scented had been in there: Finally, put them through a dishwasher on a hot wash (70°C or above). Paraffin wax and beeswax both melt a little below this, and once melted the dishwasher will remove the residue and any scent. Shifting the scent may actually be the tricky bit.
Apparently container candles are best made with lower melting point waxes anyway, so they should easily melt in the dishwasher. The earlier steps were more to protect your filters. If any scent lingers after dishwashing, either give up on that container, or wipe round with cooking oil, then fill to the brim with hot soapy water. Dishwash again after soaking.
Of course, there's no guarantee that the containers can withstand a dishwasher. Paint (or glitter etc.) on the outside would be particularly likely to be damaged.
I've recycled several glass candle containers into "whisky glasses". I put a squirt of washing up liquid in the bottom, then poured boiling water in, and left to soak/melt.
After they were mostly clean, rinsed out, then stuck in the sink, again with lots of washing liquid, and filled with boiling water.
Didn't get any breakages, and the "glasses" came out nice and clean - a couple needed a bit of extra scrubbing to loosen any "baked on" wax, but afterwards perfectly usable.
According to quite a few web sites, but specifically Science Direct, beeswax is insoluble in water, but soluble in a few different liquids.
Beeswax is insoluble in water and soluble in organic solvents, such as ether, acetone, xylol, benzene, chloroform, and tetrachloromethane. In order to completely dissolve the beeswax, the temperature must be increased beyond its melting point (Stefan, 2009).
For paraffin based wax solvents, the Chemistry SE suggests other solvents, specifically toluene or xylene. I have a quart of toluene in my garage, which implies that it was purchased a local big-box store.
Ethanol and acetone are not non-polar organic solvents. Each one has a slight dipole moment; due to the difference of electronegativity between H and O in ethanol and between C and O in acetone. Wax is composed of heavy, long-chain alkanes. And as "Like dissolves like" try to dissolve your wax in toluene or in xylene.
In many of the links I viewed, elevated temperatures are indicated. Not explosive level heat, just warmer than room temperature, perhaps hot hot water temps.
