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I have a shell that develops a white power on the exterior, and when I wipe it off, it comes back eventually if it sits without being touched.

It looked exactly like white powder mold. Oh no!

But after reading on the forums, I learned that it can be something from in the wood being exuded outward, and it may not be mildew after all.

So here's what I did to determine that this white powder is in fact wax, not mold:

1. First, I wiped some of it onto my finger tip, and rubbed my fingers together, and it felt sort of slippery, like wax, or oil. Not dry, as I would expect from mold spores.
2. Then I took square of paper tissue, napkin, etc. Wiped the powder gently, using one finger to press, so that it accumulates in a small area of the paper under the fingertip. Not pressing too hard, just lightly. *
3. Looked to see if the paper looks smooth and shiny where the finger was pressing. If so, it may be wax.
4. Hold the paper up to the light. If the light shines through easily, almost as if it were oily, then it may be wax.
4. Finally, I twisted the area of paper which I had wiped, so it formed a tight wick, and burned it with a lighter (outside). I smelled the smoke, and it smelled like a candle, which confirmed that it was in fact wax.

So why does wax come out of the wood? Perhaps it has been treated with wax to preserve it, or perhaps it naturally contains waxes (many tropical woods do, to help them be water resistant), and they are seeping out over time.

*:There is a risk that if the wood surface is oily or waxy, it could rub off onto the paper too, so theoretically, it could pass this test, even if there was mold. So careful not to rub too hard, just lightly enough to collect the powder, not enough to rub off wax on the wood.