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Re: A question

If you can extract a bit of the powder onto some clean paper (waxed paper
might be better), put a drop of vinegar on the powder; bubbling (generation
of carbon dioxide) would indicate that it is most likely sodium bicarbonate
(baking soda) or calcium carbonate (chalk). If you can't get enough powder,
though, it might be difficult to observe the bubbling. Also, make sure that
your "clean" paper does not itself cause bubbling!
You can tell these two powders apart because baking soda will dissolve
completely in a moderate amount of plain water, but chalk does not dissolve
at all.

Other substances fizz in vinegar too, but these two would be the most likely
If it does not fizz, it might be, as you suggest, mould or fungus, but it
could also be talcum.
-Kevin Martin
 the Papertrail

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