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Yes, Tony is right. The vial MUST have a small amount of the air *always*. Otherwise, the vial can open... :S

I use vegetal paper - just in the case I use ethanol! - for vials (inside I put ethanol 70%) and write with Staedtler pencil. Even if the alcohol fall on the paper, the writing remains the same.
I never sent dry specimens though. (that's why I asked for that in another thread... and crex.;)
If somenone wants to have vegetal paper let me know! This is the best for wet :) specimens. I put the vegetal paper inside the vial with ethanol and I never had problems! Of course, I have other labels outside the vial. But it works.