>They always say glass (and some metals) are the gold
>standard in lack of permiability, but even they let
>helium though.

The problem here is that a helium molecule is the smallest molecule there
is, so it is going to find its way through the cracks and crevices in any
other molecule come what may. There is just no way to stop this, unless you
can work out how to make a solid vessel out of helium ...

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