[ Nicholas Simons wrote in 060525 MMDigest:

> When I asked about the origin of Gold Fleece I was referred
> to the back end of a horse!

This may be akin to something called "gold beaters' skin" which
for centuries was used as a surface on which to pound out gold leaf.
Very thin, very tough, it is commonly derived from the caecum of cows.

As for airtight, will hydrogen do?  Gold-beaters' skin was used to
line the immense silk gasbags (no, not politicians...) that provided
lift for the Zeppelin airships.  Since each piece was about 8" x 24",
hundreds of thousands of cows gave their all for each Zeppelin.  There
were about 125 Zeppelins built for WW1, so this was a fundamental
reason for the close relationship between Germany and Argentina during
that era.

A. B. Bonds