  Vacuum seal foil. 
This very soft indium foil is used to create seals between glass components in high-vacuum systems.  The indium is soft enough to deform into any pits and irregularities in the glass just like a rubber seal would.  But unlike rubber, indium doesn't give off all sorts of vapors and leak like a sieve.  ("Leak like a sieve" being a relative term: In most applications rubber would be considered airtight, but some air and moisture does still make it through, and when you're trying to create a really good vacuum it would be completely inadequate.) 
Source: Ethan Currens 
Contributor: Ethan Currens 
Acquired: 30 October, 2006 
Text Updated: 19 November, 2007 
Price: Donated 
Size: 2" 
Purity: 99%
 |  
  |  
 |  
  |