  Gore-Tex filter. 
The name Gore-Tex is best known for its use in clothing, but there are many technical and medical applications for this Teflon-based fabric as well.  This rather large filter bag is one, though I have no idea what it is meant to filter. 
(The purity figure refers to the percentage of fluorine in pure Teflon.) 
Source: eBay seller fabriclauncher 
Contributor: Theodore Gray 
Acquired: 8 February, 2009 
Text Updated: 8 February, 2009 
Price: $7 
Size: 48" 
Purity: 76%
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