Pure indium or indium alloys are used to form hermetic seals that also conduct heat. Indium washers are often soldered around lenses which are metallized with gold.
From my perspective, the key to hermeticity is to have very clean preforms. The trouble is that indium readily forms a thin layer of oxides when it is exposed to air which should be removed prior to soldering a hermetic seal.
The recommended procedure by the Indium Corporation is to etch the indium preforms using a 5-10% HCl solution for up to 1 minute. I have heard of other methods though such as etching in different acids or plasma cleaning and I'd like to hear feedback from those of you cleaning indium be other procedures.
Please let me know what you think!
Mr. Clean picture retrieved from http://www.croatia.org/crown/articles/9421/3/Croatians-in-America—-photo-collection-by-Vladimir-Novak,-part-1