All this discussion of precision use of aluminum foil reminds me of Phil Hartmann's old SNL skit - The Anal-Retentive Chef...

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(Okay... so it's not exactly on topic, but after reading about how finicky the foil is in these controlled conditions, I can't see it holding up in real world survival applications. I think you've gotta bite the bullet and carry an unlined tuna/soup can if you're gonna actually boil water - I'll bet the weight isn't much different than the HD foil either - and if you want light - maybe even a pop can might work, if you can support it's light aluminum shell with other survival supplies packed inside.)