[Arkcon]

I used to play like this myself, back in the day.  When household aluminum foil meets hardwarestore muritatic acid, it puffs and bubbles, releases steam and chlorine, and what's left has the visual properties of aluminum hydroxide in water -- a grey, gelatinous precipitate.

Kemistry Kaiser: the correct way to conduct an investigation is to not enter with pre-conceieved notions.  This applies as much to my pre-concieved notions as it applies to yours.  Are you sure its still acid?  What happens when you evaporate and rehydrate?  What is really left, and what is really given off?  Hydrogen gas is odorless, is this reaction of yours odorless?  Can you write balanced chemical reactions for all possibilities, and see which you have evidence for?

[Kemistry Kaiser]

I used to play like this myself, back in the day.  When household aluminum foil meets hardwarestore muritatic acid, it puffs and bubbles, releases steam and chlorine, and what's left has the visual properties of aluminum hydroxide in water -- a grey, gelatinous precipitate.

Kemistry Kaiser: the correct way to conduct an investigation is to not enter with pre-conceieved notions.  This applies as much to my pre-concieved notions as it applies to yours.  Are you sure its still acid?  What happens when you evaporate and rehydrate?  What is really left, and what is really given off?  Hydrogen gas is odorless, is this reaction of yours odorless?  Can you write balanced chemical reactions for all possibilities, and see which you have evidence for?

You make a very good point Arkcon. The amount of unknowns here is simply unmanageable. I'll write up the possible formulas and conduct several more reactions and test, to the best of my abilities, for individual compounds. In the meantime, does anyone have any suggestions for ways to test specifically for certain compounds? Any good analytical methods for this kind of wet chemistry? All professional opinions appreciated!

[Intanjir]

I sealed some finely powdered reagent grade Al(OH)3 with some ~30% HCl overnight. No noticeable dissolution.
I guess the Al(OH)3 that they use in antacids must have a very a large surface area. Or there is something special floating around in the stomach to speed things along.
I don't have a fume hood so I am not keen on increasing the temperature, but what I read makes me confident it would dissolve in about an hour.

Anyways it definitely seems then that the Aluminium Oxide/Hydroxide remains mostly intact in the foil experiment. Of course it only takes one small hole for the solution to have access to the aluminium metal so this seems consistent with the observation that any large piece of foil is broken up. Increased temperature from the exothermic reaction would then help attack Oxide/Hydroxide perhaps speeding things up for a time, but this would not be expected to progress too far toward completion given the time it still takes for the it to react in even hot solution.
The grey color would then just be some bits of aluminium small enough that a perfectly impermeable passivation layer was possible.