Aluminium foil on windows: what is the residue, and how to remove it? I put aluminium foil on some windows' glass pane wetted with plain tap water, as a typical trick to stop sunlight. After 5 days, I removed the foil to find some kind of whitish pattern on the glass. The pattern follows the imperfections in the aluminium foil: bubbles, wrinkles, even a very slight embossed pattern in the foil.

To remove the pattern, I tried window cleaner, rubbing alcohol, vinegar, washing soda. Nothing worked.

Finally I tried caustic soda, at a solution stronger than that recommended for drain opening, and it mostly worked; but still there are patches where you can see the pattern if light reflects in the glass in just the right angle.

My question is: what caused this pattern? And, how can I get rid of its remains?

My next ideas are to try bleach or a stronger solution of caustic soda, but I'm afraid of etching the glass - what would be the maximum concentration advisable?

Also - if aluminium foil reacted to tap water like this, what happens when wrapping food and even cooking with it?

(I asked basically this same question in the DIY SE, but it's not getting actual answers, so I am hoping to get better insights here)
Finally I used a glass scraper. Bleach didn't help, and I didn't dare to use even stronger caustic soda. 

Fuller details at the answer I wrote at the DIY SE site: https://diy.stackexchange.com/questions/151160/how-can-i-remove-residue-left-after-removing-aluminum-foil-from-my-windows#answer-152162