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)




I did a little bit of research on this online, and I really could not find much useful information on the topic; however, I will try to give my best educated guess as to what is happening. My guess is that because the window was wetted, aluminum oxide was likely formed; aluminum oxide is present on aluminum foil in a very thin layer; however, it is a very tough material, so it usually stays on the foil. My guess (remember this is just a guess) is that the glass (silicon dioxide) is somehow more more adhesive in respect to the aluminum oxide than the actual aluminum. If this was the case, as the glass pulls aluminum oxide off the aluminum and onto itself, it would expose a fresh layer of aluminum that could be oxidized again and undergo the same process (form aluminum oxide then adhere to the more adhesive silicon dioxide). This would likely explain the reason for the white residue. The reason why this doesn't form on food, however, is because food is not likely as adhesive towards aluminum oxide as aluminum itself is, so no aluminum oxide will leech onto your food. Now to your question on how to remove the aluminum oxide: My guess is that using an acid (possible dilute HCl) would be more beneficial to removing the Al2O3 than using a caustic base. It probably doesn't matter which is used but to me an acid seemed slightly more preferable due to the following reasons. The reason for this can be seen the reactions between the two chemicals and aluminum oxide: 


$$\ce{Al2O3 + 6 HCl -> 2 AlCl3 + 3 H2O}$$


$$\ce{Al2O3 + 2 NaOH + 3 H2O -> 2 NaAl(OH)4}$$


As you can see, the reaction between the caustic base forms sodium aluminate, a soluble substance that can readily combine with other available cations like calcium and precipitate out, if you were using tap water with this, there might be a very small residue left from this reaction if the aluminate ions were able to undergo a double replacement reaction with a cation such as calcium; however, the reaction with an acid made a completely soluble substance that will not likely leave any residue. Again, I am not an expert in this field, but I hope this guess as to what is happening is helpful. 


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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> 


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This is very interesting, in a too-complex sort of way. Aluminum is an active metal; glass has a complex surface; and water is reactive by itself, and also from minerals and chlorine that could be in it.


The suggestions for cleaning might be workable, but I'm going to suggest an entirely different track: elbow grease. With toothpaste. There are some cooktop cleaners that involve very gentle abrasion and might be better than toothpaste, but I don't think a uniform application of some mild liquid is going to clean the entire surface equally clean (so that you can't see the original pattern). 


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