Let's see whether this is a question for DIY or for Chemistry.
I put up aluminium foil on my windows for a few days, to test whether it was a good way to avoid the heating by the morning sun. I put it up on the inside, by wetting the glass with plain tap water and letting the foil cling to the glass. Some of the windows were done with water + a bit of dish washing soap; there was not a big difference in the result. The foil was applied with its shiny face to the glass.
After 5 days, I removed the foil (easily, as expected) to discover that the glass was patterned according to the imperfections in the way the foil had been applied. For example, if the foil got wrinkled while applying it, that was visible in the glass. Furthermore, the foil itself has a very slightly embossed pattern; this pattern has been transferred to the glass too, in patches.
The patterns have appeared in all windows, no matter whether I used plain water or water with dish washing soap.
Thinking the pattern had to be related to water hardness, I tried wetting them with vinegar. No result at all. There were also tries with window cleaner, rubbing alcohol, washing soda. No effect.
Assuming that the pattern is caused by some aluminium salt/oxide/whatever, finally I tried with caustic soda, hoping it would react with the aluminium. It worked, mostly: there are still some patches where you can see the foil pattern in the glass if there is light reflecting from just the right angle.
My question is: what else can I try to remove those last patches? The options I can think of are a stronger solution of caustic soda, or bleach, which also attacks aluminium.
For reference, the caustic soda solution I have used up to now was 1 litre of water + 3 tbsp caustic soda. According to the packaging, 1 litre + 2 tbsp is used for drain opening. I'm hesitant to go stronger because supposedly at some point the glass will get etched.
Also interesting: some of the window panes were much easier to clean than others, even if the foil was applied in exactly the same way.
Bonus points if anyone knows what actually got deposited in the glass. If this happens with tap water, I wonder how can this be used for food...