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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...

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    Have you tried a razor scraper? I would not use stronger caustic soda. I would not have used caustic soda at all for fear of damaging the window frames and for safety reasons. The most caustic agent I would use would be dilute ammonia solution. The pattern may have come from some lubricant or coating on the aluminum foil. – Jim Stewart Nov 23 '18 at 11:57
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    Yes, razor scrapers are your friend, work ahead of them with Windex or a weak soap solution to lubricate the blade, and don't spare the blades. You want sharp: dull is more likely to respond to error by digging in and scratching the glass – Harper Nov 23 '18 at 16:56
  • Bleach or sodium hydroxide are quite good at dissolving aluminum foil. Don't leave it on for too long, and rinse obsessively. Both chemicals will etch glass if you leave it on for 6months or a year. – Wayfaring Stranger Nov 24 '18 at 1:43
  • Bleach did nothing. A razor scraper is working. So, if any of you turns the comment into an answer, I'll accept it. – hmijail Nov 29 '18 at 13:11
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Sodium hydroxide or the main chemical in draino will dissolve aluminum. The green coating on the pellets is aluminum when in contact with water it causes a boiling effect , but I would use razor blades. Years ago when tinting windows was big we had to get the windows perfectly clean or bubbles in the tint showed. We used Windex and razor blades then a final wipe with a micro fiber cloth. The Windex or other window cleaner actually created a lubricant effect and the blades lasted longer but when debris are shaved off it helped to float the dirt and residue off the glass.

  • Sodium hydroxide is caustic soda, which as stated in the question has already been tried. One can buy it with higher purity in cheaper drain opening brands than Draino; in this way one also avoids the aluminium pellets or other non-helpful components. – hmijail Dec 1 '18 at 8:25
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The aluminum has left aluminum oxide on the window glass. Clean the glass as if you are cleaning corroded aluminum, with vinegar and water. Or use an aluminum pre-cleaner product.

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    As mentioned in the question, vinegar was the first thing I tried. No effect whatsoever. Regarding "aluminium pre-cleaner product", can you give some example of brand name or chemical composition? – hmijail Nov 29 '18 at 13:09
  • Perhaps a chelating agent like Oxalic acid might work. A common brand is Bar Keepers Friend. – JimmyJames Nov 29 '18 at 15:28
  • Reading about it, sounds too generic and mild for the described case. – hmijail Dec 1 '18 at 8:16
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Finally I used just a glass scraper, which removed almost everything. Notably, I tried "lubricating" as others have mentioned with soapy water or windex, but that doesn't seem to help at all while making the residue invisible, so I stopped doing it.

As a safety test, I tried to purposefully scratch the glass with the blade. I didn't manage to leave a mark, so it feels quite safe. So, another reason not to care for "lubrication". (Maybe if the blade was really forcefully applied? or maybe it depends on some glass type?)

Tip: I got a scrapper with a ~7 cm wide blade, which turns out to not actually reach the glass in the center. I guess a thinner scrapper would have better contact and scrape better.

For completeness: bleach did nothing. I didn't try with an even stronger solution of caustic soda.

If I had to do this again, since the results seem to depend on the particular glass pane, I think I would first start with caustic soda on a corner. If the result is perfect, then go for it. If less than perfect, then forget the caustic soda and go for the scraper, since the soda would just make that final layer harder to see.

(Self-answering because the original commenters who were in the right path didn't make their own answer when requested)

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