How can I remove residue left after removing aluminum foil from my windows?

Question

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...

Answer 1

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.

Answer 2

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.

Answer 3

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)

Answer 4

Blue sos pads (wetted down) worked on mine. You have to scrub a fair bit but I don’t see scratches on the glass at all. Maybe test in a corner.

Answer 5

In case there’s people still interested I used a cream cleaner for Electric/induction hot plates kitchen and scrub it off with a normal dishwasher foam (on the rough side) and perfectly clean with no major effort. No need big quantities, just a little bit and that’s it.

Good luck everyone.