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I'm trying to turn a 6″ diameter mirror (I have spares) into a two-way mirror. It shouldn't be too hard to remove the backing, but then I need to remove most, but not all of the Aluminium plating.

My current plan is to remove the backing with paint-stripper, then soak the mirror in citric acid. However, I'm not sure how long it needs to be left in the citric acid, or even if that will work at all.

Are there any household, or easily acquired, chemicals I can use that will react with Aluminium in a controlled fashion?

Any help or pointers in the right direction would be appreciated.

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Citric acid will not etch pure aluminum on any reasonable timescale, at room temperature, though commonly-used aluminum alloys, high temperatures, or the presence of salt can change that. However, it may be better to try some other chemicals: here is a decent resource for finding metal etchant solutions. The two that are probably accessible with household chemicals are hydrochloric acid (can be found at hardware stores with pool supplies) and sodium hydroxide (often found at grocery stores as lye). Neither is super dangerous, but you'll want to take precautions to avoid getting them in your eyes, on your skin, or on your clothes and HCl produces hydrogen gas as it dissolves aluminum, so don't put big pieces of aluminum in concentrated HCl unless you have adequate ventilation.

It will probably take some experimentation to get the concentration right: you want an etch rate fast enough that you're not there for hours etching, but slow enough that you can control it. The big problem with wet etching aluminum is that most etchants take a long time to get through the surface layer of aluminum oxide (and the time it takes is not really reproduceable due to differences in thickness and such) so when you put a piece in the etchant, nothing may happen for several minutes, then you look away for a few seconds and turn back to find all the aluminum gone. This can be avoided by adding HF, which happily removes the oxide, but is very nasty stuff that is rather dangerous to work with.

It might be possible to do this by picking a low concentration etchant and checking the piece frequently, but you may run into issues if the surface is dirty (I recommend washing the surface with acetone, isopropanol, and deionized water in sequence) or depending on how the aluminum was deposited. Some deposition techniques produce large variations in film thickness across the surface and you might run into the situation where the aluminum will be completely etched away in one place but is still totally reflective in another.

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  • $\begingroup$ Thanks a lot! That's really helpful. So there's no way to slow down the reaction with $\ce{Al}$ without affecting the reaction with $\ce{Al_2O_3}$? $\endgroup$ – Zaz Nov 23 '15 at 20:43
  • $\begingroup$ @Zaz I'm not sure on this point. I've never bothered timing it with commercial etchants, but it makes sense that they would be related. Waiting a few minutes isn't really a big deal if it means you get a controllable Al etch rate. Increasing the temperature can also be used to increase the etch rate, so it might be possible to start hot for a bit, then cool it down, but it's probably pretty difficult to control. Your best bet is just to watch carefully. $\endgroup$ – Michael DM Dryden Nov 24 '15 at 18:33
  • $\begingroup$ Thanks. Am I right in thinking paint stripper isn't going to do much to the aluminium? $\endgroup$ – Zaz Nov 25 '15 at 3:13
  • $\begingroup$ An ordinary solvent-based stripper should be fine for aluminum. $\endgroup$ – Michael DM Dryden Nov 25 '15 at 18:21

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