Taking off a mirror coating

The best way to take off a mirror coating is by means of etching. Aluminium etches away quickly in a solution of Copper(II)Chloride.
The easiest way to make a CuCl₂ solution is by dissolving Cu in Hydrochloric acid (HCl_aq). You can purchase a 10% concentrated solution in most building markets, where it is sold as means to take out cement stains from walls. As a source of copper the easiest is to put in some CopperSulphate (CuSO₄), however this is not what I did.

An alternative way to get Cu dissolved in the HCl_aq solution is by means of electrolysis. What you need for this method is a glass container, two beefy copper electrodes and a current source able to deliver at least 1 Ampere. It will operate at only a few Volts.

The reaction at the anode will slowly dissolve the copper electrode in the acid. The cathode will attract surplus copper from the solution, which can be seen as a deposit on the electrode. A part of the copper remains dissolved however, and gradually colours the fluid green (see right image). When the current decreases, wiggle the electrodes a bit (especially the anode) to get the current up again. At some point the current drops quickly after wiggling; this indicates that the solution is saturated with copper. The solution is ready for use!

First make a tape rim around the mirror edge, so that the fluid will stay on the mirror surface. I used paper painters tape for this. Then pour the CuCl₂ solution on and move the mirror a little. Do not to spill a single drop, because it will etch any metal. I use a large plastic bin to keep the lot in. After only 15 seconds the coating becomes translucent, and after 1 or 2 minutes the process is completed. Pour the solution back in the container for future use and rinse thoroughly. You can neutralize what is left with e.g. sodiumcarbonate (NaHCO₃) aka baking powder.