How to remove aluminium coating?

[theodog (Jeff)]

I have a 10" mirror in a sad way. Its coating is nearing 10 years and is badly degraded.

Can anyone give me the chemicals/proceedure for removing the old coating? It is not overcoated and I have some experience with chemicals.

Your help is appreciated.

[Satchmo]

A weak solution made up from Sodium Hydroxide ( available from the hardware ) will remove bare aluminium. Works better if the solution is warm ( and use rubber gloves ) . I can't tell you how much to put it, I don't think its that critical, but perhaps a heaped teaspoon in a litre of water and put the mirror in a dish with the face submerged.

Soak for a few hours and gentle movement of cotton wool over the surface will keep refereshing the face, and work on tenacious bits of coating that are lagging behind. . This won't damage Pyrex , but plate glass can get a lemon peel finish if left overly long.

Better still , get your recoater to do it, he will have amuch better idea of what he is doing.

Mark

[marki]

Caustic soda (sodium hydroxide) is available at bunnings in the cleaning section. However you should be very careful as it can etch the pyrex if its too strong. This is why its never placed in a burettes in analytical labs. As aluminium is amphoteric you would do just as well using a dilute solution (2.0 mol/L) of hydrochloric acid which will not affect the glass and will remove the Al quicker without heat.


Mark

[Glenhuon (Bill)]

Unless you are planning on recoating yourself or doing some other work on the glass, leave it up to the recoater. Its part of the service anyway.

Bill

[scopemankit (Chris)]

I don't like any sodium comppounds, as they can etch some glasses. I use ferric chloride (used to etch printed circuit boards) to remove aluminium coating. If the mirror has a quartz or silicone monoxide overcoat, refer it to an aluminizing plant because acids are required.

[rmcpb (Rob)]

I would say its best left to the recoater. They have all the gear and do it all the time.

[marki]

As I said I would probaly not use the NaOH as it can etch the glass. Dilute HCl will not touch the glass. You could leave it to the recoaters but it would not be as much fun. Ferric chloride would also work well and does not have the problem of dilution.

Mark

[Satchmo]

Sodium Hydroxide only etches glass if its left a long time in the solution. Pyrex is pretty robust. If you just remove the coating quickly and don't leave it sitting in there longer than necessary theres little risk. Plate glass is a different story.

I've removed a number of overcoated surfaces with a thing called `Green river ' solution. Its a mix of HCl and Copper Sulphate. The Copper Sulphate makes it much more active at removing coatings . You'd find the recipe on the internet. The process starts with a light buff with cerium oxide on a chamois to put koles in the overcoat , without touching the mirror surface. HCl is pretty aweful stuff to work with . Let the coater remove your coating if you can, he will have loads of experience doing it.

Mark

[marki]

Yep, that will work as well Mark. CuSO4 does a good job of increasing the reaction rate. My concern with the NaOH is that I have seen it destroy pyrex burettes which are calibrated to +/- 0.05mL. How then would a mirror figured to wave lengths of light fare? Again working with any of the strong acids (e.g. nitric, HCl or sulfuric) when concentrated takes a bit of knowledge and a lot of care. If you don't know how leave the stuff alone.

Mark