[jacquesm]

Bare aluminum would oxidize in a very short time to something dull. It would still reflect light but not quite as good.

Aluminized Mylar would definitely work, but that's not bare aluminum but aluminum with a shiny layer over it to keep it clean.

Very useful stuff, I built huge solar concentrators with it. 1000 suns on an area the size of a poststamp. You can do some pretty crazy stuff with that kind of energy density.

[kragen]

You might need a micron of thickness of aluminum or silver. This piece looks like it's about 300 mm x 300 mm, which would work out to 90 mm³ of silver (or aluminum), which at 10.5 g/cc would work out to 950 mg of silver. Silver currently costs US$25.25 per troy ounce, so this would be 0.12¢ (US$0.0012) of silver, or somewhat less of aluminum.

Both silver and aluminum will tarnish if exposed to the air, silver more slowly but much more completely.

The process for silvering things is a lot easier to do at small scales than the process for aluminizing them. Aluminizing things normally requires a fairly good vacuum, and, moreover, a vacuum chamber large enough to fit whatever you're aluminizing. Perhaps someone will come up with some kind of convenient wet process for doing it but I'm not hoding my breath.

By contrast, you can silver glass with Tollens' test, using distilled water, silver nitrate, concentrated aqueous ammonia, hydroxide of potassium or sodium, and a reducing sugar (almost any sugar that isn't sucrose, for which you can substitute numerous other chemicals, such as formaldehyde, formate, isopropanol, or tartrate). This is commonly done as a classroom demonstration in chemistry labs nowadays, and it was done on a large scale almost 150 years ago for telescope mirrors. Nitric acid is beneficial but, unless you have to make the silver nitrate, not essential.

This is why it's much more common for amateur telescope makers to silver their mirrors rather than aluminizing them.

[jacquesm]

I've silvered a lot of copper plates for photography (Daguerrotypes).

[kragen]

Well, as usual, you probably know more than I do about what I'm writing about. Am I overlooking something significant? I don't even know if you can use the Tollens test to silver copper.