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I had always thought why you could not make the blank from aircraft grade aluminium, highly polished, and then coated and overcoated. Lightweight, thermal expansion should be acceptable (I would think).
Or are there simply some substrates that do not work well with silver coating? I suppose the aluminium would oxidise over time, but probably less than the silver anyway. Anodising would slow this somewhat.
Acrylic is another I would have thought stable and suitable, or perspex. (that does stabilise well at extremes of temperature). Acrylic HAS been used in refractors (cheap and nasty ones), the downside being UV crazing and fogging. Wouldn't matter so much with a mirror.
Interesting ideas. I guess we just traditionally stick to glasses.
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