Quote Originally Posted by Struan Gray View Post
I have not made or seen Lippmann emulsions, but I am a physicist. You don't need a metallic mirror to get a strong reflection (jewel beetles and morpho butterflies do very well with lamellae of chitin and air). However, the silver lamellae in a Lippmann emulsion are created with a metallic reflecting surface on the back of the emulsion, and it seems clear that reproducing that metallic coating will maximise the wavelength-specific reflection when viewing. The phase shift when light reflects off the back of the emulsion will be the same with or without a metallic layer, but the strength and wavelength dependence will certainly be different.
I guess there's a difference between the Lippmann lamellae and say those of a Morpho butterfly wing. In the Lippmann case you're dealing with a volume recording medium. From what I read about the Morpho butterfly structures, I got the impression they mainly involved surface relief effects (perhaps akin to a "blazed hologram").

Quote Originally Posted by Struan Gray View Post
An aluminium front surface mirror will be clouded and/or etched by quite weak solutions of either acid or alkali.(...)

Al ion in solution also interfere with the balance of Ag redox reactions. Most darkroom books advise against using Al trays for this reason. I'm not enough of a darkroom chemist to say how bad the effects are, but the warning makes sense to me.
I don't know either. But regarding front surface mirrors I'm sure most of them have a very thin protective(MgO, SiO2 etc.) coating on the aluminium layer.

Quote Originally Posted by Struan Gray View Post
However, the silver lamellae in a Lippmann emulsion are created with a metallic reflecting surface on the back of the emulsion (...).
... a "bunch" of silver nanoparticles, Ag or AgCl/HgCl2, maybe even HgAgCl2 (after a mercuric chloride bleach) within a gelatin matrix. In any case this is not plain silver.