Andrew Gabriel wrote:

Depending on the quality of the back of the mirror, the acetic
acid given off during curing of silicone can wreck the silvering.
There is a special mirror equivalent which does not risk damaging
the mirror silver. A glass shop will have it, but I think I've
also seen it in one of the sheds.

I fixed some mirrors to the plastered walls of a bathroom some years ago.
The mirrors were fixed with several spots of silicone and the walls were
then tiled. Before fixing the mirrors I covered the backs with a self
adhesive foil to stop any reaction with the silicone. I bought the mirrors
and foil from a local glazier.

Assume you won't ever get the mirror off without wrecking it.

Yep.

--
Nige Danton - Replace the obvious with g.m.a.i.l