The “Ecailles de miroirs” used to be laid in fresh plaster in the past in India, but this technique should be avoided because modern pasters cause rapid oxidation of the zinc layer at the back which forms the mirror. Silicon adhesives are also unsuitable for the same reason.
A polyurethane glue should be used, with a blob placed in the hollow of each scale.
They can be placed on the very dry substrate by pressing lightly.
Once they are laid and grouted, the scales are quite resistant to pressure but they still have low resistance to shocks.