We know that the image is formed by the silver layer, so why don't we use silver layer on some other material in household mirrors?
closed as off-topic by ACuriousMind♦, Diracology, user36790, John Rennie, Gert Jul 11 '16 at 0:58
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In a mirror the thin layer of metal at the back provides the reflection.
If this metal is exposed to air it will 'tarnish' - ie become dull due to formation of oxides on the surface. It can also be scratched quite easily. Both these reduce the quality of reflection. Glass provides a layer of protection for the metal. Glass is transparent and also hard, not easily scratched. It is also stiff and brittle, so it does not easily bend and distort the image.
The layer of glass can actually make the reflection worse, by generating multiple reflections (ghost images) at the air-glass surface - most noticeable at large angles of incidence.
Protection of the coating layer that reflects and is easier to clean. And the other reason is glass is cheaper to make with small surface defects.