The discoloration of the silver portion of the mirror occurs over the time. It is a common issue seen in most of the old mirrors. Our shower mirror
also has the same issue.
In general, mirrors are created with silver nitrate which gives the reflection or the mirror feature. This silver nitrate is protected with copper sulfate and paint
to prevent the damages. But with ages, the paint
gets off and the silver nitrates and sulfate oxides which cause the black edges. The moisture, mirror cleaning
products, air can be the reasons for the mirror rot
You can avoid such black edging with the help of a frame. Because the black edges happen only at the edges of the mirror. Thus adding a frame to the mirror can prevent mirror rotting.
You can also re-silver the mirror to make the mirror look new. The silver repairing kit is available in the market. You can repair it yourselves. Clean the edges with distilled water
and apply the silver coating over the edges. Again give a second silver coat over the first coat and paint it with any mirror repair paint.