how to remove silvering from a mirror?

how can i take a mirror and remove silvering leaving a 3" clean glass border with a straight defined line between the glass and mirror?
thank you

There are two different types of mirrors, the classic silver nitrate version and the modern, cheap way.
The later often uses a Gallium based alloy.
Either way, both will be protected by a quite resilient paint coating, which is the worst part of the problem.
If you don't want to use more or less dangerous chemicals I only know one good way of doing this:
Get some quality vinyl foil - often for free from places that do custom stickers for cars and trucks or advertising signs, just ask for some discarded off cuts.
Cut some really straight strips, or whenever possible use the existing edges of the vinyl!
Work now one rim at a time!
Take the vinyl and remove the backing so get it like a sticky tape - make sure not to stretch it!
It works best to ony peel off a little bit to place and align it all before setting it in place permanently.
The straight edge show from the border between clear and mirror glass.
Make sure the vinyl is on there really good and without bubbles near the border.
Now take some steel wool and under running water rub off the coating and mirror finnish.
If the coatig is really strong you might need stainless steel wool as used for cleaning pots but be careful as in some cases it can create little scratches.
Once the worst if off always go back to normal steel wool.
Check against some light if all is clean and the border sharp and well defined - if not you can always use another strip or cut a mm off and start rubbing again - this time making sure you only work up and down the border but never towards it ;')
When done clean and dry everything properly before placing the next strip, repeat until all four sides are done.
On a cheap mirror this should not take longer than 30 minutes (excluding the drying and cleaning) for a good mirror it might be two hours.

iceng13 days ago

You understand the mirror "silvering" is on the back side of the glass !

I'd etch it off with muriatic acid, by supporting the mirror in a tank with 3" of acid in it.