If it has a beveled edge it will be at least an inch wide usually and you can see it just looking at it from the front !! It will be completely around the mirrors edge !! If you can not see a beveled edge I would just have another cut to fit and replace it !! Since I don`t know how big this mirror is I could not say what it would cost !! I would hunt thrift markets for one larger than the size you need and the cutting by a glass shop cost should be minimal !! You can certainly put a backing on it yourself using a heavy piece of cardboard !! Just cut to fit and use push points to hold it in !! Might cost you an hours work and a $2.98 pkg of push points !! I have never heard of anyone becoming ill from a mirror however,,since you don`t know what was used on this one its best to either cover it up with a backing or replace it !! When I had my shop and got a piece with a beveled mirror that was unusable,,I took all of the old silver off with paint stripper cleaned it well and had a 1/8 inch mirror cut and just put it behind the clean beveled glass !! When you looked at the front you couldn`t tell that it had been done !! It did make a heavy mirror though !!
