Go to your local hardware store. There are several glue removers in stock at such a store. I have used in the past, a product called GOO GONE.....it is a liquid and works on everything except suede or silk etc.

Read the back label of this product before buying it....they mention this product easily removes contractor's glue and I suspect that this might be contractor's glue on your mirror. Good luck..

Also try 'white spirit' available at all DIY stores and is cheap.....apply using a rag.

Try nail polish remover...

Burning it? Lucky it didn't crack.

So I'm trying to restore a mirror I got from a hotel (they threw it out while remodeling). There's some kind of industrial glue stuck on the surface (the side you look at). I don't know what it is. It looks like two strips of it are on there about the size of my finger in width. It's very thick and sticky. It doesn't peel off easily. I tried burning it and it takes a while (hey I live in a dorm, don't judge). Anyways, if you know what I can use to remove it, please let me know. I'd hate for this thing to go to waste. It's HUGE and really cool.