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Also I would suggest that you use duct tape or more of the blue masking tape and cover large parts of the mirror over. This will keep the glass intact so it does not fall all over the place if it shatters.
Remember the mirror is not really that expensive, and as it ages it gets more brittle with age. Be sure to work with eye protection and sturdy gloves and long sleeve shirt. or jacket.
A sharp razor blade scraper or perhaps a sharp wood chisel may be able to scrape this tuff stuff off.