Why is the mirror rotting in my stained glass project?

Mirror is made when one side of a piece of glass is coated with silver nitrate, making it into a mirror. A coat of copper sulfate is applied over the silver nitrate to protect it from damage, then a coat of paint is added to seal the back of the mirror from the elements. This prevents things like moisture and contaminants from coming into contact with the silver and copper coatings. The problem occurs when we cut up those pieces to use in our panel, mosaics or stepping stones. We break the layers of protection and expose it to our process of grinding, chemicals and water.

“Mirror rot” happens when those layers of silver and copper oxidize, and the result is black creeping in on the edges on your mirror. It is irreversible so we need to prevent it.

Mirror rot doesn’t happen overnight but you must reseal it as soon as you can. So, as soon as you cut the mirror you need to do something to protect it. Sometimes we can’t control how fast we complete a pattern but with mirror there is a clock ticking. Once mirror is contaminated you can’t reverse the effects.

After you cut the piece out try to grind the mirror as little as possible or use a mirror grinder bit to help minimize the damage to the backing. In addition you will want to flip the mirror over when you grind it so that the mirrored side is up, as not to scratch the mirror backing as you move it on the grinder surface. Clean each piece thoroughly with a non-ammonia cleaner. I use Sprayway cleaner (I have found it at Walmart on the top shelf). Then you need to spray or paint each piece on the edges and back (try not to get any on the front because it’s hard to get off) with mirror edge sealant. Don’t miss any spots or it makes this whole process pointless. You will need a well ventilated area. I lay all my pieces face down on paper outside because whoa, it has a smell! Then I spray with CLR mirror edge sealant but you can use the paint on kind as well.  Let the pieces dry fully. Now you can take your time with your piece and foil, lead or insert into your project where needed.