Score them as you would glass, and put the score line on the edge of a table and left the mirror and slightly tap it, and it w/break.

Are you serious, that's all it takes? Thanks slow will give it a try asap!

It would be a good idea to seal the back side of the mirror with any kind of sealer. They do make a mirror sealant just for stained glass. It keeps the backing from scratching when grinding or working with mirror. Hopes this helps, Sally

oh that tapping thing freaks me out. My stained glass teacher made me do that to learn the sound as it changes. Every time some thing would break loose and fall I would startle!

I'm not a tapper; I just manhandle it. After scoring, I put it on something solid (usually the freezer although a table is good LOL). I make sure the score line is very close to the edge but in the air. I lean on it so that my forearm is all on the glass holding it down then I pull the other side straight down. I'm not sure this makes sense. I'm barely awake!

I didn't mean tap the glass - I meant when you lift the mirror w/the scored line right on the edge of a good solid table, just lift the mirror and bring it down w/a little force onto the table and not SLAM it, just tap it enough to break it. Works every time for me. Be sure you score it all the way across w/the same pressure - where you skip, it'll come out jagged.

I Just did the exact same thing for my son . He had a mirror the size of a closet door - ¼" thick . I laid a T square on the glass - scored it -- and took it to the edge of the table and took the small end & gave it a snap over the edge. - Just don't be afraid - follow through with the snap down past the table edge. It breaks clean.
( I'm not a tapper - seems when I tap the score line - it breaks all chipped up )

Check this out

Here is a link that might be useful: Glass break