If you like a more subtle look though, here are the easy, harsh-chemical-free steps...
You can do this to the underside of any glass you have. I tried it on my glass cabinet doors, but the possibilities are pretty endless: the underside of glass tabletops (you'd want to seal that), a regular picture frame with glass to be made into a mirror, interior french doors for privacy (I'd give the inside a fresh topcoat to match the doors though), glassware and ornaments for a mercury glass look, etc, etc.
You'll need only a few things to get started: Krylon's Looking Glass spray paint (I found mine at Hobby Lobby - Michael's stopped selling it), a half and half mixture of white vinegar and water in a spray bottle and some paper towels.
After taping off just the underside of the frame of my cabinet door, I lightly misted the vinegar mixture on the glass. If I could go back I would have really doused the glass the first round and especially focused on a few spots, rather than doing a uniform spray.
Then working quickly while the vinegar is still wet, spray the Looking Glass all over.
Wait a few seconds (maybe 10 or 15?) and then wipe off the glass.
You'll be left with a cool, patchy look. I almost wish I had just stopped here and spray the whole thing black for a great contrast against the silvery mirrored spots, but you know, live and learn.
I sprayed another light mist layer of the vinegar mixture and repeated the layer of Looking Glass spray and wiped it down all again. This is what it looked like after the second round:
Pretty, right? Again, definitely should have stopped at this point. But I lightly misted the whole thing with just the Looking Glass spray as a final step.
Even though I knew my mistake right away, I went ahead and repeated the steps for the matching door on the desk area cabinet, just so the two panes would match. I think it's still really pretty though. So much better than the clear glass! It's all sparkly and bright, just like real mirror. And I can actually use the cabinet for storage and not just pretty display stuff now. Total bonus.
When I tackle these big cabinet doors in a week or so, I'll probably go a little more dramatic with the mirror. I'll be sure to report back on how that goes.
Have you ever tried this project before? I've seen other tutorials that use these steps on the outside of glassware, but the end finish is pretty textured, so I'd probably stick to doing just the inside or underside of your glass if you're trying this on a bigger surface.