So my friend recently had a yard sale, and let me tell you, I was buying. (Plus she gave me lots of stuff too.) Anyway she had this star shaped tea light holder that I thought had potential as just a decorative element. I thought it would look great with a mercury glass treatment.
The main ingredient needed is the Looking Glass spray, by Krylon. It's a little expensive, but it's awesome stuff. I've used it to make mirror before. You just follow the instructions on the can, but they basically entail spraying the back of the glass. I like it because it created the aged mirror look that I love. While making the mirror, I found it sped the process along if you sprayed black paint on the back. (Otherwise you have to put on a lot of coats of the Looking Glass.)
So to get the Mercury glass look, I found it works best to use the looking glass first, a couple light coats. While the Looking Glass is still a little wet, start spraying, other metallic paints - I had chrome and gold- on the backside of the glass. It's also great, while the paint is still wet, to hit it with some sprinkles of water. (It reacts cool with the paint and makes it more Mercury-like.) I forgot this step on my star. I finish off with a light coat of black paint. I just used chalkboard because that's what I had on hand.
Here's the finished product ... cloudy-mirrored, loveliness. Useful, no. Pretty, yes.