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Years ago we took over a showroom formerly occupied by a clothing chain that had mirrors all the way around - the cheapest, nastiest, smellyist fence paint held onto it for about 8 years before we smashed it all up in our refit.
Buy a big pipe and a pound or two of tabacco and sit for a few weeks blowing smoke at it.
Maybe not.....
For an oil paint glaze. mix brunt umber and black heavily diluted with a linseed oil/turps mix. The turps will make it flow better, but the linseed oil is what binds it to the surface, so be generous with the linseed. Clean the mirror first with turps and fine wirewool (it won't scratch) to degrease it. Try and get the glaze heavier in the corners.
With a bit a practice you will get the 'golden glow'. The glaze will take a few days to dry, so if you don't like the result you can wipe it off and have another go.
There are transparent paints for stained glass, which may work better, but I have never used them.
I collect and sell antique frames and frequently antique new mirror glass to put in them. I use solutions from here - http://www.antiquemirrorps.com/. It takes some practice but it works. Antiquing mirror glass is a great service to offer - one more thing to raise you up above the fray.