There are some wonderful effects when glass and metal are combined.  Here are some examples of glass fusing which incorporates silver and bullseye glass.  The effects are unpredictable and can be very beautiful.  Some glass is made with the addition of sulphur or selenium.  When silver is laid on top of the glass there is a reaction.  The red and green pieces (see image left and below) have silver leaf  on them and have then been capped with standard Bullseye glass.  The effect is seen around the square of silver (black smudging) which makes an interesting effect with the glass.