Okay - here ya go - this is one of the projects I worked on this weekend. (I am making lots and lots of Christmas ornaments lately)
I know, you are thinking - thats a craft punch from the crafty-store! And you are thinking - you can use that with glass?!
Yes. Copper is fusible (it expands at relatively similar rates as the glass) and copper foil is relatively easy to work with. One way you can use it is with crafty punches, making letters and shapes and then sandwich these pieces of copper in between sheets of glass.
You must cover the copper in glass, both front and back... basically the glass must completely encapsulate the copper. The copper will turn colors in the kiln... the cause is the heat in the kiln consuming all the oxygen and when this oxygen depleted air comes in contact with the copper the color changes. As the glass melts and the copper is trapped, it stops changing color, however, before the glass starts moving (which doesn't happen until around 1400F) any copper that is exposed to the air in the kiln will change color. In the picture above you can see how the copper changes. This is a big 17" glass bowl, just clear glass, with a big copper circle for the color. You can see how the edge of the copper turns almost black (this is where the oxygen depleted air changed the color of the copper) and towards the center where the air could not get, the copper stayed copper colored.
Copper is not terribly predictable in glass fusing (as the color changes so easily and inconsistently) and sometimes its much more successful than other times (simply due to inconsistency) but seriously its beautiful and do-able and oh-so-tempting.
So go check out your local crafty-store and see what stamps they have. Then find some copper foil and have a blast. Or just cut the copper foil with an exacto blade - that works well too.
Just remember, you must sandwich the copper in between glass, and don't get your hopes up that the copper will look copper colored in the end. It won't. But it will look cool. Oh and try not to get too many finger prints and wrinkles in the copper, all this will show in the finished fused piece of glass (eek!)