Several years ago I read an article by Bill Helwig on Plique a Jour using copper sheet and a backing of copper foil. Not until recently did I find the time to experiment with the art of plique a jour and remember this article to review as I began my exploration. I could remember him saying enamels do not adhere to copper.
There is still some part of that statement I have questions on and will get to those later. But for now wanted to share the process. You can read the whole article in Glass on Metal www.glass-on-metal.com/pastart/bridging_the_gap.htm but to wet your appetite here are several photos of his technique.
One of the wonderful properties of copper and enamels is the warm qualities it give the enamel! In image one he has cut out the copper sheet and in image two he has secured the copper foil to the back of his design. From here he has filled the cells and fired to the point of completion and of course there is fire scale to deal with but he has created a beautiful piece!