Fusing glass onto copper sheet
I agree with everyone who commented. However, it is marginally possible to consider backing your already fused glass piece with copper FOIL sheet (very thin, like the foil used in stained glass construction, but in sheet form), and only taking it to a very low tack fuse …. around 1300 degrees. I know the foil is available in sheets with the adhesive on it, but not sure where to get it without adhesive. You could try disolving the adhesive and cleaning it well before the attempt.
Cynthia Ann Swan, Terra Pax Studio, Los Angeles, CA