My experience is that red/orange/and yellow enamals are the most
difficult to use. Fire the piece with just a little too high a
temperature or for too long a time and they go black. Too little
heat or time and the coating is "orange peal" rather than smooth.
My experience is that torch-firing directly onto a copper surface is
difficult, except for beads. The enamel powder is blown away for
one. Also, in my case, torch firing directly onto a previously fired
piece seems to alter the colour of the enamel.
My solution has been to fire on only one side. To prevent warping
due to the different expansion coefficients of the enamel and copper
I try to find some way of building in strength to the sheet, usually
by fold forming.