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My response is based on intuition and experience with enamel and
copper, but I'm still guessing as I've not done any work on forged
Your first guess, that the uneven thicknesses could cause problems
could be true. Copper expands and contracts more than the glass will.
You could have created an exacerbated situation by adding torque, if
the forged metal has twist to it. However, this would generally
result in enamel popping off cleanly when cooling.
Another real concern is metal contamination. When you are annealing
and then pounding on the copper, what are you inadvertently adding
to the copper? Chemicals or particles could be forced into the
annealed metal and then trapped when the metal is being formed and
hardened. I have a suspicion that you have a contamination problem.
Your description of the porous nature of the enamel sounds like
contamination, possibly chemical.
What to do?...First, I'd check the enamel itself by firing on clean
copper sheet. If you get the same problem, there's your answer. If
the enamel is ok, then I would inspect the area where the forging is
I would clean the working surfaces of everything the copper is
coming in contact with really well! I would then wash it down with
distilled water to remove any traces of, well, anything! Don't forget
the hammers. Try a small forged piece. Clean with penny brite and
rinse well. Then, try thinner layer than you were using. Make sure
you fire completely. Sometimes, you can burn out some contaminants.
If you're getting blisters or little holes, you can file the surface
to open them up and refire and they might heal.
Sometimes you'll do this more than once. adding a light layer.
Good luck. You can reach out to the Enamelist Society if you have
more questions. Also, you might consider the TES Conference this
August. I believe the Conference is now open for registration on our