A question was asked on enamelforum @ Yahoo about enameling flaking
on forged copper. My response to the question, “Is it possible,
because the copper has been forged to different thickness, that the
metal is work hardened unevenly and therefore when it’s heated during
the enameling process, it ‘torques’, i. e. expands and contracts at
different rates and directions?”. Is it likely that annealing the
copper would make the process possible?
Here’s the original post from [name masked]: "Hello again enamellers
I have another challenge for you (brought to me courtesy of a Metal
Arts Guild colleague). Enamelling on forged copper. Can it be done?
and if so, what are the things you need to think about?
I have uploaded some pictures of the project [password protected URL
removed]
The close-ups show where the enamel is flaking off (showing very
porous enamel underneath) These are the samples we made with liquid
enamel 1070 white. The larger pictures show the challenging shapes
he is working with.
The copper is forged thick in some places and thinner in others in a
long sinuous curve. We have tried a couple of options (he wants them
to be yellow)
He tried yellow (1235 Ivory I think) and also 1010 white for a base,
but he had quite a bit of chipping. He tried allowing it to cool
down in the kiln, but it didn’t help. I tried using a liquid
brush-able enamel and it chipped less - but it still chipped. It is
not chipping off completely but flaking. I am actually assuming that
this is because of the different thicknesses which are expanding and
contracting at different rates - in which case it will be virtually
impossible to do - but I thought I would just run it by the experts
here before giving up …"
Hope that someone out there in Orchid village has a solution.
I am actually assuming that this is because of the different
thicknesses which are expanding and contracting at different rates
- in which case it will be virtually impossible to do
Not different rates. Copper has only one rate for thermal expansion
but, thermal expansion is expressed in inches per inch of thickness
per degree F (mm per mm per degree C) so thicker areas are going to
expand more than thin ones. If there are major variations in
thickness is a good guess that this is the root of the problem. But I
might also look at how clean the work is. If it was hot forged there
can be a lot of copper oxide on the surface that may not have all
gotten removed.
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
copper.
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
being done.
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
website.