Today, the enamel chipped during setting, so I started all over
and the second time the enamel chipped again. I do not see a
solution.
What you are doing is no different from the problems posed by prong
setting fragile stones, even though your prongs are at the back. The
key to most good prong setting is the seat and the preparation of the
prong. For the seat, first be sure the opening in the front sheet is
beveled appropriately to fit the surface of the enamel, so it’s not a
sharp edge in contact, but rather decent surface contact. Then you
need to fit the prongs. If you think for a moment what happens when
you bend a piece of metal, you realize the outside surface of the
metal stretches, while the inner surface compresses. The outside,
while stretching, sometimes gets an orange peel look, but otherwise,
unless it cracks, isn’t important. But the inside, when it
compresses, well, the metal has to go somewhere. It bulges inwards
toward your stone, or your enamel. What this does is to make the
prong contact the metal down low on the prong before you’ve bent the
prong all the way. Further bending can create fairly high and
damaging pressures at the point, even though the prong might have
started out not a tight fit to the stone/enamel, and isn’t yet
“down”. The key is to shape the inner surface of the prong so that it
takes this compression into account, as well as controlling where the
prong bends, and when. Visualize what the prong would look like if
fully set and finished, how it would be shaped on the inside. Then
imagine what it would look like if you then pulled it back. You’d
see a hollow sort of space there. So before bending the prong, use a
bur (ball bur, etc) to hollow out the inner surface of the prong a
bit, especially a bit near the bottom where it will first contact the
stone/enamel. Then thin the prong’s end, sides and back a bit. This
allows you to control where the prong bends. Otherwise all the
bending is as close to the base of the prong as possible, and then
the enamel or stone will be forced to act as a fulcrum for the prong.
The aim here is to shape the prong so that, if you were to push it
over without the enamel or stone in place, it would still end up as
the correct shape. Done this way, you don’t end up with this fulcrum
point of pressure halfway down the prong’s inside. That’s likely
what’s chipping your enamel. What you want is for the prong to gently
conform to the enamel without pressure as you bend it, becoming fully
tight only at the end.
Needless to say, this takes some practice in judging just how to
shape the prong so it will do this. But try a couple test prongs.
Make one of your bezel preshapes, don’t bother to pierce the opening,
but cut the bezel away to leave as many prongs as you can. Use them
just bending over air to practice getting them to bend so as to reach
the correct final shape just from the setting tools pressure, without
needing the enamel or stone there to act as a fulcrum.
One other little trick. When you hollow out the prong, use a bur
that is a smaller/tighter diameter than the stone or enamel surface.
This way, as the prong contacts the piece, it first contacts just
with it’s edges, which exert less pressure, and can deform slightly,
flaring as you bend further, without putting lots of pressure on the
piece, while still letting you get the prong down flush to the
surface of the piece. Don’t hollow it out too much, as you don’t want
to make it deceptively weaker than it looks. Just enough so the
edges will contact first, not the whole inner surface of the prong.
You should also consider whether some of the problem might be that
your enamel is especially prone to cracking due to stresses in the
glass. Are you counter-enameling the piece? Do you perhaps need to
let the enamel cool more slowly after the final firing to let the
glass anneal a bit? Any other reason the enamel might be unusually
fragile?
Hope that helps.
Peter Rowe