I understand your frustration, enamel can be infuriating ! There are
several points which would cause the enamel to crack.
First - it’s a casting. If there’s any porosity in the casting, the
enamel can ping off, sometimes after several firings. Castings
should be done under inert gas avoid this. There was a discussion a
week or so ago about this, and zinc in gold alloys. Sometimes heating
& cooling several times can lessen the problem - as your depletion
process which would bring the fine silver to the surface and help
drive off some inner air bubbles. But - this is probably one of the
main reasons for your trouble.
2: the recess is too deep - take the recess and surrounding bezel
down to 0.4mm max (as you say). As long as you have no undercuts to
trap steam or air, a slighly rounded recess will affect colour depth
not adherence. If you want a clean finished edge the top bit is
You shouldn’t need counter enamel on a piece of this heftiness, with
5mm metal under the enamel.
3: GET RID of the glass brush!!! They are horrid tools, leave
fibreglass in your sink wastepipe, and all too often in your fingers.
Get a new natural bristle nailbrush (yes, as for your gardening
hands), mark it “enamel only” and use that instead. Brush out under
running water and rinse lots.
4: number of coats - six thin coats are better than two thick coats.
Also make sure your enamel is ground very fine and rinsed well.
You should let it dry almost completely before firing, rest it on
top of the kiln. With a very thin coat this is a bit less important.
If you don’t touch it with your fingers between coats & when
repacking, you don’t need to brush out.
5: firing temp - as long as the kiln is over 750C (sorry I don’t do
Farenheit temps) and under 1000C you’re in the right area. The
temperature that the piece reaches is more important than the kiln
temp. That sounds strange, but when the metal reaches the firing temp
of the enamel that’s when the enaml will fuse - so in a hot kiln this
will be very quick, and in a lower kiln takes longer. Open the kiln
door briefly, watch the enamel, shut the door gently if not fused
(USE GOGGLES - preferably ones to protect against infra red, but
always use goggles. Always). For the first few coats, underfire
slightly, this will help it build up thickness.
6: cooling - put the piece, in the mesh or firing support, on top of
the kiln to cool. With a thicker piece leave for longer before
moving - two minutes or more. Time for a cup of tea perhaps.
7: finishing - a flash fire is preferred, even if you matt it after,
as this seals the top surface. If you and the piece have really had
enough firings though, you can get a fairly good finish with 400 then
800 diamond pads (or adhesive diamond abrasive strip glued to a flat
thing) and then you can use ordinary jewellery wet and dry 1200 grit
paper - in a paper mandrel and pendant drill (Foredom equivalent) or
if you have the patience of Job, on a sanding stick by hand.
I do hope this helps. Sometimes a piece just refuses to get
finished, but you can take joy from the learning curve (after you’ve
finished swearing!) Good luck Tamizan, Bristol UK