Hello Laura -
I do not have experience with worked with personally made mokume gane
- I only have worked with the ready made material that has a sterling
backing. And I agree - this is most likely a better question for an
expert like James Binion!
However, I would like to share a trick that has worked well for me in
soldering flat surfaces of 14k to sterling or sterling to sterling.
I will very slightly dome the smaller flat piece added on top of a
larger flat section. Before describing how i do this - I want to
stress that it is a very very slight doming effect - hardly
negligible. So, to achieve this, I will tap the backside (the surface
that will be connected to the bottom piece) with a very slightly
rounded end piece of dowel - by placing it topside down on top of a
strip of leather - to do the tapping. My punch for this strategy is a
section of a broom handle - with a very shallow filed dome on the end.
This will give just the slightest dome and during soldering, it seems
to allow control of the warpage (the “relaxing” of the metal during
the soldering process) in the direction of flattening in the center
and keeping the edges flat to the surface - preventing warpage in an
undesired way. (Did that make sense, I hope!) The bottom piece is
left totally flat.
Usually, with flat to flat surfaces - I will melt the solder on the
smaller top piece - resand (clean surface) reflux - then, place where
desired. Of course the same cleaning and fluxing is done to the
surface of the bottom piece as well.
To help keep the piece from shifting . . . although, it is difficult
to use binding wire with flat to flat surfaces, it is not impossible.
Little kinks or bends in the binding wire with the pliers - can help
to secure the piece in particular areas. Binding wire was always a
must whenever I made large pieces like the porringer or cup. Yours is
definitely a larger piece. Taking the prep time ahead and making sure
everything is nicely cleaned (even the solder) will generally save
time in the long run.
Sometimes during the soldering procedure, I will even be ready to add
a little active pressure with a tweezers when the solder is at the
point of flow. With the tweezers opened, an equal and opposite
pressure can be applied - but it is really necessary to be very steady
and not shift the piece by mistake. The whole set up needs to be very
stable - if interacting with the piece this way. (Oh . . . when your
piece shifted and the flux got sticky - just reheat it again until the
flux becomes fluid again and reposition quickly.)
I will heat from below on a tripod set up - for this type of
soldering. (If it is a small piece - it can be held in a tweezers and
heated from below in the air.) For the larger size you are working
with, the containment idea of a firebrick makeshift wall enclosure set
up around the tripod would probably be helpful - especially if there
is a breeze in the soldering area. When we were doing large pieces
this past summer - having a larger bushier flame torch along with
another torch with a more focused tip - as a combo is good - actually,
essential. One person keeps the whole piece up to heat - constantly
moving the larger flame around the whole piece, and the other torch is
focused where the solder needs to flow - preferably from below. I
always watch the changes in the flux to know when to zero in on the
area of soldering. I question your idea of using a piece of charcoal
on top of the piece - as you had described that it would hold the heat
in. It seems to me that this would probably have the opposite effect
- namely working as a heat sink - drawing the heat away - and
preventing the piece from getting up to the needed temperature.
A longer lasting flux is probably a good idea too - as suggested. In
my minimal experience with these metals - there does seem to be a
great sensitivity to oxidation. I generally use good old Handy flux
(haven’t experimented that much with this type of situation since I
picked up a borax cone.)
Now, my input here is pure troubleshooting - and I hope I’m not out
of line in joining in the discussion (because of my minimal
shibuichi/mokume gane experience) but your problem sounded like a
great fun frustrating problem to solve - and I thought I’d share the
slight doming trick. Works for me.
I hope your problem was already solved by the time you get this