I hope I can help you a bit with your soldering problem. My students
sometimes do very similar projects, soldering sheets of silver or
brass together, and I think I can throw some light on your problem.
First, get yourself a bigger torch. You are going to want a flame
that is large and bushy, not a tiny sharp flame that torch of yours
is putting out. A Prestolite torch, or acytelene torch with the
largest tip they sell at a welding supply store works just great for
these types of jobs.
Run the pressure up to the max safe pressure on the regulator. Next,
I would try soldering your top (pattern) sheet to the oversize bottom
sheet by making sure they are both dead flat, and putting them on top
of a suspended heavy steel screen. Put both sheets together, and then
flux with a good borax paste flux when they are together. Don't apply
flux to each piece and assemble. After these two sheets are coated
with flux, dry the flux with a soft flame until it is dry. Be sure
you have adequate ventilation, as this creates fumes. This is a great
time to apply cut squares of solder to the back sheet, placing them
along the outer edge of the top sheet. (I prefer sheet solder rather
than wire solder) Hopefully you have remembered to cut the back sheet
larger than the top sheet, to allow for a lip to place the solder
on. With the torch in one hand, and a solder pick in the other, start
heating the sheets from beneath the screen with a continuous
circular motion, heating as evenly as possible to avoid warping.
Your soldering pick can be used to push wayward solder chips back
against the edge of the top sheet. Brass is a "dirty " metal, and
will oxydize quickly, so don't be timid with the flame. As the sheets
reach soldering temperature, the solder should flow between the
sheets, and the capillary action of the solder will fill the minute
space between the two sheets. I would not try to "sweat" the two
together by pre-melting solder on the back of one of the sheets. In
my experience, they will never lay as flat as they will if you keep
them flat to start with, and bring the solder in from the edges.
Binding wire is a mixed blessing. It can be quite helpful on
occasions, or it can nearly destroy some pieces. Use care or you can
solder it on to your piece quite easily.
After pickling, check to see if you have a continuous seam around
all edges, and if you have gaps or pinholes, tap the two together
with a soft mallet to seat the two a little closer, and reflux
/solder as before. When you are satisfied your two sheets are
soldered perfectly, then use a jeweler's saw to cut out your bottom
sheet to match the top. File, sand, etc.
I would stay away from high temperature solders, as they require so
much heat to melt, you can heat-warp or melt your sheets just trying
to get the solder to melt.
Hope this helps...
Jay Whaley UCSD Craft Center