Soldering Copper problem

I’m new to fabrication and can’t get the bottom soldered to the wall
of a round copper box. I have successfully soldered in the past, but
not something this large or that is of 20-gauge material. Here are
some other details:

-Box diameter is 3"; the sides are 1"-1/2" deep

-20 gauge for the sides and 22 for the bottom

-Air/acetylene w/#1 tip

-Ultra Flux

-Medium wire solder chips approx. 1/32" long and spaced about 3/8
apart on the outside of the box

-The soldering is done on a charcoal block and I’ve put firebricks
around three sides of the box to hold in the heat.

-The copper is clean and the fit is tight.

-The solder chips don’t melt at all before the rest of the box is
solid black. In fact, they don’t melt at all.

All help will be much appreciated.

You will need at least a # 3 tip. Preferably a #6. Do the work on a
tripod, heating from underneath.

I would do this with 2 torches, the larger one under the screen, and
a smaller one on top. LOTS of flux. The faster you get this puppy up
to temperature, and the solder flowed, the less oxidation you will

Heat! Lots of heat! There is a lot of metal here. When in doubt, add
more heat. This is where most folks have trouble, not enough heat

Bill Churlik

One step toward successful soldering is to be sure that the solder
itself is clean. If you are clipping from wire solder, wipe the
solder with fine emery paper followed by a clean cotton cloth.


Indigo Designs

You should probably be using a larger torch tip for a box this size.
The copper is soaking up the heat but not reaching soldering
temperature because your flame is too small.

Joel Schwalb

That’s a pretty good sized chunk of copper, which transmits heat
quickly. Sounds like its not getting hot enough at the join. The mass
of copper is drawing the heat away faster than you can add it. First
would try a larger tip. Reprep everything first of course.

Sometimes its beneficial to make your bottom sheet somewhat larger
than the walls, in this case maybe 3/8 inch overlap. Then you can
direct the heat onto the sheet outside the walls and let it soak thru
to the join, redirecting the heat to the walls as the join heats up.
This might ease any fire scale that develops before you’re up to
temp. If you’re heating from inside the box sometimes the velocity of
the flame in a confined area seems to divert it from the surface,
especially if you have cranked up the torch trying get more heat. I’m
unsure of how to describe it but if your flame is not 'normal’
shaped and sounding you have a problem.

If the problem is that the torch itself doesn’t develop enough heat
for this sized piece, a possible fix is to solder with the piece on a
small open kiln, or get a larger torch/tip which is probably the
wiser choice.

One other thing might be at play here. You have your solder in
harm’s way, so to speak. It may be fouling before its ready to flow,
in which case it won’t flow. Applying the solder directly from the
spool when its at the right temp will keep it cleaner longer, but
admittedly it takes a certain touch not to overload the join. I
employed a silversmith with 50 years experience. He used spooled
solder exclusively for all sizes of things.

Good luck!


What type of torch are you using? If you are using a mini torch
[surely you aren’t] then the tip would be impossibly small. I used to
do jobs like this with a plumbers torch for the hardware store. Are
you sure that you are fluxing everything well?

If everything is getting black that may mean that your flame is too
reducing. This means that you are using too much gas and not enough
air in the mix, which can smother the heat. Try going to a bigger,
stronger and bluer flame. Back off the piece while preheating
everything and then drive in and flow the solder.

Are you trying to solder from the top? You need to probably heat
from the bottom, because the high walls are choking back the heat.
You may need to use 2 torches. Then use one to keep the whole thing
hot and one to flow and move the solder with.

Good luck Dennis


Do you have access to another torch? For larger scale soldering two
works better than one.


Far too small a tip for a 3" x 1 1/2" box. I use a #1 for very
delicate stuff, a #3 for rings etc, but I be reaching for #5 for
something the size of your box (and still using the fire bricks). Lot
and lots of heat quickly before the flux burns off, and keep the
torch moving over the entire piece. Everything has to be almost at
the solder flow temperature before you even think about concentrating
on the joint. It might seem like you are summoning all the powers of
Hades but it is actually difficult to melt that much copper with a

Demand Designs
Analog/Digital Modelling & Goldsmithing


I’d definately switch to a #2 or 3 tip–probably the #3. The #1 tip
is too small for the size of your piece, meaning that it takes so
to heat up that the copper will oxidize before the solder melts. This
sounds like the problem you are having.

Also, in my own experience, I prefer to solder something like this
on a solderite board, without surrounding firebricks. Sometimes I
find that the reflection of heat gets in the way of where I’d like to
place the torch.

If I may be so bold as to offer a couple other suggestions (my
students have been working on hollow box construction and some of the
issues they ran into are fresh in my mind)…keep in mind that it
be more difficult to heat your bottom than your sides, as the sides
are easier to reach with the torch. Therefore, I would tend to focus
my heat towards the bottom sheet, or suspend the piece between two
bricks and start heating from underneath. When the flux starts to go
glossy (handy flux–a borax based paste), I heat from the top side to
bring the rest of the work up to temperature and to help minimize
oxidation at the joints.

However, with such a thin gauge bottom in copper, you may have
trouble with warping, so I would not suspend the piece. Generally, 18
gauge or heavier, especially in copper (it is so soft) may make life
bit easier.

Just my humble suggestions…best of luck!


Hi Peg

I’ve had good luck soldering copper boxes and bezels. The largest
box I have done is about four inches square and the sides were one
inch high.

I’m only using a hand held butane torch. I fit all the pieces
together and hold them with self locking tweezers or a small clamp.
Make sure the join is tight and clean as you have done. I heat only
the inside surfaces and when they are hot enough to melt regular
wire solder I just touch the wire to the inside seam and it runs
nicely around the join. I just have to make sure I keep the box hot
and don’t get the flame too close to the solder wire. I don’t bother
with chips. There is very tiny bit of solder showing on the outside
of the box or bezel. I leave a slight edge on the outside of the
piece and cut and file it off after the thing is soldered. It does
take a bit of time for the finishing, but after doing the final
filing and polishing you can barely see any solder.

The solder I use is “44” Rosin Core solder and is quite thin. If I am
careful I don’t get any on the floor of the box or bezel. My main
problem is getting the flame too close to the solder especially in
the smaller pieces. I touch the solder to the box seam… not into
the flame.

Best of luck to you
Sheila in Ontario Can