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The donut

Ok… Here is a beginning level note of discussion.

Imagine a flat copper donut. You want to solder a flat silver donut
hole into the center. You have a fairly good fit, you flux, you put
your solder chips around the seam, you torch it. The gosh darn thing
simply will not bridge the seam at cerain areas and you eventually
scorch the donut hole into unwanted reticulation.

How can I marry the hole to the donut perfectly?

How can I marry the hole to the donut perfectly? 

My DVD Coronet Cluster
contains the answers that you seek.

Leonid Surpin

It appears the issue is that the copper and silver expand at
different rates and the outer copper one (because of its shape)
expands leaving your gap.

You could try having a slighlty bigger silver disc than will fit the
hole, when the copper expands it will drop in the hole and then
solder it. How much bigger? Some, not too much. (You could of course
heat the donut and measure how much bigger the hole is when it is

you could try pumping most of heat into the silver disc so it expands
first (postential soldering problems with this route-ideal is
everything going up intemperature evenly).

Whatever you do use a ton of white paste flux. You need a long
working time with this one and that flux will give you enough. If
your flux begins to turn green put some more flux on. If the flux is
brown you will have to start over. Do not pickle as a pickled finish
interferes with effective solder flow (seams need to be scraped or
cleaned prior to soldering) You can’t have too much fluw when working
with copper (and sterling for that matter), golds are fine with a
thinner flux like Batterns or Pripps.

Tell us what worked-should hear some more from the list on this one.


If you can’t get the PROPER FIT then Pickel the article, wire brush,
so it is super Clean, Then use the hardest flowing solder you have.
It also helps to add solder with a pick so the solder does’nt get
dirty on the other side of the article. If You would have had a good
fit, you would not have all these problems, Taking your time at the
beginning saves it in the end

My theory. You’re dealing with differences of coefficient of thermal
expansion. Possibly linear differences. If silver expands in volume
more than copper you need to account for this. mMybe try making a
slight air gap, then when the whole lot is heated up to brazing temp
and you see the gap close up, apply the solder.

New Zealand

Okay, here’s my 2 cents worth…

Two points; Once the solder stops flowing properly, pickle the piece
to remove the copper oxides, then reflux and continue soldering.

If I were to do this job, I’d get as perfect a fit as possible, one
in which I’d have to force the center into the outside piece. It
should be almost seamless. I’d use a silver brazing flux. I think
the trick here is getting a near perfect fit. My personal technique
(for what it’s worth…) is I flow the solder from only one point
because I like to have almost no clean-up after the soldering.

Your problem is they have different rates of expansion. Look up the
coeffiecients of expansion and then you need to ascertain the
diameters of the 2 pieces and apply the difference of the expansion
of the 2 metals for your soldering temp to the anticipated sizes at
that temp and make your cold pieces different by that amount so they
fit when hot. Basically make 1 smaller or bigger than a dead fit
when cold.

Nick Royall

How can I marry the hole to the donut perfectly? 

Better/more flux, larger flame. But hard to say for sure

James Binnion
James Binnion Metal Arts

Did you clean the surface? Any grease or dirt from even your hands
will cause your problem…


How can I marry the hole to the donut perfectly? 

A couple options. If by chance you happen to use a circle
punch/cutter to punch the center hole in the donut, and the same
cutter to produce the plug you’re trying to fit, you’ll find they’re
better than just a “decent” fit. That will help a lot. When
soldering, use a more active flux than things like Batterns self
pickling or the like. Those are fine for gold and even silver, but
the copper will need a more aggressive flux to ensure the joint stays
clean, a major reason why solder might not flow into an area (also be
sure the metal is clean and bright before you start to solder)

To get an even better, tighter fit, or if you’re not using such a
"matched" way to produce hole and plug, then start with silver for
the plug thats slightly thicker than the copper. Then fit it as well
as you can, and once fitted into place, planish (hammer) the silver
center. It will flatten to slightly thinner while doing this, and
will also expand. Done carefully, you can get a virtually perfect
fit. In fact, if you overdo it, you can get a fit so tight that
solder won’t easily flow into it, but in the case of copper and
silver, that’s not a problem. If well fluxed, with a reducing flame,
you can just heat the two pieces without solder until the contact
between the two fuse. That happens well before either the silver or
copper melt. Same principal, mostly as used in granulation, or in
making a mokume billet. To work, the metals need to be in close
contact, preferably almost a press/interference fit. As well, when
heating, heat the center slightly more than the copper donut. That
way, thermal expansion won’t cause the donut to expand out away from
a cooler silver center. All the above also applies if you add solder
to the joint, which you may wish to do in any case. Don’t use more
than you need for a clean result. And if you want to experiment, and
happen to be using thicker guage metals, shape the inner edge of the
hole in the copper with either a channel cut into the center of the
inner edge, or chamfer the top and bottom edges of the hole. Then
when you planish the (well annealed beforehand) silver, it can expand
either into that channel, or around the chamfered edges, forming a
mechanical joint. That can be very tight indeed., and may need no
solder at all. In effect, you’re forming a very wide, thin, rivet…
This is useful too as a means to do what you’re doing when one of the
metals is something you cannot solder, such as titanium…

Hope that helps
Peter Rowe

Your problem is they have different rates of expansion. 

Silver has a greater thermal expansion rate than the copper, If the
initial fit is too tight then there could be too little clearance for
the solder to flow through. You need somewhere in the neighborhood of
.002"-.004" for the capillary flow of the solder. So if it is too
tight at room temperature than it could be way too tight at soldering


James Binnion
James Binnion Metal Arts

Hello Steve,

You have to take a step backwards…

Don’t keep trying to solder because the oxide buildup will no longer
allow the solder to flow.

You will need to pickle the oxides off and then try again. If the
problem isn’t the fit, then it is because of oxide buildup, More
then likely the copper is robbing the heat out of your piece and
making the soldering procedure difficult.

What flux are you using?

Mix up some 50/50 boric acid and alcohol (denatured) and paint this
onto the piece, the front and back, this will keep the copper from
oxidizing too fast. If you don’t have boric acid, try your local
pharmacy, just ask at the counter, they usually keep it in the back
or they can order it, which usually only takes a couple of days. The
price is outrageous compared to a bulk purchase.

If you can, after you pickle the pieces and before you solder again,
you need to compress the metal together to ensure a tight fit. I am
not sure what tools you have, but this can be as simple as using a
planishing hammer around the margins of the donut/donut hole
connection, just as long as the connection is as tight as it can be.

Good Luck

I would take a page from electronic soldering. I trick called
tinning which amounts to putting a bead of solder around both parts
to be joined. Then reflux and have your wire solder ready. Bring both
parts up to temp and feed the wire to it. That should take care of
the problem. But nothing replaces the proper fit to begin with.
Almost or a Pretty good fit don’t do it. You need an exact fit end no
gaps to bridge.

Good luck John (Jack) Sexton

I have seen similar problems solved by making sure that the heat is
applied as equally as possible to both components.

Also account for differences in weight, one component may require
more heat to bring it too the same temperature, so the heating should
start with the heavier piece.

Make sure that the heat can also get under the lower component.
Perhaps make a jig to raise it well above the soldering board.

Maybe solder a tight collar inside one of the halves so that the too
holes are kept in alignment and your pallions of solder might be
placed on the lower portion so the upper portion sits on them and
sinks down as they all melt.

Make sure your copper is scrupulously clean, copper oxidises very

Every one has also had good ideas so mix and match. You could also
fit a collar to the outer edge.

Best of luck
David. Australia