I am making a yellow brass pendant, soldering a fine silver bezel
onto it. First the bezel slumped, I filed it and straightened it
out, but then when I was soldering jump rings on, using medium
solder, the bezel completely melted onto the surface of the pendant!
I had the pendant facing bezel side down, and I was soldering jump
rings to the top edge. Is there some kind of reaction between the
brass and the silver causing the fine silver’s melting temperature to
drop? Thanks for your help!
Is there some kind of reaction between the brass and the silver
causing the fine silver's melting temperature to drop? Thanks for
your help!
Yes, there is a reaction between the two. Silver+brass=solder, which
has a lower melting point than either of the two parent metals.
Consequently, when heated to the point where solder flows, the
silver and brass like to combine and make more solder at the point of
contact.
There are in fact several different alloys that are called brass.
Several use zinc and I think that this could be the problem. If the
zinc amount is high enough under heat it could leave solution with
the copper in the brass and alloy with the fine silver lowering it’s
melting point substantially.
You are dealing with specific heat. Silver heats up fast and cools
off fast, brass heats up slow, and cools off slow. The brass is
acting as a “heat bank”, storing up more and more heat, and the
silver is always tapping into that, being connected to the brass by
solder. And the brass is difficult to solder to begin with. In a
nutshell, you are overheating it. Obviously, since the silver is
melting. But this is a constant issue with what you are attempting,
and other metals too - gold and platinum a lot. The only solution is
to be more careful, make sure the brass is clean and solderable, use
proper fluxes, etc. Once you hit the melting point of silver, silver
will melt - and overheated solder will tend to dissolve the nearest
metal, too.
Soldering silver to brass is very tricky. Silver solder is basically
fine silver and brass (copper,zinc alloy) alloyed together. So you
are in effect making silver solder at the joint and it is very easy
as you found out to completely alloy them together. Try extra easy
solder keep your temperatures low limit the number of times you
solder and work fast.
Jim
James Binnion @James_Binnion
James Binnion Metal Arts
I mentioned “Specific Heat” before, and it’s so important, I want to
expand it. Nobody much uses the term, but we all deal with it on a
daily basis. People tend to think that if you heat something up, it
gets hot. That’s not precisely true. Jim Binnion talked about BTU’s
on a related thread - they are a more common unit in the real world,
but specific heat is measured in calories. One calorie is the amount
of energy that it takes to raise one gram of water by one degree
centigrade. That is also the definition of specific heat - that the
energy required to raise one gram of water by one degree is one
calorie. Every substance in the universe has it’s own specific heat.
Thus, if you line up many things, you’ll find that one calorie will
raise the temp. of this by 1/2 degree, that by 1 1/2 degree, etc.
What this means in our world is pretty simple, really, but essential.
When you get a silver bezel and solder it to a brass plate, you get
your torch and apply heat. So, we’re heating, heating - oops, the
silver’s getting too hot, move the torch over, etc. So, both pieces
are good and hot, we solder them, flow it, etc., and we’re done. As
long as you stay at heat, everything will be fine, because the metals
were actually heated separately. Cool it off, and now we have
something that will behave as a unit - as one. The metals are
“married” - they are bonded together, but still retain their own
properties. Now, we need to reflow the solder. Let’s suppose the
specific heat of silver is 2, and that of brass is 1 - made up
numbers, but it’s a proportion. So, we put 200 calories of heat into
the piece. That means that the brass is rising in temperature by 1,
and the silver by 2. When the brass is at 500 degrees, the silver is
at 1000 - that is what specific heat means, and why it’s important in
metal work. Realize, too, that this is separate from conductivity -
the ability of metal to transmit heat. Even nonconductive materials
still have a specific heat. This is the reason why asbestos does what
it does, and why it seems to take forever to get a steel rod red hot.
And it’s why silver soldered to brass and other things seems to just
go splat all of a sudden.
I take your specific heat point, but I think the main issue here is
to do with alloy phases and the eutectic melting points of silver,
copper and zinc (the metals present when soldering silver to brass).
The melting point of a copper / silver / zinc alloy is significantly
lower than the the melting point for each individual metal, this is
why zinc is used in silver solder, and why if you want to make some
cheap silver solder you melt a bit of brass with your silver. What
happens on heating is that the solder at the join melts first and
alloys with the metals either side of it (which is required for a
strong bond), however, with these two metals, the alloying process
does not stop there - the reaction runs away with itself as the
metal at the join alloys up into the silver, which is the slump which
you would have observed. This may be compounded slightly by the
specific heat capacity issue you mention, but on two similar metals
which conduct well, this would be negligible as they will be getting
energy from each other as well as from the torch. This run away
alloying and melting is why you never put silver next to brass when
you laminate up a mokume gane billet, a mistake I made once and why I
looked into this issue. A phase diagram of the metals involved shows
what is happening pretty clearly. To avoid the problem I would
suggest using bronzes instead of brass, or straight copper.
Alternatively, use a very low temperature solder and pull the flame
away as soon as the solder starts to slump.