I have a couple of medium voltage switchgear contacts, with seemingly
silver buttons brazed to them. I have a little heat treating oven. How
can I use it to unbraze the buttons, to what temperature should I set
Man! Without knowing a bunch about the silver alloy and the brazing
alloy, it is pretty hard to know what would be the magic temperature.
If you had a barrel of them, you could do a couple tests to find
Pure silver melts at just under 1,600 deg. F. All, or nearly all, of
the silver binary alloys melt at lower temperatures, down to around
800 deg. F.
The most commonly used silver alloy fror ordinary electrical contacts
is coin silver, which is a binary with 10% Cu. It melts at around
1,430 deg. F.
Silver brazing alloys melt at somewhat lower temps. So, disregarding
the unusual low-temp silver binaries, I'd start at around 900 deg. F
and start raising the temp until you can pop the contacts off of the
There's a slim chance that they aren't brazed on at all, but just
diffusion-bonded in a furnace. To do that, you'd clean and flux the
contact area between the silver and copper, put them in an oven, and
soak them at around 1,500 deg. until the silver and copper mutually
diffuse and form a thin layer of silver/copper binary alloy. To get
them to separate, you'd want to soak them at the same temperature
again, or slightly higher, and pop off the silver as quickly as you
can, before a bigger area diffuses out.
Correction: The 1,430 deg. F melting temp for silver/copper is for the
eutectic, which is 28% copper. Coin silver, with 10% copper, melts at
a somewhat higher temperature.
The process would be the same, but it requires a higher temperture to
separate a brazed or diffusion-bonded contact. On the other hand, the
1,500 degrees I mentioned would be too high. Probably between 1,450
and 1,500 should do it, if the bond is diffused. But try lower first,
around 1,400, on the stronger possibility that the joint is brazed
with a lower-temp alloy.
If you soak it too long, you'll create new intermetallic compounds and
you may have a b&%ch of a time getting them apart. Work as fast as you
I just use a Presto-lite torch and replace with either Sterling or Fine
silver plate stock I get from a jewelry making supply store. Use their
soft sheet solder and spray flux. My contacts outlast factory contacts
many times over.
So it costs you 10 bucks in electric current and an hour or so of time
(mine runs $35.00/hr for odd work) To make about 5 bucks?
How about a simpler solution? Take the contacts, clamp them so the
buttons are hanging down. Then hit the back side of the contacts with a
MAPP torch and let the contacts fall off. Might take 5 minutes to do.
Of course you do not want to grind copper or other soft metals; they will
just mess up your stone. But sanding with a belt sander works pretty well if
you do not try to remove material too quickly (otherwise the paper heats up
It may not be worth trying to get them apart, but, for the record,
what I would do is this: Heat them to around 1,200 def. F in a heat
treating furnace, or just heat them good with a torch, to a low red
color (maybe 1,200 = 1,300 deg. F). If they're brazed, they'll pop
apart. If they don't, they aren't worth screwing with. I'd just scrap
them as is at that point.
Yeah, as a practical matter, I agree. And if it all just goes
"schlump," then don't do it again.
Brass goes "schlump" very easily when you silver-braze it, if the
braze melts at too high a temperature. But copper does not.
Some things you need to know.
Some silver contacts contain cadmium. If the contact does not, the solder
may. Don't do anything with them without a breeze coming from the side, to
carry fumes away from you.
Some silver contacts are not silver along, but sintered tungsten, filled
with silver. They will have a waffle pattern on the back side, obvious
when you remove the contact from the buss. The silver can be leached by
prolonged heating in distilled water and nitric acid. Keep the container
(beaker) covered with a watch glass to prevent losses of values.
Contacts are easily removed by heating with a torch. I've done, literally,
hundreds of pounds of them through my many years of refining precious
metals. Want to get them off easily? Hold the buss with a pliers, while
heating the contact. Apply heat directly on top, and keep watch on the
solder, below. When you see it's molten, rap the buss on the edge of a
coffee can. That dislodges the contact and the bulk of the solder, both of
which will end up in the coffee can. Done.
Don't toss the silver with the base metal. Silver is now over $30 troy