Last night, a student was trying to cast a flask using a new batch of
manganese bronze casting grain from Rio (706079). It shows a melt
point of 1700 and a flow point of 2100F. I think he had about 250g
in the crucible. We could NOT get the stuff to melt properly! It was
spitting and smoking (like an overheated metal would), with bits
literally spitting out of the crucible. The flames coming out of the
crucible were bright neon green (I figured that was probably the
manganese, or possibly some zinc in the alloy), but when the green
flames were visible the sound of the flames would change to a “hiss”
noise. It got to a sludgy point before I finally put a stop to it
due to safety concerns over the metal smoking and spitting. The
crucible itself was glowing bright red at that point, as well - and
we had let it go long enough that I am certain the metal was well
into the melt/flow range, but with very little visible melting. We
let the crucible cool with the metal inside and the metal looks like
it’s encrusted with black chunky charcoal. We were using borax as the
flux and ended up using a lot more than I would normally use with
silver… that’s kinda normal with bronzes.
Have any of you used this particular alloy? Can you give me any
insights into what was going on? Rio’s theory was that the spitting
was the “oxidation” occurring, but it looked almost like fireworks
coming out of the crucible… certainly not like any oxidation I’ve
seen!
So the other question is… we’ve got this crucible with a hunk of
cooled bronze in it. Do you think it’s safe to try and remelt it to
do a water pour and re-create casting grain from it?
I do very little work with bronze, so any help will be greatly
appreciated!
Thanks,
Karen Goeller
No Limitations Designs
Hand-made, one-of-a-kind jewelry www.nolimitations.com
Remove the metal from the crucible before re-melting (trying to do
so). Question is, if it never got liquid before, how are you going to
do a water pour. I would call RIO and ask them what is going on.
It happened to me too. Eventually I used a large bushy flame and a
lot of patience and got an so-so result. But it still was lousy, so
I bought brass brazing rods, a bit expensive, but they worked far
better.
From your description it sounds like you did not have enough heat.
What size torch were you using? To do that much bronze is going to
need a good size rosebud (multi orifice) type tip on a oxy-acetylene
or oxy-propane welding type torch. The crucible could be easily
glowing red hot and you still will not have enough heat to melt the
metal as the crucible is a fairly good insulator and it will hold the
heat better than the metal will. Also 2100F is a brilliant yellow
heat so this is another indication you did not get it hot enough.
James Binnion @James_Binnion
James Binnion Metal Arts
We were using our largest rosebud tip with our big oxy-acetylene
rig. We were using a “sharp” and very hot flame (not bushy, but with
enough oxy added to reduce the inner cone of the flame to a sharp
point with multiple little internal cones visible) turned on fairly
high. This setup has (before and since) melted the same quantity of
“regular” bronze (casting grain, brazing rods, plumbing fittings)
without issue, which is why I was so startled by the behavior of the
manganese bronze.
Rio’s technical service response was that the spitting was probably
“oxidation” (but we’re talking little bits of red-hot metal
literally popping like popcorn out of the crucible) and that the
smoke was “normal” (let’s see - black smoke coming out of the
crucible?). They suggested trying to re-melt it in the crucible (it’s
really stuck in there at this point - sludged enough to slump into
it), then do a water pour from that. We’ll try to see what we can get
out of it next Monday when that particular class meets again.
I’m also going to contact the direct metal supplier recommended by
one of the respondents to see if they have any insights to share.
Thanks!
Karen
No Limitations Designs
Hand-made, one-of-a-kind jewelry www.nolimitations.com
Manganese bronze has a very high zinc content (39 % I think). The
spitting and popping is the zinc flaring off. No doubt there was also
quite a bit of white smoke fuming off the crucible as well. When
manganese bronze is overheated, the zinc burns off - as the white
zinc
oxide fume and leaving quite a bit of slag in the crucible. In my
experience, it’s pretty hard to successfully torch melt manganese
bronze. What has helped is heating the metal gently, trying to avoid
getting any single part past the zinc flaring temp before the entire
mass has melted. A lot of flux earlier than usual to shield the
metal from the air. Be prepared to skim the slag off before casting.
BTW, just in case you dont alrady know, the white smoke is very bad
for you. Google “metal fume fever” for details.
We were using our largest rosebud tip with our big oxy-acetylene
rig. We were using a "sharp" and very hot flame (not bushy, but
with enough oxy added to reduce the inner cone of the flame to a
sharp point with multiple little internal cones visible) turned on
fairly high.
This is too much oxygen. The only time you want that much oxygen is
when you are melting or welding platinum. Otherwise in any jewelry
application you want either a neutral or slightly reducing flame. In
the case of your casting torch I still think you need a larger
rosebud tip. It is not the temperature you need to increase but the
heat. Think of trying to melt that metal with the little torch with a #3 tip. Even though the flame temperature is the same between your
rosebud and the little torch the heat output of the flames is
radically different. The biggest problem I see with people using a
torch to melt and cast with is too small a tip. You want to get in
melt the metal and get out as fast as possible and the only way to
do this is to have enough heat and that requires a big rosebud tip. A
second problem with using too small a torch tip to melt too large a
volume of metal is that you will try to push more gas through the
torch tip by increasing the pressure and this increases the velocity
of the gas which will pull more external air into the gas stream
making the flame actually cooler than if you had a lower velocity
gas stream from the torch tip.
This setup has (before and since) melted the same quantity of
"regular" bronze (casting grain, brazing rods, plumbing fittings)
without issue, which is why I was so startled by the behavior of
the manganese bronze.
There are literally hundreds of bronze and brasses with a tremendous
range of melting temperatures and flow points. Your manganese bronze
is one of the higher temperature flow points. I think the
combination of too much oxygen and not enough heat is contributing to
your not melting the charge.
Rio's technical service response was that the spitting was
probably "oxidation" (but we're talking little bits of red-hot
metal literally popping like popcorn out of the crucible) and that
the smoke was "normal" (let's see - black smoke coming out of the
crucible?).
I am not certain about the oxidation theory but I have had this
behavior with metal that was not properly grained by the
manufacturer. I have even had sterling from more than one
manufacturer that did this when we heated it in the induction melt
crucible. One batch was so bad it literally through some of the shot
out of the crucible when we melted it. The cause is water trapped in
the shot from when it is improperly grained. When heated it expands
and will sometimes pop with quite a bit of force.
Jim
James Binnion @James_Binnion
James Binnion Metal Arts
I think you’ll find that the manganese bronze was simply overheated.
All copper base alloys with a high zinc content will fume/burn when
overheated and anything containing manganese (which includes some
white bronzes) will “spark” when the sputtering jumps up into the
heat stream. This sparking looks very similar to the sparklers kids
use on the 4th of July.
Our experience is that the overheated metal is pretty much toast and
can be discarded if you can ever remove it from the crucible. We use
large #10 crucibles and if this overheating ever occurs, ususally to
temporary non attendance during melting, if leaves a very hard
compacted ashlike deposit. This crust is very tenacious and usually
results in causing the crucible to break if too much force is applied
to removing it.
My advice when using manganese bronze would be to heat it very
slowly and cast at the lowest possible temperature to achieve the
desired results. If you want to try some more, we’ve got about 100#
(1/2" chunks) laying about that a great deal could be induced.
Contact offline if interested.
Thank you for your helpful response. You confirmed what I was
suspecting, which was that parts of the metal were overheating…
while other parts hadn’t reached a workable temp. The smoke was a
dead giveaway for me. So we’ll try melting the stuff out of the
crucible using a smaller flame and more flux, and we’ll be prepared
to skim off the slag, as well.
And for future classes, I’ll make sure the students know NOT to buy
the manganese goop!
Karen Goeller
No Limitations Designs
Hand-made, one-of-a-kind jewelry www.nolimitations.com
Is there a proper way to reheat an crucible in which the metal has
been allowed to harden?
An interesting question! I had planned to heat it gently from the
top, directing the flame at an angle into the top of the crucible.
Pretty much the same way that we would melt any large bit of metal.
We’ve already “scrubbed” off most of the black gunk that we could
get to (via brass brush). I’ll be interested in hearing thoughts
back!
Is there a proper way to reheat an crucible in which the metal has
been allowed to harden? If this were a pot of hardened pitch, I have
been taught not to heat it from the bottom - for fear of creating
pressure under a cap of solid pitch. Is this similar? Does Karen need
to be careful about the way she reheats her crucible full of hardened
metal? What is the right way to do this?