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Why does my gold BOIL?


#1

Hello all that read this, I’m using acetylene and oxygen with a semi
large melting torch. And have had frequently got about 4 ounces of
gold half melted, when it starts boiling, while the other half is
still ‘congealed’. In other words, the lump in the middle of crucible
is surrounded by boiling gold. I move the torch around in a circular
motion trying to keep the heat even. I’ve always heard that you want
to get the metal melted as quick as possible to prevent oxidation. I’d
appreciate any thoughts on this subject. Thomas,
www.islandgoldworks.com @Island_Gold_Works


#2

Sure you want to melt your gold quickly, but not at the expense of
tremendously overheating it. Personally, for multi-ounce melts, I
prefer to melt any way but with a torch. There is a wonderful unit
Swest used to sell about 20 years ago that is back on the market. It
is a resistance melter that is not hand held. The crucible is taken
out of the unit be means of a lip into which a special pair of tongs
fits. Sorry I don’t remember whose selling it now, but I bet someone
else here knows. The Kerr Handi Melt works really well too. I also
have an old Jelrus Thermotrol I-3, or I-1, it’s late, I’m tired, and
I forget and I’m too lazy to go look :), that is great for melts up
to about 10 ounces. I use hydrogen for my torch melting. It’s very
clean, and you don’t get any embrittlement problems with super-white
gold alloys. I highly recommend hydrogen over acetylene. You can add
just a small bit of oxygen and get a soft but fast flame that won’t
boil your gold.

Jeffrey Everett


#3

I had the same problem a few months ago.

Pop said that I had lead my metal. (Could be, I was melting scrap
for vacuum casting) The unfilled tree set untill yesterday when I
decided to pout billets with it and see what would happen. It didn’t
boil this time. I then took the old button (I cut the tree off the
button previously) and started melting it to pout another billet.
This time, it didn’t boil, but looked like small splaters. I’m coming
to the conclusion that the metal isn’t contaminated in my case,
rather I had too much borax on the laddle or my torch is too small to
get a good even temp across all of the metal that I’m melting in the
laddle.

Try throwing your laddle in the pickle overnight, and then let it
dry for a day or two. See if that helps.

-Stan


#4
    And have had frequently got about 4 ounces of gold half melted,
when it starts boiling, while the other half is still 'congealed'.
In other words, the lump in the middle of crucible is surrounded by
boiling gold. 

Gold boils at over 5000 degrees F, so I doubt that you are boiling
it. I would suggest that you are dealing with dissolved gasses,
oxygen comes to mind. I have noticed in particular, that old gold
sometimes blisters and "boils as it melts. I don’t know if that is
because over time it has absorbed that much oxygen, but it wouldn’t
surprise me. When I have this problem, which is rare, as I usually
work relatively new gold, I will over heat the melt until I am sure
that I have driven off the zinc and add new. Not the perfect solution,
but it works for me.


#5

Sounds like gasses coming out of the crucible. Try heating the
crucible, letting the reflected heat melt the metal. The best way to
avoid oxidation is with a reducing flame, in my experience. Acetylene
is hot enough that you don’t need a bunch of O2 in the mix.

Spike


#6

I use oxy acetylene exclusively, what you are doing is not using a
broad enough flame, you need to make the blue tip about 1\2 inch
long to achieve sufficient heat distribution. Make sure you flux
your metal well and heat en masse slowly at first…the flux will
prevent oxidation… then when you start to cherry the metal bear in
and heat from the outside to the center in a circular motion… John
Henry P.S. the boiling is caused by overheating the metal without
allowing it to become a mass


#7

Thomas,

Jeffrey has a good comment there. When melting larger amounts of
gold, it is best to use a closed envronment such as an electric
melting caraff. Lacking that, you can achieve an excellent
environment by using a graphite crucible. This will absorb the O2
and keep the metal stable. You will not have to add or remove zinc
or fear the alteration of your alloy. I used to cast all my gold
using graphite crucibles in a dental caster with a temperature
controlled electric muffin oven attached. Never lost a cast that
way.

Small amounts of gold will work just great with a single gas
(acetylene) and air torch such as the Smith or Victor. Using an O2
torch on larger amounts in an open crucible is asking for trouble.
You are simply driving O2 into the metal.

Cheers from Don at The Charles Belle Studio in SOFL where simple
elegance IS fine jewelry! @coralnut2


#8

Hi Thomas,

Too hot, too fast. When melting larger amounts of gold, the surfaces
of the metal will melt and boil before the full volume of the metal
conducts enough of the heat to melt. Give it more time, a
less-intense flame, and most importantly, pre-heat the crucible to
glowing red before adding the metal. This will reduce the melting
time, as well as the risk of having a solid chunk of metal hidden
inside the molten exterior of your puddle.

If you follow the rule that the metal needs to be melted as quickly
as possible, you will invariably damage the alloy. Use a slightly
reducing flame, and back off the heat from the metal’s surface.
Heating the contents of the crucible from a couple of inches away
will allow the metal to conduct the heat without boiling. Slower
melting is better. Keeping the metal molten for longer than
necessary to pour is the nastiest error that you might make, so melt
slowly, “oscillate” the crucible to stir the contents to satisfy
yourself that the whole mass is molten, and back off the heat to
pour immediately.

David
www.davidkeelingjewellery.com


#9
I use hydrogen for my torch melting. It's very clean, and you don't
get any embrittlement problems with super-white gold alloys. I
highly recommend hydrogen over acetylene. 

Interesting, Jeffery. Our experience was just the opposite. Not
that I recommend oxy acetylene for melting any precious metals. Too
hot. but I’ve found Hydrogen to be good mostly just for platinum
melting, or on a small scale, with the “water torches” for soldering
operations. We had LOTS of frustration and problems with gas
porosity, ESPECIALLY with the super-white nickle white golds, and
also with palladium white golds and rose golds, when we were casting
with oxy-hydrogen. A little less so as the flame became more
neutral, but then it gets too hot. When we switched to propane or
natural gas, most of the porosity problems went away… I’ve been
told by several metallurgists that hydrogen causes problems with some
alloys because nickle, and copper, and perhaps some of the other
alloying metals, tend to dissolve hydrogen when molten, and it comes
back out of solution again when the metal solidifies, giving tiny gas
pockets, or porosity, in much the way fine silver can dissolve lots
of oxygen when molten, which it then spits out as it solidifies.

Peter


#10
I've always heard that you want to get the metal melted as quick as
possible to prevent oxidation 

Thomas: Try adding a bit more acetylene to your mix, Not so much
that you see black smoke but it sounds like your flame is either
oxygen rich, or too close to neutral, the softer flame is actually a
bit hotter than a flame with a lot of extra Oxy. and the extra acet.
Will help with the oxidation as well


#11

I use either a 7 tip on my Little Torch, or I use the multi-opening
long tip for my Little Torch, depending on the amount of gold I am
melting. Then, if necessary, I bump up the pressure on my
propane/oxygen supply generating a powerful fat blue flame. I add a
pinch of boric acid to the crucible, which seems to aid a cleaner
melt. During the melting process, I rock the crucible from side to
side as the gold melts, exposing the unmelted central lump. When I
get a shiny molten bubble shape, moving smoothly around the crucible
I rock it gradually closer to the lip of the crucible so that the
colder area of the crucible does not recongeal the bubble, then pour
in one smooth fast pour into the preheated ingot mold. I rarely try
to melt more than two ounces at once, so your four ounce melt would be a
challenge in any case. Good luck.


#12

Huh, Interesting. And I got this data straight from the manufacturer
of the super-white nickel gold (AAAPM). They told me to switch from
propane to hydrogen for the best results. A number of people, (Marc
Robinson being one of them) told me to use hydrogen-oxygen for
casting, which is also what we used at Nielson’s in Seattle back
when… I wonder what the difference was in our techniques. I use a
large melting torch with a large rose-bud type orifice a pre-heated
crucible, and a relatively soft brush flame. I simply don’t see
porosity in my castings, micro-fine gas, hot or cold tear porosity or
otherwise. What you say makes sense though. I don’t know what to
say.

I’ve always send out palladium-gold alloys to be cast by induction
melting, which is how we cast it back in the 80’s (Aurum induction
casting machine). I noticed that torch melted pall-au cast from a
crucible used for yellow golds turns out slightly off color.

I simply don’t torch melt rose golds. Even certain higher cu content
yellow golds (deep color) don’t torch melt well, I have to melt them
with constant stirring to avoid overheating the copper if torch
melting.

Thanks for that I’ll keep it in mind.

Jeffrey Everett


#13

I have cast over 100 grams at a shot on my ‘‘centrifugal casting
machine’’, if you hold a tight hot flame, use plenty of flux, and
don’t sit on your metal keeping tiny bits in molten stage then i
don’t agree with the theory of oxygenating the metal. I have
achieved many thousands of sucessful casts. Am I missin something?
John Henry


#14
    I have cast over 100 grams at a shot on my ''centrifugal
casting machine'', if you hold a tight hot flame, use plenty of
flux, and don't sit on your metal keeping tiny bits in molten stage
then i don't agree with the theory of oxygenating the metal. I have
achieved many thousands of sucessful casts. Am I missin something? 

I don’t think you’re missing a thing. Anything to do with melting or
soldering is about heat control, right? The one thing I’ve noticed
about casting, and I’ve also cast thousands of flasks, is once you’ve
got a feel for it, you can break just about every rule in the book
and still get quality castings.

I would like to mention though that certain alloys are very
forgiving, especially silicon additive gold alloys which like to be
cast hot. In my experience, high copper (non-silicon or zinc
additive) alloys don’t respond so well to a tight and hot torch.

Personally, I use no flux when I cast other than a glazed crucible,
and use the coolest/softest flame I can quickly melt with. My
technique has more to do with the particular high karat alloys I’m
casting than general technique and undoubtedly applies to no one
else.

Jeffrey Everett