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Graphite gold contamination?


#1

Im sathyaraj, working in su-raj diamonds as a engineer in casting
dept. at present we use graphite crucible for gold casting and
melting practices. is there any chance of graphite getting mixed
into melt and get casted. because we face a problem of black
inclusions in the product after casting. we use a closed type
casting system.


#2
is there any chance of graphite getting mixed into melt and get
casted. because we face a problem of black inclusions in the
product after casting. we use a closed type casting system. 

Graphite crucibles can indeed break down and create graphite
inclusions in the castings. If they are exposed to atmospheric
oxygen while hot this will eventually happen.

James Binnion
@James_Binnion
James Binnion Metal Arts


360-756-6550


#3
Graphite crucibles can indeed break down and create graphite
inclusions in the castings. If they are exposed to atmospheric
oxygen while hot this will eventually happen. 

And I believe I read (in orchid, most likely), that under some
circumstances you can also get the graphite actually reacting with
some gold alloys to form gold or other metal (like nickel in white
golds) carbides as inclusions in the melt. Those would result in
inclusions that would be a bit different in color, but still
metallic looking, and considerably harder than the parent metal. We
sometimes have this problem with a “soft” 18K white gold casting
alloy we get from Stuller. Nice metal, so far as 18K white gold goes,
in most respects. But those occasional harder than steel inclusions
are a real pain in the backside. You find them with a saw or drill or
bur when the tool suddenly stops cutting and becomes instantly dull
or breaks, or when the inclusions stand proud of the surface when you
polish those surfaces. They take diamond bits to grind out, prior to
then welding in metal to fill the void that leaves…

Peter


#4

To be more sure of the source, look at a new crucible next to the one
used when the inclusions occurred. See how bad the wear really is.
The hole at the bottom ( I assume a bottom pour system) can easily be
the place for release of graphite into the jewelry. As we try to keep
costs low, its easy to over use crucibles and wind up spending
"crucible money" in the polishing or repair areas. And-Jim is quite
right-extend crucible life by keeping air away from it when hot. Run
the neutral gas and keep the system closed as mush as possible.


#5

Peter,

How are you casting the 18kt white, vacuum, centrifugally, or ?

Richard Hart G.G.
Jewelers Gallery


#6
the graphite actually reacting with some gold alloys to form gold
or other metal (like nickel in white golds) carbides as inclusions
in the melt. 

AHA! So that’s the cause of that huh? I spent many hours on some
white castings with that ick on it. Most frustrating. The good alloy
would just dig out around the garbage and the more you polish the
worse it got, like watching mountains form. Got the best finish by
hand sanding then really limiting the time/pressure on the buffing
wheel. Eventually got them eye clean but I was not happy.


#7

Dear Sathyaraj, Yes you will have contanimation from carbon. The are
reasons: The type of crucible ie what is the crucible made from? What
is the grade of carbon? What are the ohm values? What is the exposure
time after heating? What type of energy is being used for heating? Is
the power loaded on the crucible? How many casting have been carried
out from the crucible? What is the thickness of the crucible? What is
the source crucible? What are the type of alloys being used? What are
the temperatures? What are the hold time after casting ie the exhaust
time? These are some of the factors. In case you want any further
guidance you can contact me at IIGJ mumbai I am consultant & head the
manufacturing department. I would recommend a detailed analysis of
the inclusions for trouble shooting.

Regards
Umesh
Mumbai


#8

Neil,

I spent many hours on some white castings with that ick on it 

Yes,those pesky spots in white gold are a form of nickel carbide
forming. Even carbon from a rod or from a sooty flame can cause it.
The best solution I’ve found is to dig out the spots, if shallow
enough then burnishe out the hole, deper, fuse in new metal.

Paul


#9
AHA! So that's the cause of that huh? I spent many hours on some
white castings with that ick on it. Most frustrating. The good
alloy would just dig out around the garbage and the more you polish
the worse it got, like watching mountains form. Got the best finish
by hand sanding then really limiting the time/pressure on the
buffing wheel. Eventually got them eye clean but I was not happy. 

Neil, re-read my post. I’m not SURE that’s what’s happening. I’m
guessing, based in part on a vague memory of something I thought I
read some time ago. I was sorta hoping someone who knew for sure
might chime in…

As to dealing with it, another note of interest. Nitric acid applied
to those hard spots instantantly turns it dark, so it’s not got 18K
gold content. For that reason alone, it’s better to dig deeper than
trying to sand it, and actually dig it out. Diamond burs work well,
and occasionally swabbing the area you’re digging at will tell you
for sure if you’ve got it all. When the chunks are still large, you
can just see the different color. When you’ve got it out, then torch
fuse, or laser weld, clean metal to fill in the cave you dug out. In
my experience, those chunks are often larger than you’d think from
their initial appearance at the surface, and though large, I doubt
they offer the metal the same structural strength as the gold does,
so such things in a shank are likely to make it weaker. Another
reason to disect them out if you find them.

But I’d still love to know exactly what they are and why they form,
or whether it’s due to improper alloying or mixing still on the
suppliers end, or what… Tried calling Stuller once about them, and
was told to expect a return phone call from one of their metal gurus.
He must have lost my number though. No return call… Maybe he
doesn’t know about this problem either…

cheers
Peter


#10
How are you casting the 18kt white, vacuum, centrifugally, or ? 

Vacuum cast on a basic vacuum table (non-perforated flasks, but one
of the tyvec flask liners for better vac draw), with the metal
melted in one of those italian electric melting ovens. Graphite
crucible, no protective gas cover beyond the simple cover of the
furnace lid, simple open air pour. The melts are generally small (and
ounce or less at a time). Generally up to maybe six rings at a time
in a 2.5x2.5 size flask. Flask temps vary depending on the items, but
most commonly, they’re at 900 to 1000 F for ladies medium to light
weight rings. The metal is at 970 C, if I recall the furnace
settings right (note I’m giving flask temps in F and metal temp in
C… Sorry. that’s what the two controllers read out in…) The alloy
is Stuller’s “soft” white gold. Don’t recall a stock number (I’m not
the one who orders it). Cast buttons generally are nice and clean
looking after breakout and pickling. Castings sometimes almost shiny
looking (which I suspect means the metal or the flask could actually
use to be a little hotter, when that happens, but the castings are
still nicely complete and solid. Only occasional problems with
porosity, which usually seem due to improper spruing or model design.
The cast items can be pretty hard if the flask was allowed to cool
too long before quenching, but they anneal well enough and are then
nicely workable. In general, a decently behaved white gold, other
than a somewhat yellower color than we might prefer (they usually
need rhodium for most of our customers). At least, neither our
polisher or diamond setters have threatened to quit over the stuff,
as has happened at least with one setter, back when we used one of
the super white alloys, which were hard enough to dent steel tools,
sometimes…

Need any other data?

cheers
Peter