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Strange casting results


#1

Last night hubby and I cast eight flasks of sterling charms. We use
an Electromelt and vacuum casting equipment, and the metal was a
50/50 mixture of home-made casting grain and re-used sprue bases from
previous castings. All of the sprue bases were pickled until clean
and stored in a sealed container.

On the 4th flask, when he peeked into the electromelt to see if the
silver was liquified, he reported seeing small tendrils of black
smoke coming from the metal. From my position a few feet away I
couldn’t see the smoke. When we poured the metal and it cooled, the
visible button was a light yellow color, not the usual black of
firescale. When we quenched the flask and got the contents out, the
sprued charms were light pink instead of black. When pickled for
only a couple minutes, the sprued charms turned that lovely white of
sterling that’s been pickled for hours. The other seven flasks all
had the usual black firescale that comes off after several hours of
soaking in hot pickle.

All we can assume is that something was on/in the metal in that one
flask that prevented it from developing firescale. But we know that
none of that metal contained the alloy that prevents firescale–it’s
all home-made (fine silver with the proper % of copper added). The
only contaminant that could possibly have gotten in there was
injection wax, as there may have been tiny shreds of it on the bench
near the area where I weighed the metal.

My question is: Could a tiny bit of wax in the electromelt have
prevented the firescale? Whatever it was, we want to do that again!

–Kathy Johnson
Feathered Gems Pet Motif Jewelry
http://www.featheredgems.com


#2

Kathy - Some years ago, perhaps even over a decade, there was a
wonderful article in Rock and Gem magazine about intentionally
putting shreds of wax into a closed container with the cast flasks.
The resulting combustion used up all of the oxygen, which was the aim
of the author after accidentally doing the same thing. The author’s
result was the same as yours - no oxidation on the castings. I’m
sorry that I don’t remember the author; you may be able to find the
article through Rock and Gem’s web site.

Jim Small
Small Wonders


#3

Dear Kathy, Here’s my opinion. I think what you did was to melt a
little hotter than you would have with premixed sterling. When making
casting grain (in gold and sterling) in order to get every alloy to
melt, mix and flow, you have to melt at the highest melting
temperature of the melting alloys. I usually melt even beyond what
they recommend. I also prefer the deox sterling for all casting which
requires a hotter melt. Shot is usually poured into cold water to
form the little casting grains. When solidified you now have a new
common melting temperature of the mix. This is lower than the
hottest alloys melting temperature. Try to figure that out. I would
think it should be the highest temperature, but what do I know? Many
times people who mix their own alloy make the mistake of trying to
cast from the first melt. This melt is a metal cast much too hot.
More true with the white gold alloys than the yellow. I can’t explain
your smoke except to think it is the melting container. The
electromelt is the best economical way to melt so you are choosing a
good way to melt. What was the temperature on the dial? Match that
next time and your results should be the same.

Best Regards,
Todd Hawkinson


#4
    My question is: Could a tiny bit of wax in the electromelt
have prevented the firescale? Whatever it was, we want to do that
again! 

Kathy, There are two types of copper oxides. The top or first layer
is black and is called cupric oxide. The layer that is typically
under that layer is reddish and is called cuperous oxide. It is this
red layer that is called “fire scale”. One possibile explanation for
your occurance is the small tendrils of black smoke that you saw
emanating. Possible contamination of your metal with wax could have
done it in a perfect world. Boric acid on a melt tends to make a
film that protects from contact with oxygen and can also collect
debris and oxides in the resultant glass like film. While boric acid
protects, it does not reduce. Wax is mostly carbon and like has been
discussed on the Orchid forum many times, the copper is the reactive
element in your sterling alloy. The black cupric oxide that appears
on most of your castings may have been reduced back to metallic
copper giving you the pink color. If carbon exists in the flask from
an incomplete burnout it can combine with the gas that is liberated
from the decomposing CaSO4 binder portion of your investment. As the
binder decomposes it becomes 2CaO and SO2 in the absence of carbon.
In the presence of carbon it can become CaO+SO2+CO if the reaction
was perfect. I don’t believe in perfect reactions in a world of
chaos however, it appears that you may have experienced it to some
degree. If you have carbon from wax present in the melt, then copper
oxide can be reduced from an oxide back to metallic copper during the
melt. Your description of a pink casting instead of a black casting
indicate that a reduction reaction has taken place not only in the
melt but likely in the flask itself. Besides being a poison, carbon
monoxide (CO), is a reducing agent.

Now if you have pink sterling castings after pickle that doesn’t
easily rinse or polish off, then you are in a pickle, (pun intended).
If this cuperous oxide migrates deep into your castings you can
pretty much write them off. It will appear as a dark grey cloud that
cannot be removed. Once this has formed, your sterling is toast. I
have seen folks that turned this into a “patina” but that was a
matter of marketing definition. To me, it was scrap.

The one unknown here is the copper and before I would commit to the
belief that you have experienced a reduction reaction I would want
to be sure of one thing. What was the source of your copper. Were
you using electrical wire by any chance or maybe pennies? I only ask
this because of the occasional presence of aluminum in these sources.
Aluminum has been used in sterling alloys in the past as a deox
element. New pennies are actually copper clad aluminum and not solid
copper. Some electrical wire is also aluminum or a copper / aluminum
alloy. The use of aluminum in electrical wire is to save weight and
cost in long expanses. Could it be? Also, in the castings that
appeared pink, was there an increase in “porosity” or other gas
related defects such as a dendritic surface as compared to your black
castings? Did your “pink” castings experience any unusual cracking
later in the process? I’m curious. You may have created a deox
alloy by a contamination with aluminum by accident or you may have
caused a reduction reaction with your wax. You may need to repeat
the experiment and eliminate variables. Use all freash metal, known
pure copper and .999 Ag. Do 2 identical flasks and contaminant one
with wax. I look forward to hearing from you on this.

Best Regards,
J. Tyler Teague
JETT Research


#5
    Kathy - Some years ago, perhaps even over a decade, there was
a wonderful article in Rock and Gem magazine about intentionally
putting shreds of wax into a closed container with the cast
flasks. ...I don't remember the author; you may be able to find the
article through Rock and Gem's web site. 

The fellow’s name is Lee Epperson, and he also wrote about this
recently in an article for Lapidary Journal (Part IV, March 2003), as
well. He has refined the technique. He can be contacted by email at:
leessilver@aol.com or (602) 993-4766.


#6

I believe that the author of the subject article is Lee Epperson, a
fellow Phoenician and Orchid contributor.

Lee Einer


#7

Hi, All pennies produced in the past 15 years or so are made out of
copper plated zinc; 2.5% copper 97.5% zinc.

Richard


#8

When casting from an electomelt, do I use flux, or not. I have been
told not to use flux in my charcoal crucible. Is this correct?


#9

All, the inadvertent genius behind burning scraps of wax in a closed
space with casting flasks, was our own Lee Epperson of LeesSilver.
Wonderful man, generous with his knowledge, and he will send his
instructions if they aren’t already in Archives. Hello Lee.

Pat


#10

Kathy,

In the days before de-oxidized gold casting alloys, if you got a
bright, shiny casting it meant that you had an incomplete burnout and
carbon had been left in the mold, which prevented oxidation of the
surface of the metal - and sometimes ruining the cast for other
reasons. Some contamination from some stray wax scraps could very
well cause your situation, but could lead to other contamination
problems. Don’t believe I would try to do it on purpose, but then
again I suppose that someone told Tom Edison that an electric light
bulb wasn’t a good idea either…

Allan Beck
Meridian, Idaho


#11

Hi, All pennies produced in the past 15 years or so are made out of
copper plated zinc; 2.5% copper 97.5% zinc.

Oops, you are right and that is an even better deoxidizer than my
mistaken alunimum to silver addition, both of which are used to
deoxidize some sterling alloys.

Best Regards,
J. Tyler Teague
JETT Research


#12

Kathy and all Electromelt users, I noticed the when I used scrap
sheet silver left over from a bead punching process that my castings
were sharper then when I started using casting shot. When I used
sheet scrap I could not completely fill the crucible so I would get
the silver melted then add more silver. This took a longer time to
get a full cruicible then when I used grain. When using casting
grain I could fill the crucible before melt. When I poured the
silver melted from grain I would get a string of silver forming on
the crucible. The upper ring of the cruicible was too cool and
chilled the silver as it was poured. The difference between the
melt processes is that melting sheet allowed the upper ring of the
crucible to heat more than it did when I melted grain.

I now heat the metal to the melt temperature and wait at least 5
minutes before I pour. I have the large Electromelt and usually have
enough silver to pour several flasks. I wait at least 5 minutes
between each pour. The one advantage in melting with the
Electromelt is that you will pour at a consistent temperature. The
temperature gauge may not indicate the melt temperature but if you
heat to a set gauge temperature the pour temperature will
consistently be the same.

Kathy, you might want to get a means of reading the gauge. That way
you will always get consistent pour temperatures. Slight variations
in the pour temperature might have some affect on the oxides. Good
luck Lee Epperson


#13

Hi All;

This goes out to you casting experts and metalurgists.

I do most of my own casting, but recently a client wanted a couple
dozen copies of a sterling ring with a hefty top. I didn’t want to
take on a project casting all these and having to melt a pound or so
of metal, and I don’t have high tech equipment, so I contracted it
out. The castings came back with a surface defect I’ve seen before
but only on white gold when the molds were too hot. Where we cut
off the sprue, the metal was white, but everywhere else, under
examination with a loupe, I could see tiny, greyish crosshatching,
like the dendritic structures one sees in meteoric iron. These
rings tarnished very quickly and will have to be re cast. The
company’s engineer said that it was the result of quenching the
flasks too soon, but that seems doubtful to me. I suspected that
the metal, being probably overheated, reacted with the investment,
which was also too hot, and the sulfer in the investment and the
copper in the sterling formed some sort of sulfide. Am I way out of
my league with these speculations, was the engineer right, or is
this something entirely different?

David L. Huffman


#14

Dear David,

What I think may have happened is that the metal may not have been
hot enough. When traditional melting non deox sterling, the colder
the melt the better. I’m talking torch melting. Was the silver deox?

The flask temperature should be 500 Fahrenheit for extra large
items. Ask what temperature the flasks were held at and for how long.

But with the newer deox sterlings it is almost better to over heat
the metal a bit to help the deox work. I got a cob web
(crosshatching) area on some casting years ago when this metal was
introduced. Ask the caster what kind of sterling he was casting.
There are several to choose from.

Anyway my best guess.
Best Regards,

Todd Hawkinson


#15

By your description, it sounds like you were the one on the money.
The metal was probably WAY overheated (if they used induction, like
maybe the pyrometer was set for gold - which could be like 500deg.
hotter). Perhaps the flask was also too hot. One test for overheated
metal would be, besides the appearance, try to size one. Which is to
say, put it on a mandrel and bang on the shank like you are
stretching it. Your metal will probably break, but at least it will
probably crack - the cracks look like the cracks in mud as it dries
out, usually - deep, grainy-edged. Your statement that it oxidizes
quickly, though, is curious. It could be that your analysis is
correct, but I wonder if it’s even Sterling Silver at all

John Donivan @John_Donivan.


#16

Hi David,

I’m just a persistent wax carver, not a casting expert.

I do have a really nifty reference: “Handbook on Casting and Other
Defects, In Gold Jewellery Manufacture”, produced by Dieter Ott and
edited by Christopher Corti. Copyright World Gold Council, 1997. I
don’t see an ISBN on it but think that it’s available from Rio
Grande. (maybe it’s out of print! I just looked through the Rio 2005
Tools catalog and couldn’t find it! But it’s a really excellent
reference. Everyone who does, or uses, precious metal castings would
benefit from having one.) I was given mine at the Santa Fe Symposium.
Thank you again, Rio! (Usual disclaimers)

From the phrasing of your question, it sounds like you may have this
book, too. On page 35 it says:

  "Case 10: Dendritic Surface - Due to shrinkage and reaction
  with the investment." 

They show good photo-micrographs of dendritic surface texture in
gold. I’ve seen a similar effect in some sterling castings I
produced. (Remember, I’m just a wax carver! :slight_smile: )

Dr. Ott writes:

  "Growth of crystallites at the onset of solidification
  manifests itself as a dendritic shape. The residual melt
  remains in the inter-dendritic space. If the melt does not wet
  the investment at the mould wall and decomposition of the
  investment gypsum binder causes the formation of sulphur
  dioxide gas, the residual melt is pushed away from the surface,
  leaving a skeleton of dendrites. Thus, the typical dendritic
  surface structure appears. These conditions are preferentially
  fulfilled in a neutral, protective atmosphere and with high
  casting and/or flask temperatures." 

That being said, would you please describe the dendritic texture a
bit more? Does it extend all the way through the cross section of
your ring, especially at the heavy top, or is it only on the surface?
Does the dendritic surface texture cover the whole ring, except
where you ground and polished the sprue? If the dendritic texture
goes all the way through the ring and occurs more on the thicker
sections of the ring than on the thinner sections, it could be due to
poor progressive solidification of the casting. That is, maybe the
thinner sections (the sprue and shank?) solidified first and the
thicker section couldn’t get enough molten metal to fill the areas
between the dendrites. If that is the case, I expect to see the
porosity go all the way through the thick section and even show up as
larger cavities near the center of the thick section.

I cannot imagine getting a flask hot enough to keep my metal molten
and to get it into to the quench tank in time to effect the surface
texture. I know that I can get it into the quench in time to cause
shanks to crack. :slight_smile: But not in time to effect the surface.

I think you probably hit the nail on the head with your first
thought - Gas reaction pushed the residual melt away from the
surface.

Chuck in springtime Asheville!


#17

David,

There are many things that can go wrong when doing a casting but the
amazing thing is that you can do many things wrong and still get a
good casting.

There are some things you can never do wrong and expect a
satisfactory piece.

The first thing I would look at is how the piece is being sprued.
The piece should be sprued at the heaviest area unless you are
placing the sprue to a large flat area. The metal needs to flow
through the sprue and into the model in a free flowing manner
without solidifying before this is complete.

When casting pieces that are very heavy you need to cast at a lower
temperature, somewhere in the 800 to 900 degree range. the button
needs to show no red before it is quenched and the metal needs to be
melted without introducing too much oxygen to the melt.

Turbulence and overheated metal are the two biggest enemies of
casting.

Good Luck
Greg DeMark
Longmont, Colorado
email: greg@demarkjewelry.com
Website: www.demarkjewelry.com
Custom Jewelry - Handmade Jewelry - Antique Jewelry


#18

Chuck you are probably correct. It sounds like mold decomposition
due to over heating of the investment. The rapid tarnishing is
another clue to the presence of sulfur dioxide in the metal from the
investment breakdown.

David do not reuse that metal as you cannot get the sulfur dioxide
out of the metal without refining it and all castings you do from
that melt will exhibit higher than normal gas porosity.

Jim
Jim Binnion

James Binnion Metal Arts
Phone (360) 756-6550
Toll Free (877) 408 7287
Fax (360) 756-2160


@James_Binnion
Member of the Better Business Bureau


#19

Hello Orchidians;

Thanks to all who have responded to my querry about the odd casting
results. As I stated before, I didn’t cast these, and unfortunately,
I don’t have them any longer for further study. The company is going
to do them over for the client, and I’ve learned a valuable lesson.
Us goldsmiths should stay away from the idea of price-point
production in sterling. It’s way too easy to end up doing a lot of
work for nothing. The margins are just too tight for errors and I’m
used to a little “swing” room. Still, I’ll be interested in hearing
any more input on the casting problems I described.

Thanks again.
David L. Huffman


#20
    David do not reuse that metal as you cannot get the sulfur
dioxide out of the metal without refining it 

Hi Jim;

I didn’t do the casting. I’m not a production caster. I do one-offs
and some casting for my accounts. And fortunately, the company that
did this also does refining, so they can salvage their silver. What
really tee’s me off about this is the precursor to this casting
mishap. I made the mold and shot the waxes. I didn’t make the
original model, and if I did, I probably would have done some things
differently. The design invited some of the problems, but it still
should have been well within the range of their expertise.

They didn’t tell me till I got the bill that they determined that my
waxes needed an additional $60 worth of improvements. I called them
and told them what I thought of that, since it’s the first time in 3
decades I’ve had a caster complain about my waxes. I’m not one of
those idiots who thinks that casting a wax can improve problems in
the wax. I’ve cast thousands of waxes, and I have a good idea of
what to expect and what can cause problems. And they had effectively
taken away my choice. If they had called me, I would have chosen to
either have them wait till I sent more waxes or told them go ahead,
I’ll take responsibility if they don’t come out and it’ll be on my
dime. Maybe I would have even told them go ahead, fix em, but keep
one to send me so I’ll see what it is you don’t like. In my
judgment, those waxes shouldn’t have caused any problems. But after
they blew it on the castings, I guess it kind of confirms what other
people have told me about their casting operation.

I had them cast a couple platinum pieces, and although they weren’t
exemplary, there were no problems that made me consider the work
unacceptable. Their engineer, or metallurgist was the one who came up
with the hypothesis that the flasks had been quenched too soon. But
he hadn’t actually seen the castings either. But from my knowledge,
quenching sterling too soon can cause cracks, but I’ve never seen it
cause this kind of problem. This ring is fairly massive on the top,
and I knew without a neutral atmosphere to melt and cast in, I’d have
at least some porosity, and besides, there were just too many waxes
and too much metal for my operation. Too bad, because they had, by
far, the best price on platinum casting. I’ll still send them my
refining, but I don’t feel like they will provide me the professional
consideration I believe I’m entitled to when it comes to casting. I
think they have devised their policy to protect them against their
worst customers at the expense of the customers they really want. I
sympathize. I can imaging the crappy waxes they get and the customers
who expect miracles, but if you’re going to be picky about the waxes,
you better know what it is you need to pick on and you better darn
well know what you’re doing when you pour the metal.

David L. Huffman