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18k white gold casting problems


#1

Dear All, In the last year we have begun to experience some
difficulties casting our 18k white gold projects. We had not
intentionally changed anything when the problems began, and I’m
wondering if anyone has any ideas or thoughts. Our castings in this
alloy are turning out with a very heavy orange peel texture. Thin
sections are not affected as badly as thick, heavy sections. Our
casting scenario is as follows:

Alloy: 18k white gold from Hoover and Strong.

Casting technique: Deep well vacuum casting

Metal melting technique: We use a Kerr Auto Electro melt for melting
the metal

Pour temperatuRe: We have varied from our standard temp for this
alloy of 1870 degrees down to 1840 degrees with no discernable
difference.

Flask temp: 910 degrees

Quenching: We have varied from waiting just until all color is gone
from the button down to almost cold, with no discernable difference.

Investment: Kerr Satin Cast

These are the specifications we have used for casting this alloy for
many years with consistently good results, and so we suspect that
there is an outside change we are not aware of. We are not
experiencing any difficulties with our yellow gold castings or even
our 14k white gold castings. These alloys cast at a lower
temperature, so we are wondering if the higher temperature is
reacting unfavorably in some manner. The only things we can think of
to explain the problem are changes in alloy, changes in the
investment, or problems with the accuracy of our instrumentation. I
have contacted Hoover and Strong to see if there were any changes in
their alloy, but they assure me that is not the case. They recommend
we switch to their 18k palladium alloy; we are not keen to do this
if we can correct whatever is causing our current problem, as we
like the color and working properties of our current alloy. Hoover
and Strong said it sounds like the metal and /or flask is too hot.
They said they would get back to us on the lowest recommended temp
but we haven’t heard yet and I will admit I’m a bit antsy; we have
all of these 18k white gold custom designs coming due…

As you see we are puzzled. Does anyone recognize this problem? Does
anyone else regularly use this product, and if so, can you tell us
the lowest temp we can cast at and still get complete castings?

Also- I remember some time ago reading posts about an unannounced
change in Kerr’s satin cast investment. Is there any truth to this
and if so, what types of troubles did it cause other people? Maybe
our orange peel texture is related? We have not contacted Kerr, but
perhaps we should.

Thank you all.
Brenda
david lee jeweler
Mason City, Iowa


#2

Dear Brenda, Here are a few thoughts:

  1. I don’t think you are melting the allow hot enough. I’ve know
    other casters who have had the same problem. I don’t think you are
    melting the deox off the metal. Are you using deox 18k? If you are
    not using deox your temperature is fine. I melt at 1950 degrees F.
    This clears off the deox and gives me good castings. I haven’t cast
    18 karat in a while, but would not change anything.

  2. I cast as cold as possible. I know this may sound nuts, but you
    set up the same ring in about four flasks. Now cast each flask colder
    and colder until one does not fill. Add 100 degrees and you have the
    correct temperature for your flask. This may be a little tricky. A
    lot depends upon your casting machine.

  3. I gave up trying to cast white gold by vacuum. I think you get
    much better results using spin casting with those colder flask
    temperatures. When you pour the metal you give it too much time
    between the melt and the flask. It may seem like a little time, but
    compare it to spinning pouring is too long a time. If you can spin
    those problem ones you may also have your problem go away.

  4. With vacuum casting the investment shouldn’t be the problem. Does
    your machine pull a good vacuum? If so, the investment probably isn’t
    the problem.

  5. If your change anything don’t make a LOT of changes. It’s hard to
    pin point what you know are doing right. It sounds like you know what
    you are doing.

  6. The refinery may also be using a different blend of deox in the
    alloy for 18k. If you are using deox you may try going back to the
    old standard alloy mix. Have them chop up some sheet 18k white gold.
    This gold is usually free of any deox material. If you use the
    standard alloy without deox what you are doing sound like it would
    work. So by getting the alloy without the deox you may NOT have to
    change anything.

  7. Spin casting wait ten minutes to quench. Vacuum casting wait
    fifteen to twenty minutes to quench.

Good Luck and Best Regards,

Todd Hawkinson
TR the Teacher
T.R.Hawkinson, Ltd.


#3

Timo Santala used to say that if nothing has changed in the process,
check the equipment. Sensors can go bad and calibration can slip,
resulting in incorrect melt and/or flask temps. Timo told a story
about being called in to trouble shoot a casting operation
experiencing the type of troubles you describe. (All of a sudden
things stopped working; they didn’t know why.) He went through the
process with them, and everything looked fine. Then he looked at the
melt itself – which was obviously far hotter than it was supposed to
be. Turned out the machine’s temperature sensors were shot. (I don’t
remember the precise details of the story, but I remember that it
was the equipment that had failed, not the process.)

Have you checked the calibration of the temperature sensors on your
electro melt? How about on the burnout oven? You could be setting
these to the correct temperature, but be getting something very
different. Your equipment’s manufacturer should be able to suggest
ways to check the calibration.

Timo often presented at the Santa Fe Symposium, and was kind enough
to present at a seminar or two I organized for MJSA. He was always
willing to share the benefits of his experience. The industry lost a
real expert and a great guy when he was killed in a plane crash last
year.

Suzanne
Suzanne Wade
writer/editor
Suzanne@rswade.net
http://www.rswade.net
Phone: (508) 339-7366
Fax: (928) 563-8255


#4

Hi David, When I experienced such difficulties in the past, I cured
it by changing my metal source or alloying the metal myself (not
pratical with white gold). Despite my sources claims that they had
not changed their formula, nor had they had any other complaints,
this cured the problem, which sounds very much like yours, a fine
impurity which gives the gold almost a sandy surface. I had tried
several other solutions to no avial, ie. replacing my crucible,
getting new investment, etc., but changing the metal is the only
thing that worked. Good luck!

Best regards,
John


#5
    Dear All, In the last year we have begun to experience some
difficulties casting our 18k white gold projects. 

Hi Brenda; It sounds to me like an effect I’ve seen when white gold
is overheated in an electric melter with a graphite crucible. When
you look at the casting under magnification, can you see dendritic
structures? Those would look like tiny, short, straight lines
pointing in all directions. This is carbon attempting to form
branching crystals within the grain boundaries of the metal. You
might note a bit of glassy hardness where those little hairs are
concentrated. If that’s the case, what may be happening is that
your electromelt needs to be calibrated, as the temperature inside is
higher than what the pyrometer is reading. Don’t know how you would
do that for your perticular melter, but you might get info on that
from MPG repair (who post here on the forum).

David L. Huffman


#6

Hey Brenda what H & S said sounds right, The first thing to look at
is your thermocouples (pyrometer probe) they can go bad and or fail
right out of the box, my experience is that as they age they read
right at low temp but are off 100 or 200 =BA F above 1000 =BA F. I kee=
p a
control or unused pyrometer just to test temperatures. I would need
to see the orange peel surface, but it might be that your oven isn’t
reaching the peak temperature. (improper burnout due to worn
thermocouple)

About the investment check with Kerr about its “Breakdown or Melting
temperature” You might find its close to the casting temp. of the
alloy. If that is the case you will have problems with the investment
forming a hard layer on the cast part.

Also Hoover and Strong just hired a new metallurgist and
Reformulated a lot of their alloys, the ones I use got better.

Hope I helped some…
James M…
Outcast & Company
Seattle