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Micro pitting in white gold castings


#1

For years and years we have had problems with micro pitting in the
heavier sections of white gold castings. We can go weeks with no
problems, but it never seems to go completely away.

We began by using a spring operated centrifugal casting machine with
a Harris torch w/rosebud tip (oxy/propane) and did for a number of
years. Several years ago we upgraded to a Degussa Motorcast that can
also safely cast platinum, it will spin at 350-450 rpm’s as long as
the door is shut, we are still using the same torch (we do get much
more dense castings with this machine, we’ve been very happy with
it).

For years we have been in contact with United Precious Metals, PM
West and anyone else who could offer suggestions as to how to reduce
or eliminate the micro pitting in white gold.

We find that the bad white gold castings look good until we go to
lap and final polish and at that point we find areas that look
unpolished because of hundreds of tiny gas pits. We can
hammer/burnish/lap them away (really just hide them) but it wastes a
ton of time. We have adjusted our flask temps, our burnout cycles,
our sprueing, the metal itself and we are still dealing with this
aggravating problem.

We are at the point of investing in an induction casting machine
with vacuum and inert gas over-pressure with the hope that the cost
will be made up with time saved by not fixing the tiny pits.

But before we do that I thought I would kick this out to the giant
throbbing brain that is orchid. Any suggestions would be much
appreciated.

Thanks,
Mark


#2

Have you tried 100% virgin casting grain? White gold seems to be more
susceptible to gunk in remelted old gold.


#3

Hello Mark;

The first logical place to look for a cause for this problems is
flask temperature. If you’ve been having consistently good results
for the most part, I’d assume you’re covering these issues. So what
may be happening is that the occasional problems arise when you’ve
got a larger flask than usual and it’s not getting enough time to
equalize to the ideal casting temperature. And, of course, you want
that kiln calibrated. I like to cast white golds as cold as I can get
the mold to fill, usually around 850-900 degrees for vacuum.
Centrifugal might allow colder that that, depending on the article.

Many assume sprueing is the paramount issue. That will be true if
you’re porosity is mostly of a shrinkage variety, and it will occur
in thick sections. But what most people don’t understand is that,
beyond a certain point, the entire casting is a sprue of sorts.

But I’m going to take a guess here. I think it’s all a design issue.
I also think what you are seeing is not actually gas porosity, it’s
very fine shrinkage porosity. Why? Because it’s in the thick
sections. The thin sections are cooling and drawing metal from the
thick sections. If it’s truly a gas porosity problem, you’d probably
see an attendant dark color or rough surface or both. And you’re
flasks, when quenched, will smell sulpherous.

Why do I say this is a design issue? Because if you design an
article that has abrupt transitions from thin areas to thick ones,
you have designed in a problem. I get wonderful results with my own
waxes in white gold, but then, I don’t set myself up for problems. I
get problems when I try and break my own rules. But I have clients
who have designs with thick to thin shifts, and no matter what I do,
either the thin sections don’t fill, or the thick one’s have
porosity, or I have to do stupid pet tricks with the sprue system,
which leads to the next possibility. Read on…

White gold, especially the higher nickel alloys (the nice white,
stiff ones), doesn’t like to be re-melted. Yellow has it’s limits
too, but it’s more forgiving. Each time you melt white gold, you’re
changing the nature of the metal, burning off zinc, dissolving in
oxides, etc. Common opinion says you can use up to 50% sprue metal on
a casting, but with white gold, I like to stay below 30%. So, if
you’ve designed objects which require massive weight in the elaborate
gates and sprues, you’ll be tempted to re-use all that metal. So, try
my approach, which is…

It’s ALL about design. Everything, every bit of it, start to finish.
Anything with a wall thickness above 2 millimeters is going to be
trouble. Keep those thicker areas away from the walls of the flask,
and away from other waxes in the flask, so that more than one area
together don’t get to form a “hot zone”. And, the higher the nickel
content, the more trouble. If it’s X-1, well, good luck with that
stuff. Also, am I nuts here or is there really a difference in
"density" between vacuum and centrifugal? Can molten metal actually
be compacted any more that it naturally crystallizes when it
solidifies? If that were true, don’t you think we’d have some cases
of specialty metals that were used in industry such as “super dense
cast steel” for armaments?

David L. Huffman


#4

In a shop i worked in we were able to use recast material up to 50%
for yellw gold, (plus some special alloyed reffreshener) but we
allways used virgin metal for white gold, it was never worth
playing around, white gold is just too dellicate for any
variation…even if it ends up looking fine, you will end up
seeing trouble, and paying for it later…


#5
For years and years we have had problems with micro pitting in the
heavier sections of white gold castings. We can go weeks with no
problems, but it never seems to go completely away. 

If you’re getting the micro-porosity sporadically or only in heavy
sections, your metal temp and/or flask temp is probably too high
and/or you might be trying to go from thick through thin to a too
thick top. If you examine the pitting under a very high
magnification, I’ll bet that you will find it’s shrinkage porosity,
not gas bubbles. Gas or other inclusions would be spread throughout
the entire piece, and probably not be localized to just heavy
sections.

A trick that kinda helps with the metal temp issue is to make a
button by melting and mixing all the metal for one flask in a regular
dish crucible with handle (as quickly and for as short a time as
possible) and quench it (crucible and metal) in a bucket of water as
soon as the metal hardens to the point it won’t spill out. The metal
button will fall out as soon as it cools or you might have to gently
pry it loose. Pickle and clean the button and it’s ready to cast. I
do this every time I cast any metal that requires mixing or stirring,
or has pieces of different sizes like using a button with fresh grain
and certainly any time I do any alloying, yellow or white. It doesn’t
hurt the crucible at all (but it will steam and hiss for a bit) and
it allows you to melt the metal in one ingot in the centrifugal
machine. A single fully mixed button melts all at once instead of
requiring you to cook and stir pieces of metal of different sizes and
melting points to mix them in the casting machine’s crucible, which
will definitely overheat the metal. The rule here is to keep the
metal hot for as short a time as possible. Bring it up to temp
quickly. As soon as it stops sticking and you can push it around with
the flame, throw it.

I use this same melting and casting procedure regardless of the
weight of the piece(s) and the metal type. I vary only the flask temp
to correct for weight, karat and color. I have found that using a
flask temp as low as possible without getting non-fills helps, but
I’ll bet you already came to the same conclusion. I use 800 - 900 or
so for thick white gold bands and stuff without thin parts (say
8-10dwt or heavier) - a lot lower than normal for lighter stuff. You
might also want to let the flask soak at the casting temp for at
least a couple of hours, maybe even three or four. Just because the
pyrometer in the oven is at 800 doesn’t mean the inside of the flask
is. How long does it take for a six pack of your favorite beverage to
get cold in the fridge? And that’s only a thirty or forty degree
change!

As far as cross section of the wax, it should go from thick to thin
only or at least have a consistent cross sectional area throughout.
If you try to go from the button and sprue through a thin section of
shank to a thicker part you will have problems, no matter how you
melt or cast the metal. Cooling metal shrinks whether it’s in argon,
oxygen or complete vacuum. Thin parts harden more quickly than thick
parts, cutting off any flow. If the heavy parts can’t pull from
heavier parts, you will have porosity. The more metal that can’t
pull, the more the shrinkage causes problems. Also, the more the
metal has to cool, the more it will shrink. Put the two together and
you end up with Swiss cheese.

These tricks haven’t completely eliminated our white gold porosity,
but now for us, it’s almost always an issue of thick-thin-thick, like
European style shanks, or getting lazy in thinning out the top that
causes us problems. Hope this helps you.

Dave


#6
The first logical place to look for a cause for this problems is
flask temperature.... what may be happening...when you've got a
larger flask... it's not..the ideal casting temperature. And you
want that kiln calibrated. I like to cast white golds as cold as I
can get the mold to fill, usually around 850-900 degrees for
vacuum. 

David, we do tend to use the same size flasks nearly always and we
have digital controllers running the kilns, but we have never
calibrated the kilns (good suggestion). We have also been casting
the white gold around 900-1000 degrees per the metal suppliers
recommendation, We could drop that considerably and still get good
fill, we will give that a try.

Many assume sprueing is the paramount issue. That will be true if
you're porosity is mostly of a shrinkage variety, and it will
occur in thick sections. But what most people don't understand is
that, beyond a certain point, the entire casting is a sprue of
sorts. I also think what you are seeing is not actually gas
porosity, it's very fine shrinkage porosity. Because it's in the
thick sections. The thin sections are cooling and drawing metal
from the thick sections. If it's truly a gas porosity problem,
you'd probably see an attendant dark color or rough surface or
both. And you're flasks, when quenched, will smell sulpherous. 

You are not alone in thinking this is a shrinkage issue rather than
a gas porosity problem, replies off list have suggested the same
thing. All of our work is custom, but even though they are our
designs, the designs are dictated by the clients requests or they
need to match an existing ring. So we are kind of stuck with
thicknesses and transitions that was often abrupt. I do think we
should go back to heavier sprues on heavier sections. That may be
helpful.

White gold, especially the higher nickel alloys (the nice white,
stiff ones), doesn't like to be re-melted. 

We tend to reuse less than 30% of the white gold. We cast everyday
usually about 2-5 jobs each day and we end up with a pile of white
gold buttons that we can’t recast. But that is another problem.

Also, am I nuts here or is there really a difference in "density"
between vacuum and centrifugal? 

By more dense I mean when casting gold we really never have an area
that is not fully filled, no matter how fine. I am not someone who
thinks spinning is better than vacuum casting, they each have thier
advantages, I was just saying that with this motorized centrifugal
casting machine we almost never have misses. Unfortunately we have
plenty of other issues to deal with.

Have you tried 100% virgin casting grain? White gold seems to be
more susceptible to gunk in remelted old gold. 

Neil, that would really help, but we then have so much used white
gold left over. We can make some into sizing stock, but not all of
it. It bugs me to scrap so much white gold, maybe I need to get over
that and just consider it part of the cost.

In a shop i worked in we were able to use recast material up to 50%
for yellw gold,, (plus some special alloyed reffreshener) but we
allways used virgin metal for white gold,,, it was never worth
playing around,,, white gold is just too dellicate for any
variation...even if it ends up looking fine,,, you will end up
seeing trouble,, and paying for it later.... 

Joseph, we use no more than 30% old white gold. What did you do with
all of the white gold that is left over? We have been scrapping most
of it and that is such a big expense.

Thanks,
Mark


#7

ive been trying to follow this thread waiting to see what others
have to say before i jumped in with any suggestions. i also have been
pleased with the previous suggestions so here goes #1 i think the
alloy you choose to work with is important and i think the the new
high nickel and what ever else is in them are designed and i mean
DESIGNED for the new generation of casting machines with controled
environments. torch casting leaves alot of variables to cause
problems and it is difficult to isolate what exactly it is that is
causing the trouble, with that said. #2 contamination can happen
much easier than you think if you read the alloy charts for steels
and other metals the percentage amounts added to the metals are in
.001% at times. you may need to look at cleaning up your area. iron
is especially dastardly effective in causing problems with white gold
i recently in the last month was having the problem you desribed
after cleaning up my crucibles soaking

them in pickle and re coating the problem has waned. you should
lookat everything start with the water you mix into your investment
is it sulphery high iron. are you next to other industry? if so what
do they produce . are you casting next to your polishing area ? rouge
will cause alot of headaches, are your hands clean when you handle
the casting material ? i read you use pm west alloys which is what i
use but i use the 100nd alloys now and since ive purged my setup of
all the unknown alloys and scrap ive had alot fewer problems. i dont
mean to sound as if i am looking over my spectacles at you but ive
not seen anyone write about this stuff on the thread and what ive
said will make or break your quality-

goo


#8

The shape of the pits or of each porosity spot can give a lot of
info.

Gas porosity, in my experience, tends to give pretty spherical pits
or voids. A sulfurous smell may indicate either an investment
breakdown due to high heat or left over pickle trapped in an old,
reused button. I think that it is better to sandblast or abrade away
casting skin from buttons prior to recasting. Pickling can open some
bad doors.

Shrink Spot porosity usually occurs in heavy areas, as many have
already said, but the spongy pores are really small tears, since that
is what is happening as the metal cools and, of course shrinks…

Contaminants such as dirt or investment are often jagged pits,
sometimes filled with the culprit, such as that pin prick of white
investment that grows and grows as you attempt file it away (and file
deeper into the plaster filled void).

A small rounded pit or depression surrounded by a bright halo is a
small bit of flux that got ahead of the pour occupied space before
dissolving out and leaving a ring of clean, oxide free metal.

One thing that I learned early on was flask temp. As others have
said, Heavy castings should be cast with a much cooler flask temp–
700- 900F, maybe. The larger mass of metal takes much longer to cool,
crystals grow larger and coarser, castings more pitted and brittle.

Years ago I was plagued by porosity – gas, I think-- around sprue
connections. I had never had the problem before and I had changed
nothing procedurally. I mentioned this to a colleague and he asked if
I steam de-waxed. I did not. I understand the value of steam
de-waxing, but it is not key to a good casting. He then told me to be
sure to keep those initial stages of burn out long and to soak it
before ramping up: let the wax have a good, long and thorough melt
out. (do wqaht steam de-waxing helps to do)

I started doing this and the problem went away. I realized that I
had been slowly ramping up the the burn out time–especially the
beginning-- and that impatience led to the problem. I’m still not
sure how this all works out during the burnout-- how too rapid wax
evacuation leads to that type of porosity. But, with casting (soooo
many variables) if I have to do a special dance and hum “Girl, you’re
a woman now” to guarantee a solid casting, I’ll do it.

Thanks to Mike in Hayfork who solved my problem.

Take care, Andy


#9

i also would like to mention for calibrating your kiln you can
obtain ceramic cones from the pottery supply that will slump and or
melt at a predetermined temperature f. apparantly pottery glaze is
extremely sensitive to slight temp increments the cones are used to
confirm the kiln stayed at the particular temp it was set. the cones
are very accurate try them

goo


#10
i also would like to mention for calibrating your kiln you can
obtain ceramic cones from the pottery supply that will slump and or
melt at a predetermined temperature f. apparantly pottery glaze is
extremely sensitive to slight temp increments the cones are used to
confirm the kiln stayed at the particular temp it was set. the
cones are very accurate try them 

Pyrometric cones are only moderately accurate and the method of
reading them is somewhat subjective. There is a time at temperature
aspect to how they slump and it requires that you be watching them
at the time of slumping to get any control of temperature. Digital
temperature controls have gotten so cheap that it just doesn’t make
sense to not use one. However if you want to use something like a
cone but with more accuracy get a tempil crayon. It is a crayon that
melts at a particular temperature and it is much clearer that you
are at the specified temperature in that the mark it makes melts and
becomes liquid at the stated temperature. They are accurate to 1% of
the melting temperature.

James Binnion
@James_Binnion
James Binnion Metal Arts


360-756-6550


#11

I would like to thank everyone for their replies, it’s just
remarkable to have so many talented and experienced people willing
to share their knowledge. I am very grateful.

I thought I would try to summarize the responses;

First, as far a identifying the problem. The consensus seems to be
that what we have is shrinkage rather than gas pits.

It was said that gas pits tend to be more spherical and are usually
found throughout the casting, while shrinkage usually occurs in
heavy areas, has spongy pores that are really small tears.

Suggested remedies;

  • Sprue from thick to thin, your objective being that you want a
    homogeneous and smooth flow rate. Spueing from thin to thick will
    nearly always cause problems.

  • Keep your flask temperature for white gold as low as possible to
    get good fill with your machine (700-900 was suggested, but that
    depends on the type of machine).

  • Try to reuse less than 30% of white gold in castings. There were
    some good suggestions on how to reuse a greater percentage and how
    to super clean the buttons for reuse.

  • Regularly calibrate your kiln. Using ceramic cones is an
    inexpensive and reliable method.

  • Use a fairly substantial button with white gold.

  • Cast heavy and light weight white gold castings separately so you
    can adjust your flask temperature down further for the heavier
    castings and a little higher for the lighter ones.

  • Let the flask soak at the casting temperature for a couple of
    hours to insure that its at that desired temp inside the flask.

  • Keep you initial burnout stages long and at a fairly low
    temperature to soak the flask well and remove all of the wax like a
    dewaxer would do.

  • Keep you crucibles clean in an effort to remove any contaminates
    that inadvertently found there way there.

  • If possible, it’s better not to cast in your polishing area. It is
    a likely source of contaminates.

If I did not include anybodys valuable suggestion, please accept my
apologies for the oversight. We’ll put the ideas that we were not
already doing into practice and see what happens. Goldsmithing is
one of those jobs where the longer you do it and the more you know,
the more you realize how little you know.

Best regards to all,
Mark