We’ve had issues casting 18k yellow and rose golds.
We use an 18k “Santa Fe” yellow (no need to name the maker), and
find the alloy to be prone to porosity. We just completed a signet
ring, had discussed spruing to the shoulders of the signet, but
decided to go with one big sprue to the bottom of the shank. We
thought we could do the single sprue because the shank was about 7 mm
wide and almost 2 mm thick.
Everything was great til the client came in, tried on the ring, and
we found we had to size down one half size. The ring cracked right
thru the center of the signet (top), where it was thinnest. We use at
least 50 % new metal all the time, and the button material is all
from the same alloy.
So, I’m looking for recommendations for 18K yellow gold alloys.
Another issue we’ve encountered (and pretty much solved with correct
annealing) with 18k rose and peach golds is extreme hardness and
brittleness. Had one ring with a fairly heavy shank just literally
shatter when we put it on a mandrel to adjust the roundness. Just
seems that particular mix of gold and copper molecules is not a
happy combination-- no problems at all with 14k pinks.
I have never used the alloy that you are referring to but my guess
would be the way the item was sprued is what is causing the problem
and not the gold. When gold is enters the sprue cavity and into the
bottom of the ring it is flowing the way it should but once it hits
the flat top of the ring it is sent into a turbulence that might be
causing the problem. You may want to try spruing from the bottom only
with the ring tipped slightly to one side and then run a second
lighter gauge sprue about 10 to 12 gauge up to the outside edge of
the top. Place this sprue along the edge that is leaning to the side
slightly.
I will not use ANY commercial alloys. I do NOT always use that much
new metal. I had an incredible 9 pointed snowflake star that I made
with a needle and alcohol lamp, each ray done one at the time or a
design element at the time, that was cast and stripped looked
wonderful, that broke apart in my pocket in the 80’s. I have never
used commercial alloys since. I am not real active, but I have had
consistently good luck using just copper and silver to alloy.999 24k.
I have had no problems with the little bit of 18k I have done. I do
not do commercial market things, but have cast for 30 years. THIS
APPLIES IF YOU ARE NOT OVERHEATING METAL WHEN CASTING, ALL BETS ARE
OFF IF IT IS A TEMPERATURE ISSUE.
Casting alloys contain a small percentage of silicon which forms a
eutectic in the metal alloy and is brittle. If you mix your casting
alloy with 18k scrap sheet, wire etc you will get a more workable
alloy. The silicon is there to reduce oxidation, lower viscosity and
help with mold filling so you will need to adjust your casting
regime slightly to keep your metal clean and flowing.
Sometimes, you can cast a metal and it will do exactly what you
want, recast the same metal and it goes porous, doesnyt fill the
mold, explodes on quenching and other disasters. We find this on
casting pure metals (tesr specimens at the day job) so it is not just
an alloying problem. I suspect that dissolved gas is the biggest
problem of this sort, it may not be visible as pores but you get a
microporosity that will weaken the grain boundaries and hence make
it much less ductile so when you go to work the metal it fails.
Fluxing generally reduces the dissolved gases but other factors
include surface area, heating time to melt, time held at melt, type
of gas torch used to melt the metal and type of crucible and its
porosity. It is a wonder we ever get anything right!
Gas porosity is caused by contamination, shrinkage by metal flow and
cooling. Gas porosity normally accumulates in areas of shrinkage, so
if your gas porosity is localized you have both problems. The Gas
problem: Dirty metal - Every time you use scrap you are adding the
gas porosity from your previous cast to the melt. This never goes
away, refine your scrap more frequently, try using only fresh metal
and examine the results. Your trees will be contaminated with
investment, acid clean, ultrasonic and then boil in potassium
nitrate to neutralize the microscopic investment particles. Melt in
clean crucibles, avoid coated crucible that might have an iron
content i.e. Salamander. Melt in a reducing environment, pure
graphite, argon gas cover, etc. Your melt might be picking up oxygen
from your torch, white gold needs a lot of heat. Consider electric or
induction melting with a calibrated pyrometer. Don’t over heat; find
the lowest temperature for both the melt and flask. For larger
objects cast both with the flask and melt as cold as possible, large
masses of molten gold especially White gold are hot enough to burn
the investment and create oxides. Different part geometry’s require
different temperatures for each. Keep detailed records for every cast
and note the results.
Burn out can be a biggest cause of gas problems. Don’t trust your
oven controller, set up a separate chart recorder (Omega) with the
pyrometer invested in a dummy flask (you can leave it in your oven
for years) and use a new chart for every burnout, you can see a
record of the entire burnout cycle, examine the chart and include it
with your notes. Look for under and over burnouts, carbon residue and
burnt investment both cause gas porosity and are the result of
improper burnout. Use the chart to determine your actual flask
temperature before casting. If necessary set up a separate oven with
an accurate pyrometer for holding your flasks at an exact casting
temperature, allow a couple of hours for your flask temperature to
settle. You might find a difference of 200 degrees or more between
your controller and the chart recorder. Consider oven mapping for hot
spots, a flask might be more then 100 degrees hotter at the back of
the oven, think what that will do to your investment. Make sure your
oven is getting enough oxygen to combust the carbon. Gas ovens with
the door cracked open are usually better for this then electric.
There are a lot of variables involved; you need to take a scientific
approach to solving them. A lot of testing and note taking. Spruing
and tree structure are the answer to shrinkage and can be just as
complicated to solve. The last thing that I would look at is the
alloy; it’s almost never the problem unless you are burning it.
My son Jason spent over 2 years of daily casting and meticulous note
taking in an effort to improve our casting, we sent samples to
Germany for testing, we tried everything we could think of before we
finally got shrinkage and gas porosity under control. Clean metal,
controlled temperature and spruing solved most of the problems and we
cast with ternary alloys, no additives. I prefer to vacuum cast large
objects, but I doubt that it makes much difference unless your pieces
are really heavy. Professional quality casting is not easy to do; it
takes a lot of work and time. By the way, fancy casting machines are
not going to make any difference. You still need to do everything
right, all that you are gong to get from an expensive machine is the
ability to individually program each melt for the objects in the
flask plus an oxygen free melt and pour. It is not going to solve any
of the other problems.