I have been a subscriber for a short while now and am sorry I waited
so long. It never ceases to amaze me how often when I search Google
for a solution to a problem or answer to a jewelry question a link to
Orchid will come up and contain the I am looking for or
point me in the right direction.
My question involves Rio Grande’s Caster’s Chunk Brass (63% copper,
1% lead, and various amounts of zinc and tin). I have been
experimenting with it (as well as their white bronze and ancient
bronze) for over a year at this point and my results have
consistently gotten better. My basic casting procedure is as follows.
I am vacuum casting wax models from quarter size (25 mm) to belt
buckle size (3.5inches x 2.5 inches). I use Americast Investment by
Ransom & Randolph. I use solid flasks that I drill air passages
around the perimeter to increase the vacuum through the flask. I
steam dewax. I use a programmable controller with my electric kiln
and max out at 1350 degrees F (manufacturer recommended temp). I use
900 deg F. as my pour temp for the flask and heat the metal to 1900
deg F in an electromelt to cast. I wait for all color to leave the
button (aproximately15-20 minutes), then quench.
My problem (challenge) is this- I have been getting some castings
that come out of the quench with black stains (looks like soot) which
when I scrub clean are copper colored underneath. The copper will not
come out with pickle (I use Rio Nickle Pickle, sodium bisulfate and
potassium dichromate) and sometimes becomes darker (more copper
colored) and if left in can even spread. A wire brush will somewhat
hide it but if I clean the piece in a heated ultrasonic it will
reappear as a dark stain. A rotary tumbler with stainless shot will
not remove it either (it will become less obvious but is still
there). I have bead blasted the area as well with both 80 micron and
60 micron media. Again this will lessen the effect but not remove
it.
I have not figured out a pattern to when it appears although it a
happens more frequently when pieces are closer together in the flask.
Can anyone help?
I never meant for my first post to go on and on but I figured the
more details I spelled out now the less time would be spent going
back and forth to get those details.
Make sure you still stir the melt right before the pour. This will
mix the metal. Never assume it is. Remelts or even new melts can have
some separation with solidification. This is even more important if
you remelt sprus. This is done with a small carbon rod set into a
file handle. This is also a guide to weather the metal is ready to
pour. If the metal sticks to the carbon stirring rod it is not hot
enough. When pouring hold the carbon stirring rod on the back of the
melting crucible to keep it in place while pouring the cast. You
usually have to overheat a bit for good vacuuming casting with the
white and yellow bronze.
We would love for you to call at the Rio Grande technical support
team at 1.800.545.6566. Our team would be happy to help try and
solve some of the casting issues your seeing.
My gut feeling, without seeing any pictures, is that you may be
getting some copper rich areas in the thicker sections of the piece
as it is cooling/solidifying. The copper might segregate from the
rest of the alloy and solidify last in the heavier/hotter sections.
This came to mind when you said that you notice it more on tighter
packed trees, which tend to cause more hot spots as pieces are
closer to each other. Seeing the sprue patterns and relating them to
the dark areas will help as well.
You can ask for anyone on the technical support team and we will
find the right resources for you.
Phillip Scott
Rio Grande
Technical Support
1.800.545.6566
I used to see this on my large silicon bronze trees. This is very
likely what is called metal mold reaction. The gypsum bonded
investment starts to break down at around 1350-1400F This breakdown
includes the production of sulfur dioxide gas which is absorbed by
the metal and leaves the metal black and rough surfaced. Once you
clean it you will see the copper surface you are referring to. This
is best removed by vibratory tumbling although it can be removed by
hand finishing. The reason you see it more in heavily populated
trees is the local temperature of the investment is kept higher by
the larger amount of molten metal in an area.
The way to reduce or eliminate it is to cast into a colder flask so
that the metal cools down faster so there is less time for the
reaction to take place. There is no easy formula to decide on how
much cooler because it will depend on the geometry of your castings
as to what is too cold. But you can experiment and get an idea of
what you can get away with. You may be able to lower it by a couple
hundred degrees again depending on geometry. You can also reduce it
by making less dense trees along with cooler temperatures. Another
suggestion is go to the Ultravest Max investment which is designed
for higher temperatures.