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
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.