Dark castings

Hello all - I need some help figuring out what is going on with some
gold castings that one of our metalcrafter group members has been
doing. I will try to give you as much detail as I have and hope
that someone, with greater knowledge than I, will know what is
happening. I’m clueless.

First of all, Ron is using metal from a variety of sources: gold
teeth, 10K service awards, old rings, etc.(all yellow). The gold
was his father’s (now deceased) and he wanted to re-mold it into
crosses for his mother, brothers and sisters.

First wrinkle: I know this mixing should not be done so I
recommended melting everything together and pouring the liquid gold
into water to form shot. The shot was very clean and had good
yellow color.

Second wrinkle: we are doing steam casting. Ron is using purchased
waxes that he sprued properly for steam casting (He used 4, 12ga
wires leading from a depression in the investment to
expanion/cooling reservoir and then through a larger sprue to the
model itself). Because the crosses are small, he sprued 2 of them
to the expansion reservoir.

He used Westcast, caster’s choice investment and burned out for 3
hours (1 at setting 3, 1 at 6, 1 at high).

In steam casting, you melt the gold in a depression in the top of
the flask, then cover it with a tin can lined with wet newspaper.
Because the steam cannot go up, it “pushes” the gold down into the
flask. He used half old gold (the stuff we turned into shot) and
half new 14K yellow gold from Rio. He added borax, heated to liquid
and cast. The castings formed OK.

However two of the 4 crosses are noticeably darker than the others.
When the sprue was cut there was no color difference between the
outside and the inside. Also, bright dipping in 50% nitric acid
did not change the color.

My experience with casting is limited, so I have no idea what
caused this difference. Any ideas?

Thanks again Hanuman and listmembers for this wonderful forum.
Where else could I go with questions like this?

Debby Hoffmaster

Dear Debby, You will eliminate most of your problems by using Magic
Cast de-oxidiser with your scrap yellow gold…it really is
essential for doing the kind of thing you are doing. Ron at Mills
Gem, Los Osos, CA