I cast a lot of sterling charms last night. On nearly every newly
cast tree of charms as they came out of the investment after
quenching, the small base of the tree and a few mm. up the tree
sprue, had a surface coating of mottled green. It looked very much
like verdigris on copper. It came off in the pickle, but underneath
the green was a very heavy layer of something black that won’t come
off. Fortunately the black is only on the sprue bases, so the charms
weren’t damaged.
This has never happened before. The only thing that has changed is
that my electromelt was recently repaired–it got a new themocouple
and heating element. This is the first time it’s been used since the
repair. What could have caused the green and black layers on the
castings? What is the black stuff, and how can I get it off the sprue
bases so I can re-use them when I cast again?
Sounds like you had some dirt in your metal. The black stuff is
probably dirt encased in melted borax flux. If it won’t come off on
the pickle, take your buttons, heat them up and quench them in the
pickle. That usually gets it off. Basically I think what you have is
the borax doing its job… When you go to remelt these make sure you
give them a good stir with in the crucible so that whatever is left
over will stick to the sides encased in the borax. By the way… this
is what buttons are for. All the junk floats up into them so it’s not
in your castings.
Sounds like you had some dirt in your metal. The black stuff is
probably dirt encased in melted borax flux. If it won't come off
on the pickle, take your buttons, heat them up and quench them in
the pickle.
I don’t use borax in the electromelt. Could the black stuff possibly
be carbon from unburned wax? After cleaning all these castings, I had
a lot of incomplete castings from waxes that were carefully
inspected. I’m suspecting the flasks were not fully burned out, even
though they were bright white on the outsides. I’m still curious
about
the mottled green markings on the metal buttons though. What could
that be?