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Investment coated sterling wire

Hi All, I have an interesting observation not a problem. Maybe
someone out there might have an answer to why it happens. I cast
hollow pieces in sterling. I wax weld a sterling silver wire so
that it runs from inside the object to the outside. The wire acts
as a re-bar between the core of the project and the outside so that
if the core breaks free form the outside investment it will not
float when the metal is poured and destroy the casting. Some times
the sterling wire comes out of the quenching process coated with a
harder amount of investment that is about 1/4 of an inch in
diameter. My question is What is happening? The investment has a
very slight copper color. Once the investment is removed from the
wire and it is placed in the next melt the wire seems to take longer
to melt. Is it possible that copper is migrating from the wire into
the investment during the burn out cycle? As I mentioned I do not
have a problem but just a large curiosity. Thanks for all the
interesting info all of you have input on to the site. Lee