Thanks to a suggestion by Richard in Denver, I took the gold I had
unsuccessfully tried to cast in to an experienced jeweler/appraiser
to be tested. It was not 18K, and in fact tested below 8K. This
explains a lot in terms of how it froze 1/2 way into the mold (mold
not hot enough) and why it smoked so much when I melted it. It was my
mistake, because I "knew" that gold under 10K could not be marked
with a karat stamping, so when I saw a nice clear 8 and some other
faint marks inside the rings, I thought it was 18K. I did not know
about gold being marked 8K in Europe. The customer, also, had
presented it to me saying it was 18K, so he saw the same thing I did.
Oops. After some thought, and 3 out of 4 advisors thinking it was
probably zinc burning off, I decided to try to recast, treating it
this time as brass. Instead of a 900 degree F. mold temp, I used 1250
degrees. It cast fairly well, but had a crack which separated the
piece in two. I soldered it with 14k solder, and it is presentable.
So I guess I got out of this one OK. I still have to tell the
customer, but I expect they'll be OK with it, since they are getting
their cross at the estimated price. Thanks to Richard, Dave, Alan,
Tony, and Peter for their well-informed answers! I am very happy to be
done with this, and appreciate the support!