Susan,

Copper luster is not under the glaze so it can and does turn brown under
the right circumstances.  I have also seen tea leaf decorated wares in the
antique market that were brown rather than copper luster, which suggests
that the manufacturer may have done something wrong in his firing process
so that the luster did not develop as intended.

Peace,
George L. Miller
URS Corporation
561 Cedar Lane
Florence, New Jersey 08518