Just a ranging shot to fully understand the question.
When you say shiny sparkles, and talk about crystalline glazes, I immediately think of aventurines.
Which I believe to be quite tricky, and would expect redox issues would make copper aventurines
This picture of a copper aventurine glass clearly shows the attractions
... and the associated wiki page indicates some of the difficulties in it's production (and a glass in its tank --
unlike a glaze in a kiln -- is more or less isolated from the environmental redox).
The wiki entry suggests to me that the matrix of an aventurine glaze will always be dark, as it will still contain
some precursor oxide of the crystals (and they always seem to involve the transition elements, which have
highly coloured ions).
Your request for "just the copper sparklies" suggests that you are after a clear copper aventurine. If so, you
may be out of luck.
BTW I'm not familiar with Crystalline Ultramarine. Do you have a reference, which might give a clue as to the
mechanism behind the sparkles.