Sedveral years ago I purchased a teflon tape which prevents PMC
from sticking to a ceramic ring mold when firing. Rio Grande no
longer sells the tape, too bad, I cannot seem to find it.
I too love the ceramic ring forms. The tape you are thinking of is
ceramic tape, not Teflon tape.
You can buy the same thing and cut it into strips yourself. Use a
straight edge and a utility knife, about 3/4 inch wide works nicely.
It is just the thinnest available size of shelf paper used by glass
artists. You can buy it in 12 x12 or larger pieces from glasses
places such as:
The thicker version of the same stuff (the middle, not the thickest)
can be used on the ceramic ring forms to make the resulting ring
about a half size larger. When you tape it on, just don't overlap it,
tape it on end to end.
Metalsmith, Certified PMC Instructor
Hard to Find Tools for Metal Clay