ZYP Coatings (formerly known as MR-97) is a high temperature, anti-stick release agent for glass forming that works on most common ceramic and metal molds.  It is inert and yields 97% boron nitride at 1800° F.  13 oz. spray can.

Directions

Remove old primer residue with a stiff, nylon kitchen or tooth brush.  Shake can well.  Hold can and mold vertically, 10 to 12 inches apart.  Depending on the size of the mold, apply a two-second or longer coat in a sweeping pattern over the entire mold cavity.  Do not saturate.  Wait five minutes.  Using the same process, apply a second coat.  Wait 15 minutes.  Do not scratch primer surface when filling.  After firing, wash residual from glass casting.  Brush out mold.  Only a single coat of MR-97 is needed before each subsequent firing.