How deep is the mold? It
How deep is the mold? It would help if I had a better idea of what the mold looks like.
What COE glass are you using? Opaque? Transparent? It makes a difference.
Bullseye will typically say it’s safer to add time rather than increase the slumping temp. However, it’s also better to not go above 1200 if holding for a long time. When I have a long hold for a stubborn slump, I set my top temp at 1190 (also recommended by Bullseye). I’ve held it there as long as an hour. If you keep the temp this low, you shouldn’t have any problems with a long hold.
I have a fiber kiln, and the only variation I need to make in standard firing schedules is the cool down phase after annealing, particularly if I am creating a piece that is thicker than the typical 6mm (1/4 inch). Letting the kiln cool as fast as it can is too fast in a fiber kiln in many cases. Also, if using Bullseye glass, the current recommended annealing temp is 900.
All the temps I mentioned are different if you are using COE 96.
I hope I didn’t add confusion to all this.