You need to double the annealing time for an open-faced casting over the schedules for the same thickness, because the glass is cooling from one side only. The usual schedules are premised on cooling from both sides equally. The schedules given for 50mm thick open face castings should be used for a 25mm thick open face casting.
If you could cover your open-faced casting with something of equivalent insulation as the investment around the glass you could go back to a 1" schedule.
So an open-faced casting 25mm thick needs to be annealed using the schedule for 50mm thick castings as follows (for Bullseye glass - make adjustments for different glasses):
482°C for 8 hrs
4°C/hr to 427°C
7°C/hr to 370°C
23°C/hr to 21°C
See the Bullseye chart for annealing thick pieces.
Based on Don Burt’s work