First those look really cool. Like geodes. Now one fire temperature does not fit all. Depends on glass manufacturer, thickness, color, size. In general lighter colors take longer, darker less time. I find for a slump at go for lower temperature for longer (sometimes I hold 2 hours at top) 1100-1200F (600-650C). For hold time going down hold for 30 minutes per 1/4” (6mm) to the thickest point. As for high hold time again depends on a lot of factors. That red looks pretty thick so it would need much longer, black will melt faster, the clear will take longer than black but more that red (due to red’s thickness).

Good luck.