Max temp for kiln casting into silica/plaster molds

[glaskrafter]

I'm kiln casting glass salmon having success and am struggling with bubbles and surface scum and (what I call) alligator skin on the tail, fin and jaw ends. They are about 2inches thick, 24 inches long. Smaller castings ( half size and thickness) come out almost pristine. I am ramping up to 1515 f from 1250f hold at 600 and had been holding for 3 hours, now shortened to 2 without any visible improvement. And now the glass is almost now filling out to the extremities. Am I in danger of fusing the glass to the mold if I was to run it up to 1600f ? I was moving it up over time from 1480 to now 1515 and I wonder if I were to heat it up that would clear the bubbles. I had a thought that the alligator skin may be dealt with if used a sucker suck on the way down. Any ideas, suggestions.
Thanks