Trouble firing ceramic resin part

When I fire my part printed with ceramic resin it comes out black, cracked, and brittle. It does not shrink the anticipated amount. The firing schedule I am using is below and the furnace is very precise. The part was fired and ventilated in an atmosphere of air.

Ramp 1 - Heat part from room temperature to 240℃ at 1℃/min. (3.67hr)
Burnout Phase - Heat part at 240℃ for 6hr then ramp up to 300℃ at the rate of 1℃/min and hold at 300℃ for 1hr. (8hr) (Wall thickness of part is 6mm)
Ramp 2 - Heat part to 1271℃ at 3℃/min. (5.4hr)
Sintering - Hold at 1271℃ for 5 min. (.083hr)
Cool Down - Cool down to 900℃ at 6℃/min. Cool down from 900℃ to room temperature at 1℃/min. (15.6hr)

Is this a ventilation, sintering, burnout or other type of problem?

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