How disappointing to have a piece break after so much cutting, firing and coldwork! One thing we need to understand is if the piece broke on the way up to process temperature, or after.
If you were slumping, and the pieces don't fit together, then your piece broke on the way up. Is this the case? This would be a thermal shock.
How was your mold set up in the kiln? On stilts, or on the kiln shelf?
Do you have a subscription to Bullseye's videos? The video "Why Did It Break?" might help you determine the cause. https://videos.bullseyeglass.com/videos ... -it-break/
This video actually shows a piece breaking during a slump firing, and explains annealing shock and thermal shock.
We look forward to learning more about your piece, and helping you with this!