Quartz Inversion — 573°C
Silica crystals change structure (alpha to beta quartz), causing ~1% volume change. Cool slowly through 600-550°C to avoid dunting cracks. This is the most critical cooling zone.
Cristobalite Inversion — 220-275°C
Second silica transition with ~3% volume change. Mainly forms above 1100°C in stoneware/porcelain — minimal risk in bisque firings at cone 06. Safe to start venting here.
A kiln retains heat by design — thick firebrick insulation means natural cooling from 1000°C to 150°C takes 17+ hours. Cracking the door at 250°C safely speeds up the final phase.