Three mechanisms drive oven spring in the first minutes of baking
Oven spring is the rapid rise that occurs in the first 10-15 minutes of baking, driven by three simultaneous processes. Gas already in the dough (from fermentation) expands as temperature rises. Remaining yeast cells continue producing CO2 until killed around 60C. Water on the dough surface and inside the crumb begins to evaporate, converting liquid to steam and adding volume.
The rate of rise accelerates steeply: dough rises slowly at 50C, then rapidly between 60-75C as all three mechanisms peak simultaneously. After 60C, yeast dies (ending gas production), and the dough's proteins set (coagulation), limiting further expansion. Steam production continues, but the dough's structure is now rigid, so the rise flattens.
Temperature management shapes final loaf volume
Starting at high temperature (475F, 246C) maximizes early expansion and spring. Lower starting temperatures (400F, 204C) produce less spring and denser crumb. Steam injection during the first few minutes traps moisture on the crust, slowing evaporation and allowing more time for internal expansion before a crust forms. Once crust sets (around 15-20 minutes), oven spring ends and browning reactions dominate the remaining bake.