Oven spring is the rapid expansion in the first ten minutes of baking, driven by gases already in the dough expanding with heat, plus a final burst of yeast activity before it dies. A well-timed, well-steamed loaf can grow 20 percent taller in the pot.
The prerequisites are a fully fermented but not overproofed loaf, adequate surface moisture so the crust can stretch, and strong bottom heat so the base sets with structure rather than spreading. That is why a preheated Dutch oven or stone outperforms a cool oven.
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If your loaves burst at the score or stay flat, the usual cause is underproofing, not weak steam. If they collapse and feel hollow, the cause is overproofing. Spring is a diagnosis, not just a goal.