Bread is usually started at a high temperature, then the oven temperature is lowered halfway through baking. High heat is needed for oven spring in the first few minutes: gas and steam inside the dough expand rapidly, making the loaf rise all at once. If the temperature is not lowered, the crust thickens too early and the outside darkens before the inside is baked. Lower the heat when the crust begins to take on color and set.
The bread rises the most during the first few minutes after it goes into the oven. The gas inside the dough heats and expands, the water turns to steam, and together they push the loaf upward. The oven must be very hot for this spring to happen; in a warm oven, the dough heats slowly, gas escapes at the edges, and the bread stays flat. That is why the oven is preheated to a temperature higher than the intended baking temperature. Start preheating well before the dough goes in, and continue waiting for a while even after the oven indicator reaches the set temperature.
If high heat is maintained for too long, the crust quickly thickens and hardens. Once the crust has set, the loaf stops rising and heat has trouble reaching the inside; the result is a dark exterior and a doughy interior. For this reason, lower the temperature after oven spring is complete and bake the rest of the time at more moderate heat. This lets the inside bake without drying out and the crust bake without burning. An early-setting crust also prevents the scores from opening, so the bread may split along the sides and look misshapen.
The moment to lower the heat depends more on what you see than on the clock. When the bread has risen noticeably, the scores have opened, and the crust is beginning to color, oven spring is over; this usually happens in the first 15–20 minutes. If you are baking in a cast-iron pot, this is usually when you remove the lid. Do not leave the oven door open for long while lowering the heat, or the temperature will drop too far and take time to recover. The reduction is usually 20 to 30°C / 36–54°F; a larger drop extends baking unnecessarily.
High heat alone is not enough; moisture is also needed during the first few minutes. A moist surface takes longer to harden and gives the bread time to rise. That is why high heat and steam are used together. When the steam stops, the crust begins to form, and that is when the temperature is lowered. Consider these two adjustments together; if you use one and skip the other, the result is incomplete and the benefit is lost. The simplest way to add steam is to pour hot water into a pan on the bottom rack of the oven or spray water over the dough.
This method works best with lean bread doughs. High heat is risky for doughs enriched with milk, fat, or sugar because their surfaces brown very quickly; bake them from the start at a lower, steady temperature. Oven spring is also limited in pan-baked loaves. Doneness is still confirmed by internal temperature: look for 93–99°C / 200–210°F in lean bread and 88–90°C / 190–195°F in milk breads and other soft doughs. Lower the temperature for pan loaves too, because the sides of the pan reflect heat and the side crust browns early.