The first minutes in the oven are the most dramatic part of the bread's life. The loaf experiences oven spring, a rapid final rise driven by the expansion of trapped gases as the dough heats. To allow this rise to happen fully, the crust must stay soft long enough to stretch, and that is the role of steam. Steam in the early phase of baking delays crust formation so that the loaf can expand to its maximum volume, and a heavy lidded pot called a Dutch oven is the most popular home way to create this steamy enclosed environment without specialty equipment. Trapping steam around the loaf is what produces a crackling, sometimes blistered crust with strong oven spring.
As baking continues, two important browning reactions take over. The Maillard reaction occurs between amino acids and reducing sugars at temperatures above about 140 degrees Celsius, producing deep browning and the complex savory flavors characteristic of artisan crust. Caramelization, the browning of sugars, happens at higher temperatures and contributes additional color and flavor. Together, these reactions build the thick, flavorful crust that defines sourdough. Typical sourdough is baked at 450 to 500 degrees Fahrenheit (230 to 260 degrees Celsius) for thirty to fifty minutes, depending on loaf size and oven, and a finished loaf generally weighs 750 to 1000 grams for a standard boule or batard.
Doneness is checked in several ways. The most reliable is internal temperature: a finished loaf reads 200 to 210 degrees Fahrenheit, with 205 being a common target. Many bakers also tap the bottom of the loaf; a properly baked loaf sounds hollow. Once out of the oven, the loaf must cool on a rack for at least an hour before being cut, both because the crumb continues to set as moisture redistributes and because slicing too early produces a gummy interior. Cooling on a rack is essential so that steam can escape; otherwise the bottom becomes soggy. While a perfect ear is a celebrated mark of skill, ear perfectionism is only one sign of a good loaf and not the only measure of success.