Published January 1, 2008. From Cook's Illustrated.
How can a bread baked in a Dutch oven develop a shatteringly crisp crust?
A major breakthrough in the no-knead bread recipe published in the New York Times was to bake the bread in a preheated Dutch oven, which creates the dramatic open-crumbed structure and the shatteringly crisp crust that was previously attainable only in a professional bakery. How does this work?
First, as the loaf heats it gives off steam to create a very humid environment inside the Dutch oven. Since moist air transfers heat much more efficiently than dry air, the loaf heats much more rapidly. This in turn causes the air bubbles inside to expand much faster, leading to a more open crumb structure. As a test, I baked two loaves of bread, one in a Dutch oven and the other on a preheated baking stone. After one minute in the oven, the surface temperature of the Dutch oven-baked loaf had risen past 200 degrees, while the other loaf had reached only 135 degrees.
Steam contributes to a great loaf a second way. As steam condenses onto the surface of the baking bread, it causes the starches to form a thin sheath that eventually dries out, giving the finished loaf a shiny crust that stays crisp.
Many recipes suggest adding water or ice cubes to the oven; the problem is home ovens cannot retain moisture in the way a professional steam-injected oven can. With its thick walls, small internal volume, and heavy lid, a Dutch oven is the ideal environment to create and trap steam.