Steam

Steam does several useful things during the first few minutes of baking. It greatly increases the rate of heat transfer from oven to dough. Without steam, the dough surface reaches 195°F/90°C in 4 minutes; with steam, in 1 minute. Steam thus causes a rapid expansion of the gas cells. As the steam condenses onto the dough surface, it forms a film of water that temporarily prevents the loaf surface from drying out into a crust, thus keeping it flexible and elastic so that it doesn’t hinder the initial rapid expansion of the loaf, the “oven spring.” The overall result is a larger, lighter loaf. In addition, the hot water film gelates starch at the loaf surface into a thin, transparent coating that later dries into an attractively glossy crust.