1. In the bowl of an electric mixer, combine 1/4 cup water and yeast. Let stand until yeast is foamy, about 10 minutes. Add 1/2 cup more water and sponge; mix on low speed using paddle, about 2 minutes. Add flours, salt, and remaining water; mix 1 minute more. Using dough hook, mix on medium-low speed, pulling dough from hook 2 or 3 times, until dough is soft and sticks to fingers when squeezed, about 8 minutes. Or knead by hand, 15 to 20 minutes.

2. On a floured surface, knead dough into a ball by hand, 4 or 5 turns. Place smooth side up in a lightly oiled bowl. Cover with plastic wrap; let rise in a warm place until tripled in bulk, about 3 hours.

3. Turn dough onto floured surface. Flour top, and cut in half. Roll up one piece of dough lengthwise, flatten slightly, and roll lengthwise again. On an unfloured surface, cup your hands around the sides of the dough and move it in small, circular motions, until top of dough is rounded and bottom is smooth. Flour a piece of parchment, and place dough on it; cover loosely with oiled plastic wrap, then cover with a towel. Repeat process with second piece of dough. Let rise in a warm place until doubled in bulk, about 50 minutes. 30 minutes before this final rise is completed, place a baking stone or baking sheet in the oven and an empty baking pan on the lowest shelf; heat oven to 450 degrees.

4. Sprinkle dough with flour, and dimple the surface with your fingertips; let rise 10 minutes more. Using a baker's peel or baking sheet, slide loaves and parchment onto the baking stone or baking sheet. Pour 2 cups very hot water into the heated baking pan to create steam. Bake until dark golden brown and hollow sounding when tapped on the bottom, 25 to 30 minutes. Transfer to a wire rack to cool for at least 30 minutes before slicing.