In large nonmetal bowl, combine all ingredients for sponge.
Cover with a double layer of cheesecloth secure with an elastic band. Let stand and ferment in a warm place, free from drafts, 24 to 48 hours, depending on sourness desired. Stir once or twice a day with a wooden or plastic spoon.
When sponge is fermented and bubbly, add salt and enough flour to make a soft dough. Turnout dough onto a lightly floured surface. Clean and grease bowl, set aside.
Knead dough 10 to 12 minutes or until smooth and elastic.
Place dough in greased bowl, turning to coat all sides. Cover with a slightly damp towel. Letrise in a warm place, free from drafts, until doubled in bulk, 1 1/2 to 2 hours. Grease 2 large baking sheets or 2 (2-quart) casserole dishes. If desired, lightly sprinkle cornmeal over baking sheets or bottoms of dishes.
Punch down dough; knead 30 seconds. Divide dough in half. Shape into narrow oblong loaves with tapered ends for baking sheets or round loaves for casserole dishes. Place on prepared baking sheets or in prepared dishes. Cover with a dry towel. Let rise until doubled in bulk, about 1 1/2 hours.
Adjust oven racks to 2 lowest positions. Place a shallow roasting pan on lowest shelf; pour in 2 cups boiling water.
Preheat oven to425 F (220C). Slash tops of oblong loaves diagonally and round loaves in 3 slashes horizontally and 3 slashes vertically, in tic-tac-toe designs. Brush slashes loaves with egg-white glaze. Bake 15 minutes; brush again with egg-white glaze. Remove roasting pan from oven. Bake loaves 15 to 20 minutes longer or until bread sounds hollow when tapped on bottom. Brush loaves a third time with egg-white glaze, 5 minutes before removing from oven. Remove from dishes or baking sheets.
Cool on racks.