Sponge:
1 envelope, 1/4 ounce active dry yeast
1 1/2 cups warm water, 105 to 110F
2 cups unbleached all purpose flour

Dough:
3/4 cup coarsely ground whole wheat flour
1 cup warm water, 105 to 110F
1 tablespoon coarse salt
about 6 1/2 cups all purpose flour
1 egg white, lightly beaten
sesame seeds, poppy seeds, cracked wheat or flour, for topping, optional

Make the sponge about 12 hours ahead: In a large bowl, dissolve the yeast in 1/4 cup of the warm water. Let stand for 5 minutes. Stir in the remaining 1 1/4 cups water and the flour, beating by hand or in a mixer with a dough hook for 1 minute. Cover the bowl with plastic wrap and let the sponge stand at room temperature for 12 to 14 hours.

Make the dough: Stir the whole wheat flour, warm water, salt and about 4 cups of the all purpose flour into the sponge until the dough becomes hard to work, then turn it out onto a well floured surface. Let the dough rest while you clean the bowl. Now start kneading, slapping the dough around, scraping it up, gradually adding about 1 1/2 cups more all purpose flour and kneading until smooth and elastic, 8 to 10 minutes. Return the dough to the bowl, cover with plastic wrap and let rise at room temperature until almost tripled in volume, about 2 hours.

Turn the dough out again onto a floured surface, punch it down and tear off a small hunk about 6 ounces to reserve for the levain; see note. Divide the remaining dough in half and shape each piece into a 1 1/2 inch thick disk. With the palms of your hands, coax the sides of each disk down and under all around, stretching the gluten cloak and plumping up each loaf into a round shape. Pinch together the seams on the bottom and place the rounds seam side down on 2 lightly greased baking sheets. Cover loosely with a towel and let rise until doubled in volume, about 1 hour.

Preheat the oven to 450F. Paint the top and sides of the risen loaves with the beaten egg white to glaze. Sprinkle, if you wish, with one of the toppings and place on the middle rack of the oven. Immediately throwa few ice cubes on the oven floor to create steam. Add more ice after 3 minutes and again after 6 minutes. Bake for a total of 15 minutes. Then reduce the oven temperature to 350F and bake for 20 minutes longer, or until the bread sounds hollow when the bottom is tapped. Turn the oven off and let the bread rest in the hot oven for 15 minutes. Remove to a rack and let cool.

Note: Put the hunk of reserved dough, about 6 ounces in a covered jar with 1 1/2 cups of warm water. This is the levain, which will go into the next loaf. Let stand at room temperature for 12 hours, then refrigerate. The levain will keep for up to 10 days; freeze for longer storage.