Dough Proof: Pour the culture into a mixing bowl. Stir the water and salt into the culture with a mixing spoon. Add the flour a cup at a time until the dough is too stiff to mix by hand. Turn out onto a floured board and knead in the remaining flour until the dough is smooth and satiny.
Proof the dough overnight (8 to 12 hours) at room temperature, about 70 degrees, in a large bowl covered with plastic wrap. During this time, the dough should double in size. After the proof, use a spatula to gently ease the dough out onto a floured board.
Allow the dough to rest for 30 minutes. If marked flattening occurs during this time, knead in additional flour before shaping.
After the 30-minute rest, shape the dough. Flatten it slightly, then lift a portion from the periphery and pull it toward the center. Continue this around the dough mass to form a rough ball, then pat and pull into the shape you desire.
Loaf Proof: Place the shaped loaf in a bread pan or, for French loaves, on a baking sheet. Cover and proof at room temperature for 2 to 4 hours until it has doubled in bulk or reached nearly the top of the bread pan. Remember that proofing at higher temperatures (90 degrees) will produce a more sour loaf with good flavor but decreased leavening. Proofing at room temperature will yield good leavening and mild sourness. Proofing at temperature for the first hour, and then at 90 degrees until risen will yield a moderately sour loaf with only slightly decreased leavening.
Baking: Just before putting the loaf in the oven, slash the surface of the dough several times with a razor blade. Place the pan with its shaped, proofed loaf in a cool oven, then turn the temperature to 375 degrees and bake for 70 minutes. For a firm, chewy crust, place a pan of boiling water below the loaf. When the loaf is baked, remove it from the pan and let cool on a wire rack for at least 15 to 20 minutes before slicing.