Add warm water to sourdough culture and stir until well mixed. Slowly add the flour stirring to blend until smooth and lump-free.
Cover bowl with plastic wrap and place in a warm area for proofing. (8-12 hours)
At the end of the proofing period stir sponge thoroughly. Take out one cup and replenish the sourdough starter stirring thoroughly.
Measure 1-1/2 cups of sponge into a large warm bowl. Stir in the water (90ºF), salt, yeast and 1 cup of the flour. Combine to make a thick heavy batter.
Mix in additional flour 1/2 cup at a time until the dough starts to come together. (approx. 2 more cups)
Turn dough out onto a lightly floured work surface and knead until it becomes smooth and elastic, about 5-10 minutes. Place in large greased bowl and allow to rise until double in bulk. (1-2 hours)
Form dough into desired shape or shapes. Place on baking sheet sprinkled generously with cornmeal, cover and set aside in warm area until the dough had doubled in bulk again. (approx. 1 hour)
Preheat oven to 425ºF. Using a sharp knife, cut 2 to 3 slashes across the top of the loaf and brush with water. About 5 minutes before baking, place a baking pan with water on the bottom shelf of the oven. This will give the loaf a thick, crispy crust.
Bake the bread about 20 minutes, or until golden brown and hollow sounding when tapped. Cool on a wire rack.