In large bowl, sprinkle 1 pkg. active dry yeast over 1 1/2 cups warm water. Stir to dissolve. Add 1 cup starter and mix well. Add 4 cups flour, 2 tsps. sugar, 2 tsps. salt and stir vigorously 3 or 4 minutes.
Cover and let rise until double in size.
Mix 1/2 tsp. soda with 1 cup flour and stir into dough. Knead in the remaining 1 cup flour until dough is satiny - 5 to 10 minutes. Shape into loaves.
Place on greased baking sheets which are lightly coverred with cornmeal. Let rise until doubled.
Brush with cold water and make slashes across the top. Put a pan of boiling water on lower rack of oven. Bake bread at 400 degrees for 15 minutes. Brush with water.
Bake at 350 degrees for 35 to 40 minutes. Brush with water and return to oven for another 2 or 3 minutes.