In a medium bowl, dissolve 2 1/4 teaspoons yeast in 1/2 cup warm water. Stir in 2 cups flour, 2 cups lukewarm water, 1 tablespoon sugar, and 1 teaspoon salt. Beat smooth with rotary beater. Let stand 3 to 5 days uncovered at room temperature stirring 2 or 3 times a day. Cover at night to prevent drying.
In a large bowl, dissolve 2 1/4 teaspoons yeast in 1/2 cup warm water. Blend in starter batter, 2 teaspoons sugar, and 1 1/2 teaspoons salt. Add 3 1/2 cups flour, and beat with a rotary beater for 3 to 4 minutes. Cover, and let rise till double in bulk.
Mix soda with remaining flour; gradually mix into risen dough, adding enough to make the dough stiff. Turn out onto a floured board, and knead 8 to 10 minutes. Shape into one large or 2 medium loaves, and place on a lightly greased baking sheet. With a sharp knife, make diagonal cuts across top of dough. Let rise till double.
Bake at 400 degrees F (205 degrees C). Remove medium loaves after 35 to 40 minutes, large loaf after 40 to 45 minutes. Brush with melted butter and let cool before serving.