Dissolve package dry yeast in 1 1/2 cups lukewarm water, mix with 1 cup sour dough starter in large bowl. Add 4 cups flour, 2 tablespoons sugar, 2 teaspoons salt. Cover and let rise in warm place until double in size. Then mix 1 cup flour and 1/2 teaspoon baking soda and stir this into the dough, adding enough flour to make a stiff dough. Knead until smooth and shiny. Shape into 2 loaves and place on greased paper or sheet sprinkled with cornmeal. Let rise until double in size. Brush lightly with cold water, make 1/4 inch slice along top of loaf, bake in 400 degree oven with a pan of water under. Bake until almond brown, about 25 minutes. Brush with melted butter and hot water and put in oven 15 minutes longer.