In a large bowl, dissolve yeast in warm water. Blend in starter batter,salt and sugar.
Add 4 cups flour. Beat 3 or 4 minutes, Cover, let rise until double (1 1/2 hours) in a warm place free of drafts. (a good way to do that is to heat oven for about 2 minutes, turn it off and place bowl of dough in oven,
Leave it alone the whole time, when it has risen, Mix baking soda with 1 1/2 cups flour, stir into dough. Add enough flour to make a stiff dough,
Turn dough out onto floured boad or clean counter top. Knead 10 minutes. Divide dough in half shape into loaves of round loaves and place on lightly greased baking sheet.
With sharp knife make diagonal gashes across the top and again let rise until double.
Bake at 400 degrees for 40 minutes. brush with butter if you wish.