Dissolve yeast in hot water, pour into large mixing bowl and mix with starter. Add 4 cups flour, sugar and salt and stir vigorously for 3 or 4 minutes. Cover with a towel and let rise in a warm place until double in bulk (1 1/2 to 2 hours). Mix soda with 1 cup of the remaining flour and stir in; the dough will be stiff. Turn dough out onto a floured board and begin kneading; add the remaining 1 cup flour or more if necessary to control the stickiness. Knead until satiny -- between 5 and 10 minutes.
Shape into oblong loaves or one large round loaf (my preference). Place on a lightly greased cookie sheet, cover, and place in a warm place; let rise to nearly double in size -- time will depend on warmth of spot -- 1 1/2 to 2 hours in most kitchens.
Before baking, brush outside with water, make diagonal slashes across the top with a sharp single edge razor blade. Put a shallow pan of hot water in the bottom of the oven. Bake in a hot oven -- 400 degrees -- until the crust is a medium dark brown (about 45 minutes for oblong loaves and about 55 minutes for the large round loaf).
For a more sour bread, leave out the yeast and 1/4 c of water but include the rest of the warm water, all the starter, 4 cups of flour, sugar and salt. Set this sponge after stirring vigorously in the afternoon and allow to set over night in a covered bowl. In the morning dissolve the yeast in 1/4 c of hot water and stir thoroughly into the sponge. Then add your additional flour and soda and follow the remainder of the instructions from there.