I have been studying yeasts and bacteria in Microbiology class, and researched the history and microbiology of Sourdough Bread. It has a very interesting story, and the concept actually goes back to ancient Egypt, when flour and water were allowed to ferment before baking, as it made the wheat more tender and imparted a flavor to it.
This recipe I put together after reading literally dozens of recipes on the subject, most of which use packaged yeast in the starter, the bread, or both. But the science says it can be done without adding yeast, and this recipe bears that out.
Blend the warm water and flour in the clean crock; don't worry about a few lumps. Cover loosely and store at room temperature (70-75°). Some people keep theirs on top of the refrigerator.
Feed the starter every 24 hours like this:
Discard 1 cup of the mixture and stir in 1/2 cup flour and 1/2 cup warm water. Cover loosely and store at room temperature as described above. Feed the starter every 24 hours, this is very important!
In about 3-4 days, the starter will begin to bubble and ferment. It will smell like a brewery, or just like strong sourdough bread. This means that yeast from the air has begun feeding on the flour. This could take up to a week. Mine took exactly 7 days to reach this stage. Once it reaches this stage, it is ready to use.
If not baking bread immediately, place crock in the refrigerator and continue to feed as above once or twice a week. As long as you discard 1 cup and feed again at least once a week, it will continue to live and be active.
TO BAKE BREAD WITH THE STARTER:
Place 2 cups of the starter into a large mixing bowl (stand mixer with dough hook works best - do not try this in a bread machine). Feed the remaining starter with 1 cup warm water and 1 cup flour and replace in refrigerator.
Stir in the sugar, salt, and oil or softened butter. Mix well with paddle attachment of mixer or by hand. Incorporate in the flour, 1/2 cup at a time, switching to dough hook once it gets a bit thick. Or, incorporate flour in by hand. Knead as much of the remaining flour in as necessary to get a smooth, soft, elastic dough. The amount of flour depends on how wet the starter is to begin with. Do not add too much flour - less is better.
Turn your dough into a large greased mixing bowl, cover loosely, and set in a warm place (80°) to rise, about 1 1/2 to 2 hours. Dough may not completely double in size, but if you poke the dough, it won't spring back to fill in the hole once it's done.
Punch down the dough and let rest 5 minutes. Turn it onto a floured board or pastry cloth and knead a few times, shaping it into a long loaf. Place it on a lightly greased cookie sheet that has been sprinkled with cornmeal. Cover it with a thin towel, and allow to rise again in a warm place, until it doubles in size. This may take 1 to 1 1/2 hours.
Place bread in oven and then turn oven to 350°. You do not have to preheat the oven to bake sourdough bread; it benefits from the time it takes to reach the temperature. Set timer for 35-40 minutes.
Bread is done when it sounds hollow when tapped. Larger loaves may take longer to cook. Remove bread to a rack to cool, and wait to slice until fairly cool.
I use distilled water for my starter and for feeding it, because it is pure water and has nothing in it that might affect the action of the yeast and bacteria.
You don't have to discard the starter you remove each time you feed it; instead, use it as a base for pancakes, muffins, biscuits, or other breads or even cakes.