In a large glass bowl stir flours and water together with a spoon. Cover with a kitchen towel . Set aside to ferment at room temperature for 2 to 3 days. It will begin to smell sour but not in an unpleasant way. Stir in 2/3 cup flour and 1/2 cup water, cover and continue to ferment at room temperature. Refresh starter in this way every day for 3 days. It will become sharper and more pungent as it sits. When it is sharp enough for you, begin using it to bake with. After using the amount of starter you need, the remaining starter should be refrigerated. Let it come to room temperature each time before use.