Sift both flours into a large enough bowl. Put the sourdough in a flour well, fold in a little flour. Cover with a wet tea towel and let stand for one night.
Gradually stir in salt, caraway seeds, buttermilk or water with a wooden spoon. Knead with your hands. Make the dough a little moister to rise. Cover with a wet tea towel and let rise in a warm place for 6 hours. Dampen the tea towel again if necessary.
Turn the dough out onto a work surface sprinkled with whole wheat flour, take the indicated mass of dough from it, knead the rest for 5 minutes, form into a ball, dividing each loaf into equal parts.
Knead each and place in a loaf pan.
Cover with a wet tea towel and let rise for 2 hours.
Place a gratin dish with boiling hot water on the bottom of the oven. Preheat the stove.
Brush the loaves with oil, slide in and bake for 60 minutes.
Leave to cool on a rack.