Sift the wheat flour and rye flour with the salt into a suitable bowl. Make a well in the middle of the flour and crumble the yeast into it. Add the sourdough mixture with the lukewarm water and knead into a dough.
Place the dough on the floured surface and continue to knead for 7 to 10 minutes until very smooth. Then return to the baking bowl form, cover tightly with plastic wrap and let rise at room temperature for 1, 5 to 2 hours until it has about doubled in volume.
Place the dough repeatedly on the floured dough surface and knead extensively. Now divide into two on the spot pieces and shape into two balls. Place the two balls on a floured baking board and cover with a floured dishcloth.
Let rise again at room temperature for 1, 5 to 2 hours, until repeatedly doubled in volume.
Place the terracotta bricks or a baking stone on the bottom of the oven and preheat the oven to 250 °C (gas 5, convection oven 230 °C). Place the dough balls on the bricks. Pour 100 ml of water on the bottom of the oven (not on the bricks!!). Immediately close the oven door and turn the heat down to 200 °C (gas 3, convection oven 180 °C).
Bake the breads at this heat for 15 minutes, pouring a little water on the bottom of the oven in between so that the breads get a crispy crust. Bake for another 50 minutes at 170 °C (gas 1-2, convection oven 150 °C). The breads are cooked when the bottom side is