Mix the sourdough mixture with the rye flour and approx. 100 ml of water and cover with a lid and leave to rest at room temperature overnight (at least 8-10 hours).
Remove 2 tablespoons of the sourdough, fill into a screw-top jar and seal. Keep in the refrigerator until next time. Mix the remaining sourdough, flour, salt and water in a bowl. Then cover the dough with a lid and let rise at room temperature for 6 - 8 hours.
Now divide the dough into two parts on a well-floured work surface and roll the loaves.
Place the body with the last milled side down on a floured tea towel and fold in the corners of the tea towel. Let rise again for 1.5-2 hours.
Preheat the oven to 250 degrees Celsius (top / bottom heat). Turn the loaves out onto a parchment-lined baking sheet. Reduce the temperature to 220 degrees Celsius and bake the bread on the middle rack for about 40 minutes.
At the beginning of the baking process, I recommend putting a small bowl of water in the oven. I always throw a handful of ice cubes in the oven. That works fine too.