Mix first 2 ingredients and let sit overnight covered with plastic wrap.  The next morning, proof the yeast and mix into the dough from the previous night.  Add salt, syrup, vinegar and rye flour and knead.  Cover and let rise 2 hours.  Add 5 dl wheat flour and knead; add rest of the flour and knead.  Shape into 2 loaves and let rise 50-60 minutes.  Prick the surface of the loaves and brush with a mixture of butter and water.  Bake 1.5 hours at 350-375°F.  Let cool on a rack covered with a cloth.