Meanwhile, in another large bowl combine rye and all-purpose flours, caraway seeds, yeast and salt.
Sprinkle this flour mixture over sponge mixture and cover bowl tightly with plastic wrap and set aside in a warm place for about 3 to 4 hours to ferment.
With a dough hook attachment beat the dough for about 2 minutes at low speed then 10 minutes at medium speed.
Dough should look smooth and elastic. (If kneading by hand turn dough out on to floured board and knead for about 10 minutes until smooth and elastic)
Cover dough and let rest for about 20 minutes then place into a clean, oiled bowl.
Rub top of dough with a little oil; cover bowl with plastic wrap and let rise for about 1 to 1 ½ hours.