In a large bowl whisk all ingredients together until well combined; set aside in a warm place for about 10 minutes.
To Make Dough
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.
Combine
Knead dough for about 5 minutes then return to bowl: cover and let rise for another hour.
Preheat oven to 450°F.
Shape dough into a loaf shape. Place on an oiled baking sheet. Cut 3 or 4 slits in top of loaf from side to side. Sprinkle with caraway seeds if desired.
Bake for 10 minutes at 450°F then lower temperature to 400°F bake for about 30 to 35 minutes or until golden brown and sounds hollow when tapped on base.