Sprinkle yeast over the water in a small bowl. Set aside.
Combine rye flour with plain flour in a large bowl. Stir in the salt. Mix in the yeast mixture to make a soft dough. Turn dough out onto a board floured with dark rye flour. Knead lightly, mixing in dark rye flour. Shape dough into a fat roll and cut into 12 portions. Roll each portion into a ball; cover balls with a towel and let rise 20 minutes.
Preheat oven to 220 C / Gas 7. Lightly grease 2 baking trays.
On a floured board, roll out each ball into a flat circles about 10cm in diameter. Place rounds on prepared baking trays and prick with a fork.
Bake in preheated oven until lightly browned, 8 to 10 minutes. Cool on a rack.