In a large bowl, sift 5 cups of the bread flour with the cornmeal, semolina and salt. Add the raisins and butter, then add the yeast mixture and blend well. Transfer the dough to a lightly floured surface and knead, adding up to 1 more cup of the bread flour to make a moist but not sticky dough. Continue kneading for a few minutes until smooth and elastic. Transfer the dough to a lightly buttered bowl, cover with plastic wrap and let rise until doubled in bulk, about 1 1/4 hours.