In a small bowl, dissolve yeast in warm water. Let stand until creamy, about 10 minutes.
In a large bowl, combine 4 cups of flour and salt. Cut in the butter and stir in the yeast mixture. Beat in the remaining flour, 1/2 cup at a time. When the dough has pulled together, turn it out onto a lightly floured surface and knead until smooth and elastic, about 10 minutes. Butter a large bowl, place the dough in the bowl, and turn to coat. Cover with a damp cloth and let rise in a warm place until doubled in volume, about 1 hour.
Preheat oven to 400 degrees F (200 degrees C). Lightly grease a large baking sheet.
Deflate the dough and turn it out onto a lightly floured surface. Form the dough into a large oval loaf. Place onto the prepared pan. Cover with a damp cloth and let rise until doubled in volume, about 30 minutes. When the loaf is risen, cut a 1/2 inch deep cross onto the top of it. Brush the top with water before baking.
Bake in preheated oven for 25 minutes. Reduce heat to 350 degrees F (175 degrees C) and bake for an additional 15 minutes, or until bottom of loaf sounds hollow when tapped. Let cool on a wire rack before slicing.
