Beat the molasses and buttermilk vigorously together with a wooden spoon in a large bowl. Stir in the raisins. Combine the dry ingredients and mix them into the buttermilk mixture one cup at a time, stirring well after each addition. Thoroughly wash and dry two empty 28 oz (794 g) tin cans and coat the insides with the butter, using a pastry brush. Divide the batter evenly between the cans. Cover each can loosely with a circle of buttered wax paper and then with a circle of aluminum foil. The foil and waxed paper should be puffed up, like a chef's hat. Tie the wax paper and foil in place with kitchen string. Place the cans in a large pot with enough boiling water to come about 3/4 of the way up the sides of the cans. Return the water to a boil, cover the pot tightly, and reduce the heat to low. Steam the bread for 2 hours and 15 minutes. Remove the wax paper and foil immediately and turn the bread out of the cans if you plan to serve it immediately. Or you can leave the bread in the cans with the foil and paper in place and steam it 10 to 15 minutes to reheat the loaves before serving at a later date. The cooked loaves, with the covers intact, can safely be kept in the refrigerator for a week to ten days.