Place the dark and golden raisins in a medium bowl. Dissolve baking soda in boiling water, and pour over raisins. Let stand overnight without stirring.
Preheat the oven to 350 degrees F (175 degrees C). Grease three 1 pound sized coffee cans.
In a large bowl, stir together the flour, sugar and salt. Stir in the raisins with their water, and the oil until well blended. Batter will be thick. Divide the batter evenly between the 3 cans.
Bake for 1 hour in the preheated oven, or until a toothpick inserted into the crown comes out clean. Lay cans on their side to loosen the bread as it cools.