In a heavy-bottomed pot, combine 6 cups cold water with the beans, 1⁄4 cup of the oil, the onion half, the celery, and bay leaf and bring to a boil over high heat. Reduce the heat to medium-low and simmer, stirring occasionally, until beans are very tender, about 2-1⁄2 hours. When the beans are just about cooked, season with salt to taste. Discard the onion, bay leaf, and celery and set the soup base aside.