In a heavy kettle cook the leek, the onion, the celery, the carrots, the thyme, the bay leaf, and salt and pepper to taste in the butter over moderate heat, stirring until the vegetables are softened. Add the potatoes, peeled and cut into 1-inch pieces, the saffron, the wine, the broth, and the water and simmer the mixtures, covered, for 20 minutes, or until the vegetables are tender. Discard the bay leaf and in a blender puree the soup in batches, transferring the soup as it is pureed to a large bowl. Stir in the half and half and salt and pepper to taste. Let the soup cool, and chill it for at least 8 hours

or overnight. Makes about 11 cups, serving 8 to 10.