In a medium bowl, sift the flour, salt, sugar, and baking soda together. In another bowl, add ¼ cup (60 ml) water, the pumpkin puree, oil, eggs, nutmeg, cinnamon, and allspice. Stir well to combine. Whisk the dry ingredients into the pumpkin mixture, then stir in the walnuts. Grease a standard (8½ by 4½ by 2½-inch/21 by 11 by 6 cm) loaf pan with the butter. Pour the mixture into the pan and place on a baking sheet. Transfer to the oven and bake 50 to 60 minutes, or until cooked through. Remove from the oven and let cool.