Add the salt, sugar, cream of tartar and baking soda to the all-purpose flour. Sift into a large mixing bowl. Add the whole-wheat flour and mix thoroughly with a round-ended knife, using a lifting motion to aerate the mixture. Make a well in the center and add the milk, mixing until the dough leaves the sides of the bowl clean. Knead into a ball, flatten slightly and place on a greased baking sheet. Cut a cross into the top of the loaf. Brush the top with a little milk and bake in a 400 degree oven for 40 minutes. Remove from the oven, turn the loaf upside down and return to the oven to bake for a further 5 minutes. The loaf is done when it sounds hollow when tapped on the base. Wrap it in a slightly dampened cloth and stand on its side to cool. Cut into quarters, slice and butter generously.