Pour a thick (1/2 inch or so) layer of baking soda on the bottom of the baking dish.
Bake for 1 hour, stirring 1-2 times in the middle, or until it has changed in look and feel. Baking soda has a silky/powdery feel and washing soda is more grainy and not silky. The baking soda will need to reach the full 400 degrees for this reaction to take place, so give it time.
