You can convert baking soda (sodium bicarbonate) into very pure sodium carbonate by heating it in the oven. Baking soda is USP grade and so contains fewer impurities than washing soda. Place the baking soda in a glass or SS pan and heat it at 350 F for about an hour. Stir it a couple of times during the process. The fine crystals of baking soda will change to a fine fluffy powder of anhyrous sodium carbonate.