Baking soda & Cornstarch Ornaments 2 c. baking soda 1 c. cornstarch 1 1/4 c. cold water Mix cornstarch and baking soda, then add water. Cook over medium heat, stirring constantly, until the consistency of wet mashed potatoes. Turn onto a plate, cover with a damp cloth and let cool. Knead until smooth. Seal in plastic bag, in refrigerator until needed.