Inspired by "The Incredible Secret Formula Book" by Shar Levine and Leslie Johnstone, this is gorgeous stuff. Smooth and silky in your hands, it has a porcelainlike finish and a soft, eye-catching shimmer when dry.
For tinted clay, knead in a few drops of food coloring. Allow creations to air-dry overnight. Refrigerate any leftover dough in a ziplock bag for up to three days.
In a saucepan, stir together the baking soda and cornstarch. Add the water, mixing well.
Cook the mixture over low heat (adults should supervise), stirring occasionally, until it resembles mashed potatoes, about 7 to 10 minutes. You will need to stir it almost constantly the last few minutes as it thickens.
Remove the pan from the heat and set it aside. When the dough has cooled, turn it out onto a floured board. Knead the dough with your hands, adding more cornstarch as needed, to create a workable clay.