Martha Stewart Living, December 2008
Stir together citric acid, baking soda, and cornstarch in a glass measuring cup.
Pass mixture through a fine sieve or a flour sifter into a mixing bowl. Stir in sugar.
Fill spray bottle with water, and add food coloring. Spritz mixture lightly (it should become damp but not fizzy) until you can pack mixture with your hands.
Using pipette, add oil, 1 drop at a time, until strength of scent is to your liking. Using a metal spoon or your hands, mix ingredients until color is even throughout (mixture will begin to dry out; when this happens, spritz until packable again).
Spoon into ice-cube tray, pressing firmly. Let dry at room temperature overnight. Pop out of tray gently. Transfer to jars.
Ice-cube trays serve as molds, giving the fizzies their shape. Using a flexible silicone model lets you slide the cubes out smoothly without breaking them. Show off the colors of the bath fizzies in glass jars with screw tops, the kind used for storing sugar and flour. Pack each type of fizzy in a separate jar so the scents won't meld. Write the name of the scent on a vellum tag (choose a shade that matches the fizzies), and punch a hole in the top. Thread the tag with ribbon, and tie it around the jar. Citric acid, a common food additive, is available at wine-making-supply stores, some spice shops, and online. When citric acid is combined with baking soda and placed in water, a chemical reaction creates bubbles.