We get regular questions about this, simply because so many recipes calling for it are from the U.S.A. For all our soapers here in the U.K. we don't refer to this product as cornstarch, but as cornflour, so this should take the mystery out of what many bath bomb recipes include in their ingredients listings.
DEFINITION TAKEN FROM AN ENCYCLOPEDIA...
A dense, powdery "flour" obtained from the endosperm portion of the corn kernel. Cornstarch is most commonly used as a thickening agent for puddings, sauces, soups, etc. Because it tends to form lumps, cornstarch is generally mixed with a small amount of cold liquid to form a thin paste before being stirred into a hot mixture. Mixing it with a granular solid like granulated sugar will also help it disperse into a liquid. Sauces thickened with cornstarch will be clear, rather than opaque, as with flour-based sauces. However, they will thin if cooked too long or stirred too vigorously. Cornstarch is also used in combination with flour in many European cake and cookie recipes; it produces a finer-textured, more compact product than flour alone. In British recipes, cornstarch is referred to as cornflour.
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