Cornstarch and rice flour may both be used to thicken gravy, sauces and other dishes. Cornstarch is usually added to sauces in slurry form, while rice flour can be used both to make a slurry and to make a roux. While both starches may be used as a substitute for flour in recipes, cornstarch is more often used in conjunction with flour.
Cornstarch is made from corn, while rice flour is made from rice. While cornstarch has the advantage of thickening liquids without changing their color or clarity, it is not stable in sauces if frozen or boiled. Rice flour works in a fashion more similar to that of wheat flour.
To thicken sauces or gravy, substitute an equal amount of rice flour for wheat flour as called for in your recipe. When substituting cornstarch for wheat flour, use half as much as called for in your recipe.
Cornstarch should be blended with cold liquid before being added to hot dishes; otherwise it will clump. Make a slurry of cornstarch and cool liquid, then stir it into your sauce. Do not allow the sauce to boil, or the cornstarch will break down and stop working. It is also not advised that you add acidic elements to your sauce or gravy, as the cornstarch will react adversely to these elements.
Rice Flour Usage
You can use rice flour to thicken sauces in several ways. You can make a slurry of rice flour and cool or hot liquid, blending it thoroughly before adding it to your sauce. Or you can make a roux of equal parts rice flour and fat, gently cooking it until it is medium brown in color, and then using it as your sauce or gravy base.
Both rice flour and cornstarch are gluten-free, an important resource for those with a gluten allergy.
References and ResourcesStarch Thickeners
Epicurious.com: Rice Flour
ResourcesThe Art of Thickening