Well-seasoned, lump-free gravy is the sign of a good cook. But all too often, even experienced cooks have trouble with it. It's too thick, it separates, or it's pasty, rather than light and smooth.
Flour is the traditional gravy thickener, but cornstarch may also be used. Cornstarch-thickened gravy has a clearer, more transluent appearance. Cornstarch doesn't brown as flour does and counts on the brown drippings in the roasting pan to give it color.
One method of making cornstarch gravy is to add the cornstarch directly to the drippings. A second method is to thicken the gravy just before serving. To do this, stir together cornstarch and a little cold water and, stirring constantly, add it to the boiling liquid. Cook until it comes to a full boil.
When substituting cornstarch for flour in a recipe, remember that cornstarch has twice the thickening power that flour does. So use half as much as is called for.