At first glance, fondant and gum paste seem interchangeable because they both are pliable sugar doughs used for cake decorating. Confuse the two while decorating a cake, however, and you're likely to end up with a cake covered in brittle, cracked icing, or sugar flowers with petals that go limp. Despite their similarities, fondant and gum paste are different and serve different purposes in cake decorating.
Fondant, sometimes called sugar paste, is a soft, pliable icing usually made from powdered sugar, gelatin, glycerine and water. Many home bakers make their own softer fondant from marshmallows, powdered sugar, shortening and water. Fondant hardens slightly on the outside but remains mostly soft even after several hours. This soft quality makes it desirable for eating but more difficult to hold detailed shapes except for flat designs. Fondant stays fresh for up to two months in a plastic bag placed inside an airtight container -- but should not be refrigerated or frozen. It can be rolled out to about 1/4-inch thickness without tearing and may be used to achieve a smooth finish.
Gum Paste Defined
Gum paste is made with powdered sugar, vegetable shortening and egg whites or meringue powder, a composition similar to royal icing. Like royal icing, gum paste dries completely through so it can be rolled paper thin and hold the shape and fine details in cake decorations. Hardened gum paste is brittle and can crack easily, so all gum paste decorations must be handled with care. This confection quickly dries so you must work with small amounts at a time, keeping the rest wrapped up tightly as you work. For best results, wrap the gum paste in plastic wrap, place it inside a plastic storage bag then place the bag inside an airtight storage container at room temperature for up to two weeks. Coat the gum paste with vegetable shortening before wrapping it and you can store it for up to two months in the refrigerator.
Uses for Fondant
Fondant most often is seen as a substitute for buttercream frosting because it makes a smooth finish difficult to achieve with buttercream. The fondant is rolled to about 1/4-inch thick, draped over a cake coated with buttercream and smoothed into place with a fondant smoother, which is a tool shaped much like a clothes iron. Embellish a fondant cake with fondant cutouts such as stars, dots or letters. Cut the embellishments with fondant cutters, which look like cookie cutters, and attach them by brushing the back with just enough water to make the fondant tacky. While it doesn't dry hard enough to hold detailed shapes, fondant works well for figure modeling with basic shapes and a cartoon-like appearance.
Uses for Gum Paste
Cakes are never covered with gum paste because it dries too hard and while edible, it doesn't have a particularly good taste. When you want to make detailed decorations for cakes and cupcakes, gum paste is the best option because it completely hardens -- much like modeling with clay. Realistic flower decorations are made with gum paste. Roll the gum paste to about 1/16 inch and use shaped cutters to cut the petals. Place them in a bowl to form a cup shape until the gum paste is completely dry. Gum paste also works well when you want to make highly detailed figures such as people and animals. Rapid drying and its stiff nature show even the finest details such as strands of hairs whereas fondant only shows vague detail. Drying can take as little as one hour for very thin decorations such as lily flowers or about two days for thick decorations such as animals.