It's an oft-unstated fact in baking that things rarely happen when you need them to. Dough doesn't rise fast enough; souffles fall before you get them to the table -- and fondant rarely hardens enough to place it on the cake when you're ready. Fondant hardens on its own when exposed to air, but drying time varies according humidity and air temperature.
You can give fondant that little push it needs to harden quicker naturally, with airflow or mild heat. Or you can use additives, namely tragacanth gum and carboxymethyl cellulose (CMC), a synthetic form of tragacanth commonly sold under the name "CMC," "tylose powder," "tylo powder" or a variant thereof.
A warm oven -- one with a convection fan is ideal, but not required -- or a blow dryer, although unorthodox, radically accelerate fondant hardening. The oven and dryer are essentially doing the same thing: surrounding the fondant with warm air to expedite evaporation of water. Although both tools produce the same effect, use the oven if possible; it will heat the fondant more uniformly than a hair dryer.
- To harden fondant in the oven, set the oven to warm and let it heat for 5 minutes. Turn the oven off. Set the fondant pieces on a sheet pan lined with parchment paper and place it on the middle oven rack. Heat small fondant pieces, such as figurines and flowers, for 8 to 10 minutes. Heat large fondant pieces, such as bouquets and cake toppers, for 12 to 15 minutes. Take the pan out of the oven, and set it out to air dry for 30 minutes.
- To harden fondant using a blow dryer, set the pieces on a sheet pan lined with parchment paper. Set the dryer to its lowest setting and hold it 1 to 1 1/2 feet away from the fondant. Dry the pieces for 5 to 10 minutes, depending on their size, moving the dryer as needed to dry them thoroughly. Let the pieces sit at room temperature until hardened, about 30 minutes.
If you're drying a complex fondant piece with numerous small pieces, such as a multipetaled flower, dry each piece individually. After the pieces dry, assemble them using gum paste or CMC mixed with water.
Tragacanth powder, CMC and tylose powder produce the same results. Tragacanth powder, synthesized from the sap of several species of trees indigenous to the Middle East, is dried and processed into a powder before going to market. Tragacanth takes about 24 hours to activate, however, almost negating the goal of hardening fondant quickly. It's also less common and more expensive than CMC and tylose, and can leave a yellowish tint on light-colored pieces.
CMC and tylose powder are one and the same -- both activate instantly and are the best choice if you need to firm fondant fast. Add CMC before you add coloring to the fondant.
To harden fondant with CMC, mix 1 to 2 teaspoons of powder with every 1 pound of fondant. Use 1 teaspoon if you live in an arid climate and 2 teaspoons if you live in a humid area. Knead the fondant and use it immediately. Let the formed fondant sit at room temperature until dry.
Store hardened fondant pieces in a paper bag, rolled closed, or a cardboard box. Never store dried fondant in an airtight container. Although the fondant is dry, it still contains moisture. That moisture escapes during storage and condenses in an airtight container, creating a humid environment that renders the fondant sticky and weepy.