Homemade and commercially prepared fondants harden differently. Commercially made fondant contains corn syrup, cornstarch, artificial colorings and flavorings, preservatives, glycerin and vegetable oil. Glycerin in the fondant keeps it from drying out quickly, but it still hardens to a soft shell once placed on a cake or pastry. Homemade fondant is typically made from corn syrup, powdered sugar, gelatin and water. While you can add glycerin to keep it from drying out, homemade fondant is naturally gummy and soft. Homemade fondant, however, doesn’t harden like commercially prepared fondant which means it remains soft and pliable while topping a cake.

Unused fondant needs adequate storage or it dries out and hardens. Store unused fondant in plastic wrap inside an airtight container. Commercially prepared fondant typically comes in plastic tubs with a plastic bag inside. You can store your fondant in these without additional preparation for up to two months at room temperature. Exposure to the air turns fondant hard and homemade fondant can dry out after being exposed to the air within a few days. Fondant can't be stored in the fridge or freezer. The humidity creates condensation, which breaks down the fondant’s natural sugars -- leaving you with a sticky, gummy mess.

While kneading fondant, excess powdered sugar or cornstarch -- which help the fondant from sticking while being kneaded and rolled -- can make fondant dry and harden up. Counteract this by placing a small amount of vegetable shortening on your hands to incorporate moisture back into the fondant while kneading and rolling.

If fondant has hardened during storage or while rolling, you may be able to soften it. Place the fondant in a microwave-safe bowl or on a place of parchment paper. Heat in the microwave on your lowest setting for five seconds. Check the fondant for pliability by pinching it between your fingers. Once the fondant is soft and pliable remove it and use it right away.