A hard lump of brown sugar can throw a wrench into your baking plans. Instead of rushing out to the store for a new bag, take a few minutes and revive your existing store of sugar. Brown sugar contains molasses. As the moisture from the molasses evaporates, the sugar can dry out and turn into a cement-like lump. You can quickly add moisture with the help of heat if you need to soften the sugar quickly.
Place the lump of hardened brown sugar in a microwave-safe bowl. The rim of the bowl must sit higher than the lump of sugar.
Lay a slice of soft bread or a slightly dampened paper towel on top of the sugar. Cover the entire bowl with a layer of plastic wrap.
Heat the bowl in the microwave for 10 to 20 seconds, then check the sugar. Continue to heat in 10-second increments until the lump has softened enough to measure and use. Avoid overheating, which can burn or liquefy the sugar.
Measure out the amount of sugar needed and place the remainder in an airtight storage container. Place fresh slice of bread, a slightly moistened towel or two or three fresh apple slices in the container; otherwise, the brown sugar will harden again as it cools.
Place the lid tightly on the container and store. Remove the bread, towel or apple slices in two days once the sugar has absorbed the moisture.