There’s a quick fix that gives temporary results, long enough for you to measure some out for a recipe, and there’s a more time-consuming but longer-lasting fix that will restore your sugar to its original, manageable form.
But first, what makes brown sugar turn hard in the first place? Loss of moisture. You didn’t reseal the package tightly enough after opening it, and it dried out to some extent. It’s not your fault; it’s almost impossible to reseal an opened box of brown sugar completely. So after you use some, always repackage the remainder in an airtight (more exactly, a vapor-tight) container such as a screwtop jar or a plastic food storage box with a tight lid.
The brown sugar sold in stores consists of white sugar crystals coated with a thin film of molasses, the thick, dark liquid left behind when sugar-cane juice is evaporated to allow crystals of pure sugar, sucrose, to separate out. Because the molasses coating has a tendency to absorb water vapor, fresh brown sugar is always very soft. But when exposed to dry air, the molasses loses some of its moisture and hardens, cementing the crystals together into lumps. You then have a choice: either restore the lost water or try somehow to soften the hardened molasses.
Restoring the water is easy, but it takes time. Just seal the sugar in a tight container overnight along with something that gives off water vapor. People have recommended everything from a slice of apple, potato, or fresh bread to a damp towel or, for no-nonsense types, a cup of water. The most effective setup is probably to put the sugar in a tight-lidded container, cover it with a sheet of plastic wrap, place a damp paper towel on top of the plastic wrap, and seal it all up. After a day or so when the sugar becomes soft enough, discard the towel and plastic wrap, and reseal the container tightly.
Many food books and magazines inform you that brown sugar hardens because it loses moisture, which is true, and then go on to tell you to heat it in the oven to soften it, as if the oven somehow restores moisture. It doesn’t, of course. What happens is that the heat softens, or thins, the molasses “cement,” which then re-hardens as it cools.
Some brown sugar packages recommend placing the hardened sugar in the microwave oven along with a cup of water. The water isn’t there to hydrate the sugar, though, because in the couple of minutes it takes for the heat to do its job, water vapor from the cup doesn’t have enough time to diffuse through the mass of sugar and hydrate it. The water is there only to absorb some of the microwaves, because microwave ovens shouldn’t be operated in an empty or nearly empty condition. If you’re zapping at least a cup or so of sugar, you probably don’t need the water.
A chef of my acquaintance puts brown sugar out in his restaurant kitchen every day and it dries out rapidly. When it becomes very hard, he puts a few drops of hot water on it and massages it with his hands until it returns to its original texture. That’s fine for the pros, but massaging sugar probably isn’t the average home cook’s idea of fun.
Speaking of molasses, a former Peace Corps volunteer once told me that many years ago in Mhlume, Swaziland, they used to pave dirt roads by spraying them with molasses from the local sugar refinery. It dried out and hardened very quickly, taking a few months to wear back down to the dirt. (Note to my city’s Public Works Department: Maybe if you used molasses instead of lowest-bidder asphalt, our roads would last longer.)
Finally, if all else fails there is always Domino’s Brownulated or free-flowing sugar, which pours like a dream and never turns into a brick. Domino’s manufacturing trick is to break down some of the sucrose (Techspeak: hydrolyze it) into its two component sugars: glucose, aka dextrose, and fructose, aka levulose. (Some sugars have multiple names.) This mixture, called invert sugar, holds onto water tightly, so hydrolyzed brown sugar granules don’t dry out and cake. Brownulated sugar, however, is intended for sprinkling on oatmeal and such, not for baking, because it doesn’t measure out the same as the ordinary brown sugar that cookbooks specify.
If you’re in a hurry to soften hard brown sugar, your trusty microwave oven will come to the rescue with a quick, but temporary, fix. Just heat the sugar for a minute or two on high, probing it every half minute or so with a finger to see if it’s soft yet. Because ovens differ so widely, no exact time can be stated.
Then measure it out quickly because it’ll harden again in a couple of minutes. You can also soften the sugar in a conventional oven at 250º for 10 to 20 minutes.