Brown sugar hardens when its natural moisture escapes and the crystals lock together. It is usually still usable, and gentle moisture can restore its soft texture without dissolving it.
Choose the slow method when you can wait overnight, or the microwave method when you need the sugar immediately.
Brown sugar contains molasses, which holds moisture around the sugar crystals. In a loosely closed bag or jar, that moisture evaporates. The sugar then dries into a firm block. This is a texture problem, not automatically a sign of spoilage.
The bread releases moisture into the sealed air. The sugar absorbs it gradually without becoming wet.
Place up to 200 g hard brown sugar in a microwave-safe bowl. Cover it with a barely damp paper towel. Heat for 10 seconds, then press and stir with a fork. Repeat in 10-second bursts only until soft.
Use the sugar while it is warm, because it may firm again as it cools. Avoid long heating, which can melt the sugar and create dangerously hot spots.
Transfer it to a small airtight container or a heavy zip bag with most of the air pressed out. Store it in a cool, dry cabinet. A purpose-made terracotta sugar saver can also help when soaked and used according to its instructions.
Not for hardness alone. Discard it if it has mold, insects, an unusual odor or contamination from another food.
Slow moisture transfer gives the most even result, while brief microwave bursts solve an immediate baking problem. Airtight storage prevents the next block from forming.