Any high-sucrose syrup will crystallize under the right conditions. Temperature, agitation, saturation level and condensation all factor into crystallization, or the formation of crystals in a solution. Molasses, the syrupy byproduct of sugar refinement, is primarily sucrose, but the concentration varies depending on the source -- usually sugarcane, sugar beets or sorghum. Generally, the more sucrose the molasses contains, the more likely it will crystallize. However, just as easily as molasses crystallizes, you can de-crystallize it, too. Make it less likely to happen again by adding citric acid or pure fructose after it's softened.
Open the glass jar of crystallized molasses and place it in the center of a saucepan or pot.
Add enough water to the saucepan to reach the level of the molasses in the jar.
Heat the saucepan on the stove over high heat until the water boils.
Boil the water until the molasses melts.
Stir in a tablespoon of corn syrup or water, or 1/2 teaspoon of lemon juice using a metal or silicone-coated utensil while the molasses is hot to prevent it from crystallizing again.
Turn the heat off. Remove the jar from the water with an oven mitt and let the molasses cool to room temperature. Replace the lid, then store the molasses jar at room temperature.
Molasses that has crystallized in a glass jar can also be softened in a microwave. To do so, remove the lid, place the glass in the microwave, then heat in five- to ten-second increments. Check the molasses and stir it gently each time until it reaches the desired level of liquidity.
Don't store molasses in the refrigerator or in an area with a temperature between 40 and 45 degrees Fahrenheit. High-sugar syrups, such as simple syrup, honey and molasses, have a tendency to crystallize between 40 and 45 F.