Boiled sugar is the base of all candy, from fudge to marshmallow. As its temperature rises, sugar goes through several stages, from thread to hard crack. Each stage corresponds to a texture or consistency—the hotter the syrup, the harder the candy. If you're making candy at home, you can determine the temperature of your syrup with a candy thermometer or a glass of cold water.
The Different Stages
Boiling syrup goes through six different stages: thread, soft ball, firm ball, hard ball, soft crack, and hard crack. Before you begin making candy, check your recipe: It will tell you when to take your boiling sugar off the heat. For instance, the thread stage is perfect for syrups and candied fruit, while fudge must reach the soft-ball stage in order to develop its characteristic soft chewiness.
When making candy, you should rely on both the cold water test and a candy thermometer for the most accurate results. Only seasoned candy makers memorize temperatures since most candy thermometers will display both temperature and stage.
Hard-crack stage occurs at 300 to 310 F. If you don't have a candy thermometer, you can use the cold water test in a pinch: Drop a spoonful of hot syrup into cold water, then remove the candy from the water and attempt to bend it. If your syrup has reached the hard-crack stage, it will form brittle strands in the water and crack as you bend it.