Candy is made by boiling sugar in water. During this boiling process, the candy goes through several different stages: thread, soft ball, firm ball, hard ball, soft crack and hard crack. Each stage describes what the candy's consistency will be when dropped into cold water. For example, when a bit of the syrup is at the soft-ball stage and then dropped into the cold water it will form into a soft ball.
Different types of candy require a different stage--fudge needs to be cooked to the soft-ball stage while marshmallows are cooked to the hard-ball stage. (When caramelizing sugar it goes from a clear liquid stage to a brown liquid and then burnt sugar stage.)
As the liquid boils away, the temperature increases and the concentration of sugar becomes greater. When making candy, you should use both the cold water test as well as a candy thermometer for the most accuracy.
Hard-ball stage occurs at 250-266 F and can be read by using a candy thermometer. At this point, the sugar concentration is very high--92 percent--which means the moisture has decreased. When the syrup is lifted with a spoon it will form thick, rope-like threads.
You can also determine if you've reached hard-ball stage by using the cold water method. Drop a spoonful of hot syrup into a bowl of very cold water, then while it is in the water, use your fingers to gather the cooled syrup into a ball. If the hard-ball stage has been reached, the syrup will hold its ball shape and deform only slightly with very firm pressure. The ball will be quite sticky to the touch.
Hard-Ball Stage Candies
Common candies that need to be cooked to the hard-ball stage are taffy, marshmallows, gummies, nougat, rock candy and divinity (white, fluffy-looking confections made with sugar, corn syrup, and egg white).