The candy making process can seem somewhat simple since it is just boiling sugar in water. However, different types of candy require the boiling to stop at different times, or stages. These stages are determined by what the syrup'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.
During this boiling process, the candy goes through several different stages: thread, soft ball, firm ball, hard ball, soft crack and hard crack. 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 through three stages--from a clear liquid 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-crack stage occurs at 300-310 F. Besides using a candy thermometer, this stage can also be determined by dropping a spoonful of hot syrup into a bowl of very cold water. Then remove the candy from the water and attempt to bend it--if the hard-crack stage has been reached, the syrup will form brittle threads in the water and will crack if you try to mold it.
Hard-Crack Stage Candies
Candies that require cooking to the hard-crack stage are nut brittles, lollipops, and toffees.