Punch holes at the top edge of a thin 8-inch square pan so that you will be able to lace 7 strings from one side of the pan to the other. Place the laced pan in a pan deep enough to catch any leaking syrup.
Dissolve 2 1/2 cups of sugar in 1 cup water and cook without stirring to the hard-ball stage, 247 degrees to 252 degrees. Pour the syrup into the laced pan. It should reach a level about 3/4-inch above the strings. Cover the surface with a piece of foil.
Watch and wait. The syrup may take as long as a week to crystallize.
Lift out the laced pan. Cut the strings and dislodge the rock candy. Rinse quickly under cold water and put on racks to dry in a very low-temperature oven.