Rock candy requires only sugar, water, optional flavoring and few basic materials. The process is slow, however, and may require one week for the candy to completely form. Rock candy is formed through the slow and incremental accumulation of sugar crystals on a base material, such as a cotton string.
To begin the process, prepare the materials by cutting the string to a length that exceeds the height of a quart jar by several inches. Tie the string onto a pencil such that when the pencil is laid across the mouth of the jar, the string dangles inside, about 1 inch above the bottom. Attach a paper clip or another suitable weight to the loose end of the string. This keeps the string straight during the candy-forming process. Wet the string and roll it in granulated sugar.
Next, boil 2 cups of water in a small saucepan. Slowly dissolve 4 cups of sugar into this, stirring until no sugar granules remain. Remove from the heat, and, if desired, add 1 teaspoon of a flavoring extract, such as cherry, lemon, or mint. Allow the mixture to cool 10 minutes before pouring this into the quart jar. Lower the string into the solution, cover with a paper towel, and allow to sit undisturbed. If sugar crystals have not begun forming within 24 hours, reheat the solution, cool for another 10 minutes and pour the mixture back into the jar.
Once the crystals have grown to the desired size, remove the string and enjoy the rock candy.