To begin making rock candy, boil 2 cups of water in a pot. Add 4 cups of sugar, and stir until it dissolves and the water starts to boil again. Remove the pot from heat, and pour the solution in a glass jar. Place a piece of wax paper over the opening while preparing a string for dipping.
The prepared string should be two-thirds as long as the jar's depth. Tie a small weight to one end, and attach the other end to the center of a pencil. Dip the string into the jar, and coat it with the sugar water mixture. Remove it, and lay it on a piece of wax paper to dry. The dried string contains very small crystals that become the basis of the final, larger crystals.
When the string is dry, insert it into the jar of sugar solution so the pencil catches the lip of the jar and suspends the string in the liquid. Leave the string in the jar for seven days at room temperature. When removed at the end of the week, the string should display crystals of sugar.
Rock candy crystals occur because of precipitation. The sugar water mixture is a supersaturated solution. This kind of solution is created by heating a solution and then allowing it to cool. They are highly unstable, as they are composed of a greater percentage of solutes than liquids. The sugar emerges from the solution as a precipitate.