Why Is Milk White?: & 200 Other Curious Chemistry Questions (2013)
How do you make rock candy?
Rock candy is crystallized sugar. The crystals are usually grown slowly (over a week or two) to get large, attractive crystals. Growing crystals quickly makes a mass of small crystals instead of nice large crystals.
To get the crystals to grow slowly, you need to control the rate at which the sugar precipitates out of a sugar solution. If you boil the water out of the solution quickly, you get candy like taffy or nougat, where the crystals are so tiny you can’t feel them on your tongue. If you disturb the candy by beating it, you also get billions of tiny crystals, as you do when you make fudge.
To make rock candy crystals, start with plain sugar and water. Two cups of sugar will dissolve in one cup of water if you boil the water. If you want to make more candy, you can double the recipe.
Boil the sugar and water together until all the sugar dissolves. Remove the pan from the heat. At this point you can add food coloring or flavors like vanilla, mint, cherry, lemon, grape, or strawberry. Let the liquid cool for at least 10 minutes.
Next, take some string or some wooden skewers and dip them into the liquid. You can let the liquid dry on the string to form seed crystals, or you can roll them in granulated sugar and let them sit to dry for a bit. Drying allows the crystals to form in the crevices of the string or wood, so they are well anchored.
Fill some jars with the remaining liquid.
Now hang the string or skewers in the jars, being careful to keep them above the bottom of the jar. Crystals will form on the strings or skewers, but they will also form at the bottom of the jar and at the surface of the liquid. If the string or skewer touches the bottom, the rock candy crystals on the string will join those at the bottom, and you will get a big mass that you can’t remove.
I like to put the jars into saucers of water so they don’t attract ants. I also cover them with paper so that flies aren’t attracted to the syrup. After a week, as the water slowly evaporates, the sugar will be forced out of solution and will crystallize on surfaces in the jar, such as the string or wood, that were seeded with small sugar crystals. Crystals grow first where there are already seed crystals.
Grow your crystals for 7 to 14 days and remove them when they are the size you like. As crystals form at the surface of the syrup, you can knock them down into the jar or remove them.
The biggest crystals will grow in a cool solution that is placed in a location free of vibrations from running feet or slamming doors.