How to Make Rock Candy
Rock Candy or Sugar Crystals
Rock candy is made from sucrose, which is table sugar. Usually you use granulated sugar, but you can experiment with brown sugar or specialty sugars. Making your own rock candy is a great way to grow your own crystals, plus you get to eat what you make. You can add colors and flavors or make crystals of pure sugar.
Photo: Douglas Whitaker
What Is Rock Candy?
In its simplest form, rock candy is crystalline sucrose, or sugar crystals. It is one of the first candies ever made. If you look closely at granulated table sugar you can see that each crystal is a tiny rectangular box. This is the shape taken by the monoclinic crystal structure of the sucrose. When you make rock candy, you are making giant sugar crystals, with boxes stacked on top of each other.
Make Rock Candy
Let's Grow Sugar Crystals!
Rock candy isn't difficult to make, but the sugar solution will be heated, so adult supervision is recommended.
Materials for Rock Candy
- 2-4 cups sugar
- 1 cup water
- food coloring (optional)
- flavoring (optional)
- rough cotton string
- clean jar or glass
- pencil or butter knife
- saucepan and stove
- Pour the sugar and water into the saucepan. Start with 2 cups of sugar. Add food coloring if you want colored rock candy.
- Heat the mixture over medium heat, while stirring, until it just comes to a boil. Add more sugar until it stops dissolving.
- Try to avoid letting the mixture (called syrup) get hotter. Keep it near-boiling and stir until the sugar dissolves. This is when mixture becomes clear, without crystals floating in the liquid.
- Remove the pan from the heat. Stir in a few drops of flavoring, if desired. Set the pan in the refrigerator to cool for about 15 minutes. You want the mixture to be room temperature or cooler before pouring it in the jar.
- Dampen the cotton string with the syrup mixture. Roll the string in dry sugar. Shake off any excess sugar.
- Tie the string to a pencil or knife so that you can rest the pencil on top of a clean jar with the string not touching the sides or bottom of the container.
- Pour the cooled syrup into the jar and add the string. You can cover the top with a paper towel, coffee filter, or piece of waxed paper.
- Allow the jar to sit undisturbed for 3-7 days. Remove (and eat) any crust of sugar that forms on the top of the liquid.
- If you see a lot of crystals start to form on the sides and bottom of the jar, carefully remove the string and set it aside. Reheat the syrup to dissolve the sugar. Cool the sugar syrup below room temperature and then pour it into a clean jar (cool it to prevent it from dissolving crystals already on your string). Hang the string in the liquid.
- Remove the crystals when you are satisfied with their size.
- You can eat the rock candy immediately or you can hang the string and allow it to dry.
- Store rock candy in a dry, sealed container. The candy will stick to itself, so you may wish to wrap it in waxed paper or coat it with confectioner's (powdered) sugar.
See How to Make Rock Candy
Rock Candy at Amazon
Troubleshooting Common Rock Candy Problems
What to Do When Crystals Won't GrowThe most common reason why rock candy crystals won't grow is not having enough sugar in your solution. If you don't get crystal growth within a week, reheat the solution to boiling. Add more sugar until no more will dissolve, then try again.
Certain flavorings will inhibit crystal growth. Fruit juice and sweetened flavorings will inhibit crystal formation. The acid and other sugars in these flavors breaks apart the sucrose (sugar) molecule. The best flavorings for rock candy are oil of peppermint, oil of cinnamon, and concentrated fruit flavors (sold with spices).
If you can, try to grow your crystals in a cool, dry location. A warm spot will speed evaporation of the solution, but it prevents crystals from growing because warm water won't give up dissolved sugar.
You Can Color Rock Candy (Andreas Praefcke)
Cinnamon or Peppermint Rock Candy Recipe
This recipe takes 1-2 hours to make and yield 2 lbs of candy. This type of rock candy doesn't form sugar crystals, but if you want rock candy quicker than a week or you are having trouble with the traditional recipe, this might be a good option for you.
1 cup water
3-3/4 cups sugar
1-1/4 cup light corn syrup
1 teaspoon red food coloring (or green for mint)
1 teaspoon cinnamon oil (or use oil of peppermint or mint)
1/3 cup confectioner's sugar
- Line a 9" x 13" baking pan or a cookie sheet with foil or waxed paper. Butter the paper or foil.
- Mix the water, sugar, corn syrup and food coloring in a saucepan.
- Heat the mixture to boiling over medium heat.
- Cover and cook for 3 minutes to dissolve sugar crystals.
- Uncover the pan and continue cooking over medium heat, not stirring, until a candy thermometer reads 300°F (hard-crack stage). This wil take about 25 minutes.
- Remove the pan from the heat.
- Stir in the cinnamon or mint oil.
- Immediately pour the mixture onto the buttered pan.
- Allow the candy to cool completely, about 45 minutes.
- Break the candy into pieces.
- Coat the pieces with confectioner's sugar to prevent sticking.
- Store the rock candy in an airtight containter.
Learn More About Rock Candy & Crystals
- Close-Up of Sugar
Rock candy is sugar, or sucrose. Here's a microscope photo of a sugar crystal.
- More Crystal Recipes
Now that you've made rock candy, how about trying some other crystal growing projects?
- Grow a Seed Crystal
Really big crystals grow from 'seeds', which are small single crystals that attract more crystal growth. Here's how to make a seed crystal.
- Crystal Science Fair Projects
In addition to growing crystals for fun, how about using them for a science fair project?
- Crystal Growing Quiz
Are you a whiz at growing rock candy and other crystals? Take this quiz and find out.
- Crystal Photo Gallery
Crystals are beautiful! See photographs of crystals you can make as well as other crystals you can find in the world around.