Science project

Crystal Fudge

Research Question:

  • Why do some rocks that are made out of the same minerals have different-sized crystals in them?
  • What effect will faster vs. slower cooling have on the formation of crystals?

Fudge is one of very few desserts people make at home that is actually crystalline, or made out of crystals. This gives us a fun, tasty way to explore the process of crytalization.


  • Two bread pans (disposable 8” pie tins will also work)
  • Butter to coat pans, or waxed paper
  • Large saucepan (3-4 quart)
  • Wooden spoon
  • Candy thermometer
  • Pastry brush
  • Stove
  • Refrigerator
  • 3oz. unsweetened chocolate
  • 3c sugar
  • 1c warm half-and-half or evaporated whole milk
  • 1T corn syrup ¼t salt
  • 3T butter
  • 2t vanilla extract
  • 1c mix-ins of your choice: nuts, mini marshmallows, dried fruit… (optional)
  • Magnifying glass

Experimental Procedure:

  • Butter the pans or line them with the waxed paper.
  • Mix the chocolate, sugar, salt, half-and-half, and corn syrup over medium-low heat. Keep stirring until the chocolate is melted and the fudge begins to boil. Note: the fudge is extremely hot at this point, handle with care!
  • As soon as the fudge begins to boil, stop stirring and put the candy thermometer in. Clip it to the edge of the pot, making sure the tip isn’t touching the bottom.
  • Let the fudge cook without any stirring until it reaches the soft-ball stage, around 237 degrees.
  • While the fudge cooks, dip the pastry brush in a little warm water and use it to carefully wash any sugar/chocolate/whatever off the sides of the pot.
  • Take the fudge off of the burner and let it cool, undisturbed, until it’s 150 degress.
  • Add the vanilla and butter and keep stirring until the surface of the fudge starts to get dull. This can take a long time, but you need to keep stirring! Maybe you can get a partner to help.
  • Once the fudge has begun to dull, stir in your add-ins, a quarter-cup at a time, if you’re using any. Make sure they’re at room temperature or a little warmer if possible.
  • Spoon half of the fudge into each pan. Put one pan in the refrigerator and leave the other one out at room temperature. Allow both of them to cool completely.
  • Cut each panful of fudge into one-inch cubes. Pick up a cube from each pan and examine them closely. Use your eyes and the magnifying glass: do you see any differences in texture? Use your tongue: does one seem more smooth and waxy while the other is more grainy? Is there a difference in flavor? The fudge that cooled more slowly, at room temperature, should be grainier and have noticeable sugar crystals in it. This is like a plutonic igneous rock that has cooled and solidified slowly, under the surface, like granite. The one that cooled more quickly, in the refrigerator, should be smoother and have much smaller crystals, probably too small for you to see even with the magnifying glass. This is like a volcanic igneous rock that cooled quickly above the earth’s surface, like obsidian.
  • Now offer samples of each to your family and friends so they can decide which they like best!

Terms/Concepts: igneous rock; crystallography, crystal formation

References: Dig It!: Over 40 Experiments in Geology, by Lockwood DeWitt and B. K. Hixson, pp. 150-152 (Loose in the Lab Science Series, 2003).

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