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Anne Marie Helmenstine, Ph.D.

Rock Candy Anyone Can Make

By May 18, 2008

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As the weather warms up I get more and more email from people having trouble making rock candy. Why? I think it's because warmer temperatures combined with higher humidity can sabotage your best crystal-growing efforts. You may think you're doing your crystals a favor by placing them in a warm sunny window, but the increased rate of evaporation may not be enough to make up for the increased solubility of sugar when the container warms. Don't worry... I've got instructions for rock candy that will produce crystals even in the heat of summer when it's raining (tried and tested). I've written out a more detailed version of these instructions, but here's the summary:
  • Heat 3 cups of sugar and 1 cup of water just to boiling, stirring continuously. Make sure all of the sugar dissolves, leaving you with a clear solution. You can add food coloring or flavoring if you like.

  • Set the pan of hot syrup aside to cool (it's ok to put it in the refrigerator for ~ 15 minutes). In the meantime, dampen a cotton string (preferably with the saturated syrup) and roll it in sugar. Set it aside to dry while your solution is cooling.

  • When your solution is around room temperature, pour it into a clean glass or jar. (If you happen to have any leftover syrup, pour it onto a saucer. It will form crystals overnight, leaving you with a sparkly rock candy disk to eat or suspend in a window to catch the light or whatever.)

  • Suspend the string in the glass so that it isn't touching the sides or bottom of the container.

  • Set the container in a location where it won't be disturbed, away from temperature changes and vibration.

  • You'll get rock candy, probably within a few hours, but wait 3-10 days until the crystals are really big before removing the candy from the liquid and allowing it to dry.

  • You can help your crystals grow by removing (and eating) any sugar 'crust' that forms on top of the liquid. If you notice a lot of crystals forming on the sides and bottom of the container and not on your string, remove your string and set it aside. Pour the crystallized solution into a saucepan and boil/cool it (just like when you make the solution). Add it to a clean jar and suspend your growing rock candy crystals.
If you follow these instructions and still experience problems, there are two likely causes... not using enough sugar (at least two times more sugar than water) or not dissolving the sugar completely. Another possible problem could arise from the type of flavoring you would add to the solution. Flavors like oil of cinnamon or oil of peppermint tend to be work well, but if you add natural fruit juice (like from a lemon or orang), the acid and fructose may prevent your sugar from crystallizing.
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