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Making Rock Candy

  1. Jan 27, 2016 #1
    So I am trying to make rock candy. I made my saturated solution of sugar about two and a half days ago and so far there is almost no crystallization on the skewer that I have sitting in the middle of the solution. But there is a layer of sugar crystals covering the top. Should I break that top layer so that there could be evaporation? I don't see how it is going to crystallize if nothing evaporates. If I am wrong, please correct me.
  2. jcsd
  3. Jan 27, 2016 #2


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  4. Jan 28, 2016 #3
    Well I am new to this whole "internet forum" thing, but if my suspicions are correct I have been mentioned to give a helping hand.

    The problem with super-saturated sugar hydro solutions is that they are VERY stable (irritably so). The reason you have a layer of sugar crystals at the top of your solution is because some dust particles might have fallen in and have become the centers for crystal growth. But even if no dust has fallen stationary supersaturated solutions develop different gradients of concentration thus spontaneous crystallisation may occur at the wrong area.

    In your case you should break the surface layer of crystals, mix the solution a bit and leave it some more. It would help to know what method, exactly, you are using, but the main points you should consider are:
    1. Cover the solution with a paper napkin of sorts, to prevent dust falling into it
    2. Do not create a solution that is "too super saturated" - the more super saturated a sugar hydro solution is the harder is for it to evaporate, you would be fine starting with quite a "lightly" super saturated solution, leaving it to form crystals on the stick/skewer (which should happen a lot faster) and then repeating the process. You can even use a more saturated solution from the second time onwards, since the already formed crystals will act as seeds for new crystal growth.
    3. Temperature. Cooling a super saturated solution will also entice it to form crystals. For home purposes taking the solution off from the hotplate and leaving it at room temperature should be enough, but if you have a colder room available - it would help.
    4. Stir the solution periodically (once or twice at day would suffice) if the desired effect is not observed after 12 hours or more.
    5. The type of skewer/stick is also important, some materials are better suited to house crystal growth than others and you cannot grow crystals on some. The very basic requirement is that there are scratches or other imperfections, it is very hard to impossible to grow crystals on a perfectly smooth surface, as crystals need a "centre" (embryo center in my language, unfortunately I do not know the proper term in English) to start.

    That is what I can think of at the top of my head, I hope it helps! :)
  5. Jan 28, 2016 #4
    @chemaster That more than helps! I am just trying to grow some rock candy to eat with my girlfriend (we love doing things like this), so I didn't want to over complicate my method. What I did was:

    1) Dissolved as much sugar as I could into boiling water. In fact, I threw in too much, to the point where the sugar stopped dissolving. I did not filter our anything that didn't dissolve

    2) I poured the hot solution into a mason jar and suspended a skewer inside, not touching the bottom

    3) I let it sit for a few days now

    Some of my observations:

    1) There was undissolved sugar sitting at the top of the solution. I am guessing these acted as seeds for crystal growth, maybe explaining why I have that top layer of crystals.

    2) Crystal growth has started on the bottom of the glass and a bit on the sides

    3) Some crystal growth has started on the skewer. It is very little, but increases each day.

    In order to speed up the process, would you still suggest breaking the top and stirring? Considering the growth has started (albeit very slowly)
  6. Jan 28, 2016 #5


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  7. Jan 29, 2016 #6
    Well that is unfortunate, but not unfixable.

    See, crystals "compete" each-other for growing, this means that each crystal is trying to take sugar from the solution and apply it to itself, thus reducing the speed (and ultimate size) at which your main crystals grow. Since it's a home experiment I wouldn't worry too much about it, I advise you to break the top layer of crystals, along with any other crystal formations, that are not on the skewer, stir and let it sit. Repeat at a later point, if necessary.

    There are crystals forming on the sides of the jar probably because there were some minor scratches from usage, which become ideal nests for crystals to grow into. From what you said I'm supposing you are using a wooden skewer? I ask because smooth surfaces are not suited for crystal "nests" and if you were using a metal one, it probably would not work, but since you have some crystals on it - it's all good. :)

    Just some tips for your next rock candy:
    1. Always remove undissolved sugar, that will become a start for crystal growth, which will compete with your main objective.
    2. Try to use as brand-new and scratch-less jar, as possible.
    3. Don't dissolve too much sugar into the water, all the sugar you put in the water will end up on the skewer in an almost perfect 1:1 ratio, so I do not think you would need that much (depending on sweet tooth :) ) I'm supposing your solution right now has a yellowish/brownish tint and is quite viscous, next time you might want to stop when the solution just starts to gain a coloring, but ultimately that is up to you to "feel" as different sugars will affect it differently.
    4. Stir the solution periodically - even if all is going well a periodic stir (say, once a day) will help things move along.

    I mentioned before that reducing the temperature would coerce crystals into forming. Since you already have that, in your case you would want to put the solution somewhere warm - that would speed up the evaporation of water, which is your ultimate goal.
    Note: warm, not hot!
    Too high a temperature is going to make the crystals want to dissolve again.
  8. Jan 29, 2016 #7
    Thanks for the great replies, @chemaster !

    I did what you said, and left the jar for about 8 hours. When I checked on it again, much more sugar had come out of solution, but the crystals had virtually no size, i.e. it was basically powdered sugar. I realize that this batch is probably a lost cause and I will have to try again, but out of curiosity, why did I get "powderized crystals"?
  9. Jan 30, 2016 #8


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  10. Feb 1, 2016 #9

    Hm, that should not be so. I cannot tell you for certain with this data, especially without seeing the crystals, but the first thing I can think of is the type of sugar you are using. Different kinds would dissolve in a different manner and, subsequently, crystalise under different forms.
    It would be easy to test out this theory, though - take a small amount of water (a single water glass would suffice), dissolve sugar in it in a 1:3 ratio in favour of the water (for such a small ratio you needn't event heat it) and let it evaporate at room temperature (you can pour it in a dish, for more comfort). If the crystals that result from that are akin to those you see on the skewer - it's the sugar's fault (don't blame it, though - it's doing it's best :) ), if the crystals look nothing alike and they resemble their original state, prior to dissolution, then the culprit is something else. Unfortunately if it is not the sugar, then I wouldn't be able to help you get to the bottom of things without a lot more information about the experiment, it's conditions and the way it was conducted. :)
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