How to Grow Crystals

  • What is a crystal?

    A crystal is a solid that consists of various atoms, ions, or molecules arranged in a uniform three-dimensional repeating pattern. This results in the material having a specific shape and color, and having other characteristic properties. Diamond (used in jewelry and cutting tools) is an example of a crystal. It is made of pure carbon. Salt and sugar are also examples of crystals.
    Recrystallization is a process that has been used to purify solid material by dissolving the solid (called a solute) in an appropriate liquid (called a solvent) and then having the material precipitate out of solution in crystalline form. Depending upon conditions, one may obtain a mass of many small crystals or one large crystal.

    Crystals are characterized by type, shape, form, clarity, and color.

  • What is the crystallization material and how can I get it?

    Middle and High schools and Teachers

    Middle school students and home-schooled youths (ages 11-13/14) will grow crystals of alum, KAlSO4.12H2O (potassium aluminum sulfate, dodecahydrate).

    High school students home-schooled youths (ages 14-18) will grow crystals of cupric sulfate pentahydrate CuSO4.5H2O.

    Middle school teachers will grow crystals of potassium dihydrogen phosphate (KH2PO4; KDP).

    High school teachers will grow crystals of potassium dihydrogen phosphate (KH2PO4; KDP).

    The  2019 Handbook describes the crystallization procedures applicable to all three compounds.

  • How to grow crystals

    The crystal growing period is from March 1 to April 30, 2019. The judging will take place on May 10, 2019 by a panel of judges. The following crystallization information is also available as a PDF.

    RULE 1: The maximum amount of starting material that may be used for each given crystal is limited to 100 g.
    RULE 2: So that all students have an equal preparation time, crystal production must conclude within six weeks after receipt of starting material.

    First Stage: Grow a Seed Crystal

    Cupric sulfate:

    Check out a video by the X-ray Laboratory Teaching Assistant Brian Dolinar about cupric sulfate crystal growing.


    Check out the three videos by Prof. Jason Benedict (University of Buffalo): introduction, part 1(how to grow a seed crystal), part 2 (how to grow a large crystal).

    Watch a YouTube video that tells you how to grow a crystal with Johanna!

    The idea is to grow a single crystal, not a bunch of crystals. You will first need to grow a small perfect crystal that will become your seed crystal, around which you will later grow a large crystal. It is therefore essential to avoid excessively rapid growth, which encourages the formation of multiple crystals instead of a single crystal.

    What You Need

    1. Substance to be crystallized
    2. Distilled or demineralized water
    3. A shallow dish (e.g., Petri)
    4. Heating plate or stove
    5. Fishing line (1 to 2 kg strength)
    6. A small wood rod (e.g., popsicle stick)
    7. A magnifying glass (optional)

    Important Things to Know

    1. How much substance you have to work with, which you can determine by weighing it on a balance.
    2. The solubility of the substance in water at room temperature, which you can obtain from a chemistry reference book.
    3. It would also be useful to know the solubility of the substance at elevated temperatures, which is information that may also be available in a reference book such as Handbook of Chemistry and Physics, 45th Ed (1964-5).

    What to Do

    1. Warm about 50 mL (1/4 cup) of water in a glass container.
    2. Dissolve a quantity of the substance to produce a saturated solution at the elevated temperature.
    3. Pour the warm solution into a shallow dish.
    4. Allow the solution to cool to room temperature.
    5. After a day or so, small crystals should begin to form.
    6. Remove some of the crystals.
    7. With a magnifier select a beautiful and transparent small crystal. This will be your seed crystal.
    8. Tie the seed crystal with the fishing line by using a simple overhand knot.
    9. Suspend the seed crystal in a shallow (1 to 2 mm deep) small volume (about 1 to 2 mL) saturated solution (for example, in a cover or a Petri dish) for some time (1 to 2 days).
    10. Check with the magnifier that the seedling crystal is well-fixed to the line by its beginning growth. This step is very important because one can lose several days of growth if the ‘beginning growth’ is not regular or not along the structure of the seedling crystal. It is worth checking properly before going on with the regular crystal growth.

    Second Stage: Grow a Large, Single Crystal

    Now you are ready to proceed with the preparation of a large single crystal. Once you have mastered this step, you may be interested in trying to grow single crystals in the presence of introduced ‘impurities” that may give different crystal colors or shapes. In recrystallization, one tries to prepare a solution that is supersaturated with respect to the solute (the material you want to crystallize).

    There are several ways to do this. One is to heat the solvent, dissolve as much solute as you can (said to be a “saturated” solution at that temperature), and then let it cool. At this point, all the solute remains in solution, which now contains more solute at that temperature than it normally would (and is said to be “supersaturated”). This situation is somewhat unstable. If you now suspend a solid material in the solution, the “extra” solute will tend to come out of solution and grow around the solid. Particles of dust can cause this to occur. However, this growth will be uncontrolled and should be avoided (thus the recrystallization beaker should be covered). To get controlled growth, a “seed crystal”, prepared from the solute should be suspended into the solution. The supersaturation method works when the solute is more soluble in hot solvent than cold. This is usually the case, but there are exceptions. For example, the solubility of table salt (sodium chloride) is about the same whether the water is hot or cold. The rate at which crystallization occurs will affect crystal quality. The more supersaturated a solution is, the faster growth may be. Usually, the best crystals are the ones that grow SLOWLY.

    Thus, if you heated the solvent to near the boiling point to get a highly supersaturated solution on cooling back to room temperature, crystals may start to form before the solution had completely cooled. This is where the “art” of science comes into play. One has to experiment a bit to get the right conditions. A second way to get supersaturation is to start with a saturated solution and let the solvent evaporate. This will be a slower process.

    The above will apply to most situations. It is necessary to match the proper solvent with a given solute.

    WARNING: the solubility of some salts is quite sensitive to temperature, so the temperature of recrystallization should be controlled as best you can. There have been reports in the past of students having a nice big crystal growing in a beaker on a Friday, the room temperature rising in a school over the weekend, and by Monday morning the crystal had totally gone back into solution. Consider insulating your crystallization vessel inside a Styrofoam box.

    What You Need

    1. Substance to be crystallized
    2. A seed crystal of the substance to be crystallized on a fishing line
    3. Distilled or demineralized water
    4. A small wood rod or popsicle stick
    5. Thermometer
    6. Balance
    7. Plastic or glass container
    8. Heating plate
    9. Beaker of 2 to 4 liters volume
    10. Thermostated bath (optional)
    11. Slow revolution motor (1 to 4 rotations per day) (optional)

    Important Things to Know

    1. How much substance you have to work with, which you can determine by weighing it on a balance
    2. The solubility of the substance in water at room temperature, which you can obtain from a chemistry reference book
    3. It would also be useful to know the solubility of the substance at elevated temperatures, which is information that may also be available in a reference book.

    How to Prepare a Supersaturated Solution

    To grow your large, single crystal, you will need a supersaturated solution. The amounts of substance and water to be used will depend upon the solubility at room and elevated temperatures. You may have to determine the proper proportions by trial and error (just like the first scientists did).

    Method One

    1. Place about double the amount of substance that would normally dissolve in a certain volume of water at room temperature into that volume of water. (e.g. if 30 g (about 1 oz) of X dissolves in 100 g (mL) of water at room temperature, place 60 g of X in 100 mL of water.) Adjust the proportions depending upon how much material you have. Use clean glassware.
    2. Stir the mixture until it appears that no more will go into solution.
    3. Continue stirring the mixture while gently warming the solution.
    4. Once all of the substance has gone into solution, remove the container from the heat.
    5. Allow the solution to cool to room temperature.
    6. You now have a supersaturated solution.

    Method Two

    1. Select an appropriate volume of water.
    2. Warm this water to about 15-20 deg above room temperature.
    3. Add some of your substance to the warm water and stir the mixture to dissolve completely.
    4. Continue adding substance and stirring until there is a little material that won’t dissolve.
    5. Warm the mixture a bit more until the remaining material goes into solution.
    6. Once all of the substance has gone into solution, remove the container from the heat.
    7. Allow the solution to cool to room temperature.
    8. You now have a supersaturated solution.

    Now you can grow your wonderful crystal!

    Since the solubility of a substance varies a lot with temperature, it is very important to control the temperature carefully. If the room temperature is stable then you might be able to leave your apparatus out in the open. If it can vary by even only a degree or two, then it may be necessary to place the apparatus into a thermostated bath set to a few degrees above room temperature (if available, but this is not mandatory). You could also place the growing apparatus inside a Styrofoam or picnic cooler.

    Also, for the seed crystal to grow, it is absolutely necessary that the solution never be unsaturated at the temperature of the experiment (usually the room temperature).

    Getting Started

    1. Carefully suspend your seed crystal from the stick into the supersaturated solution, being careful not to let the crystal touch the bottom of the container.
    2. Cover the container in which the crystal is growing. This is to:
      • keep out dust, and
      • reduce temperature fluctuations.

      This can be done with plastic wrap or aluminum foil. If you want to allow the solvent (typically water) to evaporate (see step #4b below), then use porous paper (e.g., filter paper or coffee filter).

    3. Observe the crystal growth. Depending upon the substance, the degree of supersaturation and the temperature, this may take several days before the growth slows down and stops. A couple of different things can happen at this stage. The questions and answers below can help you.
      • Why does the crystal stop growing?
        A crystal will only grow when the surrounding solution is supersaturated with solute. When the solution is exactly saturated, no more material will be deposited on the crystal. (This may not be entirely true. Some may be deposited, however an equal amount will leave the crystal surface to go back into solution. We call this an equilibrium condition.)
      • Why did my crystal shrink/disappear?
        If your crystal shrank or disappeared, it was because the surrounding solution became unsaturated and the crystal material went back into solution. Unsaturation may occur when the temperature of a saturated solution increases, even by only a few degrees, depending upon the solute. (This is why temperature control is so important.)
      • How do I get crystal growth restarted?
        Step 4 below will give you the details.
    4. Resupersaturate the solution. This may need to be done on a daily basis, especially when the crystal gets larger. But first, remove the crystal.
      a) One way to resupersaturate the solution is to reduce the amount of solvent. This may be done by heating the solution for a while and then cool it to the original temperature. b) Alternatively, you can just let the solvent evaporate from the solution; this may be a slow process, but has the advantage of getting a better quality crystal. c) One can also supersaturate the solution by warming it somewhat, then adding and dissolving more solute, and finally cooling it.
    5. Each time the solution is saturated, it is a good idea to ‘clean’ the monocrystal surface, by
      • making sure the crystal is dry.
      • not touching the crystal with your fingers (hold only by the suspending line if possible).
      • removing any ‘bumps’ on the surface due to extra growth.
      • removing any small crystals from the line.

      It is a good habit to clean your hands after each manipulation.

    6. Resuspend the crystal back into the newly supersaturated solution.
    7. Repeat steps 4-6 as needed.
    8. To get improved symmetry and size, slowly rotate the growing monocrystal (1 to 4 rotations per day). An electric motor with 1 to 4 daily rotations might be difficult to find (consider one from an old humidity drum-register or other apparatus). This option becomes useful only when a monocrystal gets rather big.
  • How the Crystals Are Judged

    Each school is encouraged to submit one crystal for best quality and one for best overall. It is recognized that several crystals from a school may be of roughly equivalent overall quality. If this is the case each school may submit several crystals.

    Judging Criteria

    One single crystal will be judged only on the basis of quality as outlined below. The other single crystal will be judged on the basis of combining mass and quality factors as outlined below.

    The quality is judged by experts who will rank the crystals on a scale of 0 to 10. A score of 10 will be given to a perfect gem-quality crystal that fits the ideal crystal structure known for the chemical.

    1. The crystal is weighed, and the mass Mo recorded. The crystal must be a minimum of 0.5 g to be eligible for judging.

    2. The quality of the crystal is judged on a scale of 1 to 10, with 10 representing a perfect crystal.

    The following factors will be considered in judging quality:

    • match/mismatch with crystal type (out of 2)
    • presence/absence of occlusions (out of 2)
    • intact/broken edges (out of 2)
    • well formed/misformed faces (out of 2)
    • clarity/muddiness (out of 2)

    Total Observed Quality Qo = x.xx (out of 10)

    3. The Total Score is then determined as follows:
    Total Score = [log (Mo+1)] x Qo

    The logarithm of the mass is chosen so that large poor quality crystals don’t swamp out smaller good quality crystals. The value 1 is added to the mass so that crystals weighing less than 1 g get a positive score.

    A 100 per cent yield crystal made from 100 g (Mt) that scores a perfect 10 on quality (Qt) would get a theoretical maximum of:
    [log (100+1)] x 10 = 20.01

    The actual score is expressed as a percentage of the maximum. The crystal with the highest Overall Score is the winning crystal.
    100 x {[log (Mo+1)] x Qo} / {[log (Mt+1)] x Qt} = Overall Score %

    For example, the best overall crystal in the 2001 contest with 150 g starting material weighed 46.53 g and had a quality of 8.65. Its overall score was:
    100 x {[log (46.53+1)] x 8.65} / {[log (150+1)] x 10} = 66.6%

    This score is nearly an absolute score that could be used to judge different types of crystals grown from differing amounts of starting material.

  • Safety

    Potassium dihydrogen phosphate is safe but the usual safety precautions should be exercised. Gloves and goggles are recommended. The SigmaAlrdrich Safety Data Sheet for potassium dihydrogen phosphate provides exhaustive safety information.

    Potassium aluminum sulfate, dodecahydrate is safe but the usual safety precautions should be exercised. Gloves and goggles are recommended. The SigmaAlrdrich Safety Data Sheet for potassium aluminum sulfate, dodecahydrate provides exhaustive safety information.

    Cupric sulfate pentahydrate can be purchased  from Flinn Scientific or Farm & Fleet. The SigmaAldrich Safety Data Sheet for cupric sulfate pentahydrate  provides exhaustive safety information.

    The crystallization materials for the competition are purchased from Flinn Scientific.