Make Your Own Crystal Growing Kits
add a bit of interactive educational fun to the classroom
Crystals are fun and easy to grow, and it’s a great introduction to the sciences. Anyone can grow beautiful crystals, you just need a little bit of time and a couple of ingredients. I decided to write this post because my son’s teacher wanted to add some crystal growing experiments to her class. There are tons of examples online, but I wanted to add my own personal touch.
There are only a few steps involved in crystal growing, and the procedures are nearly identical for all the different types of crystals described below. All the crystallization experiments described here form crystals from the precipitation by evaporation. This is not the only way to grow crystals, but it is one of the simplest types to do.
To make all the types of crystals listed in the main sections, use the sugar instructions and apply it to the other materials.
I’m targeting K-8 teachers for this post. Add comments if you would like to see other crystal systems, or more experimentation details. I have personally done all these experiments, so I know they work. Also, all the sizes of containers, measurements of ingredients, and lengths of time are approximate. You are guaranteed crystals if you follow the just the basic instructions, but quality and time-frame of crystallization will vary. It just takes a little practice to get good.
- hardware: pot (2qt), stirring spoon, measuring cups, measuring spoons, jars (12oz)
- optional: sticker thermometer (where to buy), tweezers, thread, superglue, food coloring, pencil/stick (for suspending thread in jar).
- chemical: water (tap water is fine), reagents (sugar, salt, alum, etc. as described in the sections below)
- Please follow all warning labels on the ingredients you buy. If in doubt, don’t eat it.
How does crystallization happen?
Briefly, the crystals that you will be growing are organized arrays of atoms. Let’s start with sugar as an example. When the water that separates all the atoms evaporates away, it leaves behind a higher concentration of sugar molecules. This creates a great deal more interaction between all the sugar molecules, and this increases the energy of the system. As the water continues to evaporate, there becomes a point where there is no more free water to keep the sugar dissolved. At that point, the sugar undergoes a phase state change from being freely moving molecules dissolved in liquid, to a solid crystal. This crystallization process releases energy in the form of heat, and once again, the energy of the system decreases. So crystallization represents a minimum energy state for the substance, which is why crystals are so stable. Once one crystal begins to form in the solution, other molecules begin to self-assemble on the crystal, and the crystal grows larger and larger.
Crystals are ordered patterns of atoms, where this pattern can repeat itself infinitely in all directions. The arrangement of the patterns are dictated by the bonding geometry and electronic structure of the atomic assemblage. This pattern of atoms always has a symmetry, and this symmetry is shown in form of the crystal. So, if the pattern of atoms in a crystal form a cubic array, then the form of the crystal that you hold in your hand will be a cube, or a derivative of the cube form (like a tetrahedron).
SUGAR CRYSTALS — edible rock candy 😉
These crystals are a great starting point because there is the obvious reward of eating them in the end.
- Make a starting solution. Dissolve as much of the reagent (sugar in this case) into water as possible. You will need to get the solution completely saturated with sugar. To do this, keep pouring sugar into a constantly stirring (gentle hand stirring is fine) pot of near boiling water. When the sugar doesn’t seem to dissolve anymore, and just sits at the bottom of the pot, you have reached saturation, and you can stop adding sugar. We will call this solution the ‘saturated solution.’
- Pour (gently)the saturated solution into a jar (or multiple jars, for larger classes) while trying to avoid getting any undissolved sugar into the jar. The undissolved sugar will act as a seed, and crystallization will start at that point. If you have a lot of undissolved sugar particles in the jar, then you will likely get a lot of small crystals.
- Cover the jar with plastic, but allow some air ventilation, something like 1/8 of the opening diameter should be open, but this can vary considerably. Let the jar sit undisturbed for as long as you can stand. Crystallization should happen within a few days, but could take several weeks.
- variation 1: Tie a thread to a pencil, lay the pencil across the top of jar, and let the thread hang into the center of the jar, away from the sides, wherethe thread goes down approximately 2/3 into the liquid. This thread will act as a nucleation point, and crystals will form on the thread. Then the crystals can easily be removed from the jar. Adding a small seed crystal to the end of the thread (use a tiny amount of super glue) will enhance crystallization at the point of the seed, and seeds are good way of trying to grow very large crystals. You can use any sugar crystal as a seed.
- variation 2: Add some food coloring. This will change the color of the crystal as the food coloring dye gets incorporated into the crystal.
- Remove the crystals from the fluid. You can use a spoon, fork, stick, fingers, etc., and be careful not to break the crystals. Sometimes the crystals can be difficult to remove as the crystals can stick to the sides of the jar.
- You can now call the experiment over, or you can take one of the large single crystals (found anywhere in the jar) and use it as the seed for the next batch. By repeating this process, you can get really big crystals, and even change the color by adding a different color food dye (which will create a zoned color change effect).
A great way to add a bit more science is to keep records on the crystallization process. How much detail you want to record is totally up to you and the class. You can also not record any data, and just watch the crystals grow over the course of several days, but recording data gives you something to do while this is happening.
- Weigh the saturated solution at the start of the experiment, and be sure to subtract the weight of the jar. Weigh the mother liquor (the solution left over and separated from the crystals) at the end of the experiment too.
- Weigh the amount of reagent (sugar in this case) you add to the water, to create the saturated solution.
- Mark the outside of the jar for the starting fluid level. Over the course of days, keep measuring and marking the new fluid levels as the water evaporates. Keep recording fluid levelsand crystal sizes.
- Record the start day of crystallization, and where the crystals are forming in the jar.
- If using food coloring, record any color change of the fluid as the crystals are growing.
- Record temperature of the fluid daily, and record the temperature of the air daily. Fluctuations in temperature could change crystallization rates and possible the crystal size and shape. Crystallization is an exothermic reaction (releases heat). If possible, record the humidity of the air too. Humidity will influence evaporation rates.
- Store records in a notebook. Depending on detail of your measurements, you can also plot the data on a graph and quantify the changes over time (water level vs. days, temp vs. days, temp. vs. water level, etc.)
Growing a crystal geode
Here, you need to make a hollow chocolate ball. Once the ball has thick walls, you then pour in the sugar saturated solution (as described above) into the ball. (Use the link in the box below.)
Follow the steps for sugar, but instead use borax, readily purchased in the laundry section of the grocery store. Here are some other ideas.
This is normal table salt, and is edible only when you add it to regular food. Halite is cheap and easy, and will forms cubic crystals. Sometimes you can get ‘hopper’ crystals, which are unique crystal growth features. I sometimes add a little sodium sulfate (about 1/8 of the amount of salt added) to the solution which seems to promote hopper crystal formation. Sodium sulfate is commonly used as a mordent for fabric dying, and can be obtained from Amazon or other online shops.
These are epsom salts that you can get by the pound at a the grocery or drug store. They form beautiful crystals, but they will dehydrate in a few days. Epsom, or epsomite, is a magnesium sulfate heptahydrate (having 7 water molecules per formula unit), and will transform to 5 hydrate when left in the dry air. It’s still crystalline after this slightly dehydrated form, but will have undergone a transformation process and may look white. The white color is caused by the interface of light with all the cracks and faults throughout the dehydrated crystal. As the crystal dehydrates, it shrinks a little, and this adds strain to the crystal. The stress is relieved by cracking and breaking.
COPPER SULFATE CRYSTALS (chalcanthite)
You can get copper sulfate at just about any gardening store. These crystals can get very large with near-perfect crystal forms. Plus they are a beautiful electric blue.
Chalcanthite is the natural form of the copper sulfate pentahydrate crystal.
In 2008, artist Roger Hiorns took crystal growing to a whole new level. In his work titled Seizure, an old apartment was filled with liquid copper sulfate and left to crystallize for months. The result is an amazing mass of synthetic calcanthite crystals that cover everything.
This is pickling salt, which you can buy at the grocery store. Alum forms very nice square bipyramid (i.e. octohedron) crystals. Just follow the sugar instructions.
Alum is hydrated potassium aluminum sulfuate, and defintely not edible.
More Crystallization Ideas
A little more of a challenge. Large crystals can be grown, but seem to only grow big at around 40 degrees Fahrenheit. Crystals are very temperature sensitive, but can be grown very rapidly.
- In my experience, a good way to start is with hanksite crystals. These can be purchased online, or you can collect them yourself at Searles Lake in California, during Gem-O-Rama.
- Dissolve the hanksite crystals in water. You are not making a saturated solution in this case, you just need to dissolve the crystals completely. Making a saturated solution will actually not be good, because you’ll get too many crystals growing, and there won’t be any room to grow big crystals.
- Cool down the solution. This can be done in a refrigerator, or leave it outside in the cold. The crystals will begin to grow when it starts getting cold. The slower it cools, the better the crystal shape will be. You’ll see blade-like crystals forming, and these are the mirabilite crystals. Note that you will not re-form hanksite by this method.
A little more of a challenge as you’ll need multiple ingredients, but can be used as a good discussion for climate change, and how it can affect marine life.
- Three ingredients are needed: CaCl2 (calcium chloride), Na2CO3 (sodium carbonate), and an acid (vinegar can work).
- Dissolve a good amount of calcium chloride in a bottle (something like 1 Tbsp. per cup of water), and a good amount of sodium carbonate in another bottle.
- In a third bottle (or cup), add the sodium carbonate, then mix in the calcium chloride, and you instantly form CaCO3 (calcite) and NaCl (halite, which stays dissolved until the water evaporates).
- The formation of calcite in this water is analogous to the formation of calcite shells by animals in the ocean.
- If the oceans become acidic, then the calcite dissolves. Now add the acid (vinegar). Mix in a little acid to the bottle with calcite, and then stir. Now watch the calcite dissolve. You also might see CO2 (carbon dioxide) bubbles form and escape the water. This is analogous to the process of ocean acidification.
SUCCINIC ACID CRYSTALS
from Baltic amber using dry distallation.
NOTE: I haven’t tried this one yet, but will soon.
Aaron Celestian is the Mineralogy Curator at the Natural History Museum of Los Angeles. He researches how minerals interact with their environments and with living things, and how those minerals can be used to solve problems like climate change, pollution, and disease.