How to Grow a Big Alum Crystal

Alum – potassium aluminum sulfate – KAl(SO4)2 is one of the more popular crystals to grow. This may be because it makes a nice octahedral crystal that is visually appealling, or because it isn’t terribly toxic. It probably also has a lot to do with its very convenient solubility curve. The biggest drawback, in my opinion, is that the crystal will be colorless.

Here are solubilities for anhydrous alum at a few temperatures:

Temperature (Celsius) Solubility (grams alum per 100 grams water)
20 12.0
50 36.8
70 94.4
80 195.0

If you are working with a hydrated alum sample (indicated on the label), multiply each solubility by (N*18.02 + 258.2)/258.2 where N is the number of water molecules in the formula. (258.2 is the molar mass of alum, 18.02 of water)

To grow your crystal, you need to make a supersaturated solution of alum. This is done by making a near-saturated solution of alum at a high temperature and allowing it to cool. If you want to grow a single suspended crystal, hang a small “seed” crystal from a thread. It should be near the center of the container to avoid merging with any smaller crystals that may form at the surfaces.

If you need detailed steps, try this:

1. Choose your container (probably a beaker) and fill it 2/3 – 3/4 full.

2. Measure the volume of the water and calculate its mass (1 mL ~ 1 g), or if you were clever, you weighed the beaker before filling it, and you can weigh it again to see how much the water added.

3. Multiply the grams of water by the solubility at 80 degrees C (or the solubility at a lower temperature if you don’t want to use as hot a temperature) and measure out about that amount of alum. (It’s okay to use less, but don’t go over.)

4. Heat your beaker of water to 80 degrees C (use a thermometer) carefully. You don’t want it to boil.

5. Stir in your alum while the water is hot, keep stirring until it is all dissolved. Remove the solution from the heat.

6. Tie your seed crystal to a thread (sometimes you can get by with just rolling the thread in small alum crystals/powder).

7. Tie the other end of the thread to a pencil, glass rod, or something else that can reach across the top of the beaker. Make the string long enough that the crystal will hang halfway between the top of the water and the bottom of the beaker.

8. Suspend the seed crystal in the hot solution.

9. Cover the beaker and let it sit for days/weeks. Keep an eye on it, but don’t disturb it until you’re ready to take the crystal out.

10. When you’re happy with the crystal you’ve made, or wish to begin again, just lift out the crystal using your string.

Hint – Grow an even bigger crystal by repeating the procedure, but using your current crystal as the seed crystal.

Hint – Crystal growing takes a lot of patience. You can’t rush it when striving for a big crystal. On the plus side, these crystals will last. My father helped me grow one when I was 8 (23 years ago) and I still keep it hanging in my room. You can also help protect it from moisture by spraying it lightly with hairspray.