You have to make sure you get the right kind of alum powder, potassium aluminum sulfate. Some commercial alum contains potassium, and some doesn’t. If you get the kind without potassium, your crystals will not grow.
Eggshells or plastic eggs
We used a combination of real eggshells, broken in half, and plastic Easter eggs.
You’re going to coat the inside of every eggshell or plastic egg with glue, so you’ll need a lot of it.
You need this to paint on the glue.
Also: A box, bowls and whisk to mix, and a measuring cup.
Before You Can Grow Crystals
The day before you want to grow crystals, you have to prepare your eggs. You have to do it the day before to allow the glue to dry.
First, paint the eggshells with a thin layer of glue. If you want crystals to grow around the edge, make sure to paint a little glue on the outside of the rim of the eggshell.
While the glue is still wet, sprinkle the egg with alum powder.
Don’t neglect the glue on the outside of the egg. You can turn the egg over into a small bowl of alum powder to get this part.
Let those eggs dry overnight.
If you want to use dye, stir it into 2 cups of water.
Heat the water to almost boiling.
Pour 3/4 cup of alum powder into the water and stir it for longer than you think is necessary. If there are any crystals in the bottom, you need to either stir longer or reheat the solution or both.
The idea here is that you want a saturated solution. In other words, you want the water to be holding as much alum powder as it can possibly hold.
There’s a fine line between saturated and crystals sitting around in the bottom of the bowl.
If there are crystals in the bottom of the bowl, they will draw the alum away from your geodes, and the crystals in your geode won’t get as big.
If there’s anything else in the bowl – dust, bits of glue, unidentified floaters – strain the solution with a strainer. It’s important that there aren’t extraneous bits floating around in there.
When you’re satisfied that the mixture is sufficiently saturated, drop an eggshell or two into the mixture. We did all of ours two at a time.
Now, you wait.
Leave it alone until tomorrow. Pick up the eggshells tomorrow, and see how you like them. If you want the crystals to grow bigger, carefully put the eggshells back in the solution, and wait until the next day.
If you’re happy with the size of the crystals, then there you have it! Your very own beautiful, home grown geodes.