This has to be one of my favorite science experiments! We have made crystals in different ways, but the crystal formations from one method we tried was just amazing.
Directions for these "geodes" can be found all over the internet, and these are made from Epsom salts and water. The photograph above shows the results of two different curing methods. The carton on the left was grown in the refrigerator. The one on the right was left in the kitchen window to dry out. These pictures were taken a couple of months after we made them, so the original color has faded somewhat.
The refrigerator method gave much larger, more impressive crystals. For both batches we used these basic instructions. On another blog I read the idea of putting them in the refrigerator to crystalize, but I'm afraid I can't remember what blog it was. Another hint I read from the unnamed blog was to make sure you have a good slurry. It's the slurry you want in your shells, not the thin top liquid.
Here is a close up of the air dried version. The crystals were much finer, and sparse. It actually took about a week longer for the liquid to dissolve in the air dried shells versus the ones in the fridge!
It's a simple experiment, calling only for a few ingredients and about ten minutes of actual work time. I had been saving eggshells for a few weeks prior, rinsing them and letting them dry in the kitchen window. The fun came in watching the crystals emerge over the next two weeks.