Grow [LARGE] Alum Crystals

A quick search on the internet provided me with several sets of instructions for growing alum crystals, but none of them worked the way we had hoped. We kept ending up with big clusters of crystals, but not a single big crystal. After a little trial and error, however, we figured out how to grow some big ones!

What You Need:
alum (from the spice isle of your grocery store)
2 jars

What You Do:

  1. Pour 1/2 cup of hot tap water into a clean jar.
  2. Slowly stir in 2 1/2 tablespoons of alum, a little at a time, until it stops dissolving. You likely won’t need the whole amount – just enough to saturate the water.
  3. Loosely cover the jar with a coffee filter or paper towel to keep dust out. 
  4. Allow the jar to sit undisturbed overnight, or even for a couple days.


  1. The next day (or a couple days later,) pour off the alum solution from the first jar into another clean jar. (Do not pour the tiny crystals in with the liquid, yet.)
  2. You will see several small alum crystals at the bottom of the jar and along the sides. These are the ‘seed’ crystals which you will use to grow big crystals.
  3. Pick 1-3 of the largest ‘seed’ crystals and place them in the bottom of your fresh jar of alum solution.


  1. Repeat part 2, moving your growing seed crystals to the fresh jar. 
  2. Pick off any tiny crystal that have affixed themselves to your big crystal.
  3. If the alum water gets too low, make more solution and add it to the jar. (Wait until it cools.)
  4. Don’t be concerned if it seems like your crystal is not growing properly on one side – just turn it over and it will repair itself. 

When it is the size you want, store it in an airtight container.

You can see the size difference between our tiny seed crystal (left), and our finished crystal (right).

These crystals have gotten cloudy due to being exposed to the air for a couple months.

I hope you try this out and let me know how it worked!

Special Study – Minerals
Natural History Rotation

2 thoughts on “Grow [LARGE] Alum Crystals

Leave a Reply

Your email address will not be published. Required fields are marked *