How to Grow a Big Alum Crystal

Potassium Alum Crystal
Alum readily grows large clear crystals. (Ude, CC 3.0)

Alum is a great choice for a beginner crystal-growing project because it’s safe, readily available, and easy to grow into big crystals. Alum [KAl(SO4)] is sold as a pickling spice in the grocery store as a white powder or white crystalline granules. When dissolved in hot water, it grows into clear, prismatic crystals that resemble diamonds.

Alum Crystal Materials

Technically, all you need to grow alum crystals is alum, hot water, and a container. You don’t actually need to measure the water and alum, but this “recipe” is a good starting point:

  • 2-1/2 tablespoons alum
  • 1/2 cups hot tap water
  • Nylon fishing line
  • Pencil, butter knife, or ruler
  • Clean jars
  • Spoon
  • Paper towel or coffee filter

If you want a particular crystal shape, add a bit of monofilament nylon fishing line and a pencil or butter knife, so you can hang your crystal in the liquid. You can use sewing threat instead of nylon, but it acts as wick, causing crystal growth along its length.

Grow Alum Crystals

  1. Pour the hot water into a clean jar.
  2. Stir in the alum, a bit at a time, until it stops dissolving. You may not need to add the full amount. This makes a saturated solution.
  3. Loosely cover the jar with a paper towel or coffee filter to keep out dust. Let the jar sit in an undisturbed location.

Crystals start to appear within a couple of hours. If you like, you can let them grow until you are satisfied with their size. Then, pour off the liquid and enjoy the crystals.

If you want a large single crystal:

  1. Only let the crystals grow for a few hours to overnight.
  2. Pour the liquid into a second clean container. Collect the crystals and examine them.
  3. Select one or more crystals to be seed crystals. Either drop a seed crystal into the jar of liquid or tie fishing line around it and hang it in the liquid. Take care that the seed is covered by the liquid and is not touching the sides of the container.
  4. Let this crystal grow until it reaches the desired size. During this time, remove any other crystals that appear because they will compete with your seed crystal for resources.
  5. When the crystal is ready, remove it from the liquid and set it on a paper towel or coffee filter to dry.
  6. If you like, you can coat the dry crystal with a coat of clear nail polish to preserve it.
Common Alum Crystal Shapes
Alum crystals can take a number of forms, depending on growing conditions.
Growing a Large Crystal
To grow a large crystal with the octahedron shape, suspend a seed crystal from a nylon thread in the solution. ( André Lütken, 1878)

Alum Crystal Shapes

Under ideal conditions, alum forms a perfect octahedral crystal. The best chance of getting this shape comes from suspending a seed crystal in the growing solution. Sometimes the top and bottom of the octahedron are truncated. Crystals grown on the bottom of a container typically form truncated octahedrons, pyramids, cubes with a couple of extra flat faces, and masses of crystals.

So, you may wonder how to select for a particular shape.

  • Grow seed crystals and select the desired shape.
  • For the perfect octahedron, tie a line around an alum seed crystal and suspend it in liquid so it can grow on all sides.
  • Grow flatter alum crystals on the bottom of the container.
  • Control temperature and humidity (evaporation rate). Basically, this controls the rate the crystal dissolves versus its rate of adding mass. If an alum crystal grows too quickly (the liquid cools or evaporates quickly), you’re more likely to get a cloudy crystal than a clear one.
  • Expect some trial and error. You may need to grow several alum crystals to get the shape you want. You can suspend multiple seeds in a solution, as long as they aren’t touching each other or the container. Take notes of growing conditions so it’s easier to identify the factors that affected the crystal growth.
  • Save leftover liquid and “mistakes.” If you don’t like any of your crystals, dissolve them in a bit of hot water to start the process again.