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Grow an Alum Crystal

Updated on June 14, 2011

Grow Alum Crystals

Have you ever wanted to try your hand at growing crystals? Alum crystals are just about the easiest crystals you can grow. They also grow very quickly and are non-toxic. Alum crystals are clear, somewhat resembling diamonds, and can grow very large. All you need to grow alum crystals is a kitchen spice and some hot water, so let's get started growing crystals!

Photo: Large single alum crystal (Anne Helmenstine)

Grow Alum Crystals

Easy and Quick Crystal Growing Project

Alum crystals are so easy to grow! You will need:

  • 2-1/2 tablespoons alum

  • 1/2 cup (4 fluid ounces) very hot water

Alum is found as a pickling spice in the spice aisle of the grocery store. It will be a white powder.

All you need to do is to stir the alum into the hot water. Set the container somewhere where it won't be disturbed. You should see the beginnings of crystal growth in 1/2 hour to an hour. Big crystals will grow overnight or in a day. When you are happy with the crystals, simply scrape them out of the container with a fingernail and set them on a paper towel to dry. The crystals are non-toxic, but they aren't really a food, so don't eat them. If you expose the crystals to water, the alum will dissolve, so keep the finished crystals away from moisture.

See Alum Crystals Grow

This is a time lapse video I took of alum crystals grown using these instructions.

Alum Crystal Pyramid

Alum Crystal Pyramid
Alum Crystal Pyramid

Alum Crystal Problems

Not getting any alum crystals? There are a few reasons why this could happen:

1) Water Not Hot Enough

If your water isn't hot enough it won't be able to dissolve enough alum to produce good crystals. Be sure your water is at least 140°F. Boiling water is not necessary, but it works fine.

2) Not Enough Alum

You need 2-1/2 Tablespoons of alum per half cup (4 ounces) of water. Most cups and glasses hold more than a full cup of water, so you will need extra alum if you are filling the container all the way. One way to make sure you are getting enough alum is to stir in alum until it stops dissolving. It is fine if there is a small amount of undissolved alum in your crystal growing container.

3) Too Much Alum

While a little undissolved alum is fine, too much undissolved alum either means your water wasn't hot enough to dissolve the alum or it means you added too much alum. Too much alum hinders crystal growth because it provides a large surface area over which tiny powdery crystals can grow. If you have a lot of undissolved alum, simply filter out the extra alum by pouring the solution through a paper towel or coffee filter. The crystals should grow fine after that!

Alum Crystal Success

If you add too much alum, you can filter out the undissolved powder by pouring the solution through a paper towel or coffee filter.

Beautiful Alum Crystals!

Alum crystals can grow very large. If you are careful about preventing parasite crystals (small crystals) from growing on your large crystals, you may get something like this.

Alum Crystals as Simulated Diamonds

Alum crystals are clear so they are often found in crystal growing kits as "frosty diamonds" or "simulated diamonds." They aren't really diamonds, of course, but they do look sparkly and beautiful! You can grow alum 'diamonds' like the ones in this photo. Set a small rock into a cup or small bowl. Sprinkle a few of the small crystals from the alum crystal project onto the rock. Cover the rock with fresh alum crystal solution. Wait several hours. There you go!

Alum Crystal Diamonds

This crystal kit contains alum "diamond" and another crystal which resembles purple amethyst. The Smithsonian crystal growing kits contain all the materials you need (except water) and are very reliable.

Learn More About Growing Crystals

Did you try this easy crystal growing project?

Easy Alum Crystals Guestbook

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      anonymous 4 years ago

      Hi, I would love to grow alum crystals to make jewelry. I know it's not a good idea because they would dissolve and it might even irritate someone's skin, but I really want to try it, is there any very good varnish or something I could use to make the crystals last... forever? To make it a pendant or something? Thank you

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      anonymous 4 years ago

      I heated the 1/2 cup tap water, stirred in the 2 1/2 tbsp alum. I've been waiting for the solution in the jar to grow a crystal for 2 days now. What is wrong? I read it could take up to 3 weeks. Do I have to wait for all the liquid to evaporate?JulieB

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      anonymous 4 years ago

      I tried even boiling the 1/2 cup tap water then added the 2 1/2 tbsp alum, dissolved it and I've been waiting 2 days so far and NO crystals grew. I have read where it could take up to 3 weeks for it to occur. Do I have to wait for all the water to evaporate? help! julie