Week 4: Alum Crystals

We’re going to be growing crystals, which sounds mystical and mysterious, but is actually an entirely natural process that’s happening around us all the time. Whether it’s water crystalizing into ice over a few hours, or carbon crystalizing into diamond over billions of years, crystals are the natural process by which atoms and molecules arrange themselves into regular patterns. However, just as all atoms and molecules are not the same, neither are all crystals are the same; the shape of the atoms that make up a crystal can create a dazzling variety of shapes, colors and sizes.

Below are instructions for growing your very own crystals from alum powder, which is a common (and harmless) ingredient that can be found in the spice aisle of the grocery store. Kits are available at the community library now, but instructions are also written out below:

Supplies Needed
  • (2) 8 oz mason jars
  • 25 grams of Alum (Ammonium Aluminum Sulfate)
  • 1 short length of fishing line
  • 1 craft stick
  • water
  • spoon

Step 1: Prepare the Alum Solution

Fill one of the glass jars with roughly 4 oz of water. Place the water in the microwave for 30 seconds, until its hot but not boiling. CAUTION: glass is a terrible insulator and will become the temperature of whatever liquid is inside of it. Use a towel or potholders whenever handling the jar, and if you accidentally over-heated it, allow it to cool in the microwave before attempting to remove it.

Step 2: Supersaturate the water with alum

Once the water is hot (but not dangerously hot) stir in most of the alum powder. Try to save some back for later , and stop adding if you notice that the alum is no longer dissolving. Set the jar someplace cool and allow it to sit for 24-36 hours.

Step 3: Look for seed crystals

After 24-36 hours, you’ll notice that several small crystals have formed at the bottom of the jar. Gently pour all of the water from the solution jar into the second jar, leaving the newly formed crystals attached to the glass.

Step 4: Select a seed crystal

Our goal is to pick out the best looking one and re-dissolve the rest. Its up to you to choose your favorite one, but generally speaking, the clearest and most symmetrical seed crystal will give you the best results. Try to avoid using lump of multiple crystals fused together, these usually don’t grow as nicely. Using something like tweezers, remove you selected seed crystal and set it aside. Scrape the remaining crystals from the glass and onto a clean plate. Then, using the back of a spoon, crust the crystals back into a powder. Add your freshly ground crystals to the water in the second jar.

Step 5: Supersaturate water again

Reheat the water in the jar and re-dissolve the alum crystals (except for your seed crystal). You can add the remainder of the alum powder at this point.

Step 6: Grow the seed crystal

Tie the fishing line around the seed crystal. It can be difficult, depending on the size of your seed crystal (I had the most luck using two slip-knots). Tie the other end of the line to the craft stick. Once all of the alum has dissolved and the water has returned to close to room temperature, add the seed crystal suspended by the string. You want the seed crystal to sit in the middle of the jar, as far from any class surface as possible. This time, as the alum molecule return to a solid, rather than forming their own crystals, they’ll latch onto your seed crystal, making it grow larger and larger every day.

Step 7: Care for your seed crystal!

Every few days, check to see if any new crystals have formed on the bottom or sides of the jar. If they have, repeat steps 4-6, removing the seed crystal, pouring the water into the other jar, and scraping the undesired crystals from the glass. Those are taking alum that could be used to grow your crystal, so don’t let them! Once the seed crystal is the size you like it, pull it out and show it off! You can even put a coat of clear nail polish over them to protect them and make them shine!


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