How to Make Glow-in-the-Dark Alum Crystals

Glowing Crystals You Can Grow in Your Kitchen

You can grow alum crystals with a phosphorescent chemical so they glow in the dark.
You can grow alum crystals with a phosphorescent chemical so they glow in the dark. Gianluca Gerardi / EyeEm / Getty Images

Alum crystals are among the quickest, easiest, and most reliable crystals you can grow. Did you know you can make them glow in the dark by adding a common household ingredient to the crystal growing solution?

Glow in the Dark Alum Crystal Materials

  • Fluorescent highlighter pen (I used yellow, but you can use another color for different colored glowing crystals. Check to make certain the highlighter will glow under an ultraviolet or black light. Pretty much all yellow highlighters glow, as do most other colors. Many blue pens will not glow.)
  • Alum (sold as a pickling spice)
  • Water

Grow Glowing Alum Crystals

  1. Carefully cut open the highlighter and remove the strip that contains the ink. You may want to wear gloves since highlighter can stain your fingers.
  2. Pour 1/2 cup of hot tap water into a clean container.
  3. Squeeze the highlighter strip into the water to color it with the fluorescent ink. Discard the ink strip when you are finished.
  4. Slowly stir in alum, a little at a time, until it stops dissolving.
  5. Loosely cover the jar with a coffee filter or paper towel (to keep dust out) and allow the jar to sit undisturbed overnight.
  6. The next day, you should see small alum crystals at the bottom of the container. If you don't see crystals, allow more time. You can let these crystals grow, though they will compete with each other for material. Alternatively, you can use one of these crystals to grow a large single crystal.

Growing a Large Single Crystal

  1. If crystals are present, pour the alum solution into a clean jar. Collect the small crystals, which are called seed crystals.
  2. Tie nylon line around the largest, best-shaped crystal. Tie the other end to a flat object (e.g., popsicle stick, ruler, pencil, butter knife). You will hang the seed crystal by this flat object into the jar far enough so that it will be covered in liquid, but won't touch the bottom or sides of the jar. It may take a few tries to get the length just right.)
  3. When you have the right string length, hang the seed crystal in the jar with the alum solution. Cover it with the coffee filter and grow a crystal.
  4. Grow your crystal until you are satisfied with it. If you see crystals starting to grow on the sides or bottom of your jar, carefully remove your crystal, pour the liquid into the clean jar, and put the crystal in the new jar.

Making the Crystal Glow

When you are satisfied with your crystal, remove it from the crystal growing solution and allow it to dry. Just shine a black light (ultraviolet light) on the crystal to make it glow. Depending on the ink you used, the crystal may glow under fluorescent light or sunlight.

You can display your crystal or store it. You can wipe dust from a display crystal using a cloth, but avoid dampening it with water or else you will dissolve part of your crystal. Crystals kept in storage may be wrapped in paper for added protection from dust and changed in temperature and humidity.

True Glow in the Dark Crystals

If you want the crystals to really glow in the dark (no black light), then you stir phosphorescent pigment into a solution of alum and water. Usually, the glow will remain on the exterior of the crystal rather than get incorporated into the crystal matrix.

Alum crystals are clear, so another way to make the crystals glow is to mix phosphorescent pigment with clear nail polish and simply paint regular alum crystals. This also protects the crystals from damage by water or humidity, preserving them.

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Your Citation
Helmenstine, Anne Marie, Ph.D. "How to Make Glow-in-the-Dark Alum Crystals." ThoughtCo, Feb. 16, 2021, Helmenstine, Anne Marie, Ph.D. (2021, February 16). How to Make Glow-in-the-Dark Alum Crystals. Retrieved from Helmenstine, Anne Marie, Ph.D. "How to Make Glow-in-the-Dark Alum Crystals." ThoughtCo. (accessed April 1, 2023).