Crystal-growing kits are a classic DIY science purchase, but you don’t need a kit for a great crystal growing experiment. With Epsom salt or alum from the grocery store, warm water, and a little patience, you can grow a sparkling crystal garden overnight. Then you can go further and grow one large, clear single crystal over a week or two.

Like every experiment in the 25Pages collection, this one follows a simple loop: predict, test, record, explain.

Make a prediction first

Before you start, answer two questions in your notebook. Will crystals grow faster in a cold refrigerator or at room temperature? Will fast-grown crystals be bigger or smaller than slow-grown ones? Write down your reasoning, too.

What you need

• Epsom salt (magnesium sulfate), about 1 cup
• Alum (potassium aluminum sulfate), sold in the spice aisle for pickling. Optional, for part 2.
• Distilled water, warm but not boiling
• Clean glass jars and a shallow bowl or saucer
• A spoon, a pencil or stick, and nylon fishing line
• A magnifying glass
• Labels and a notebook

Part 1: Fast Epsom salt crystals (3 hours)

This method is adapted from the Penn State Materials Research Science and Engineering Center.

1. Start with 1 cup of warm distilled water, not boiling.
2. Add Epsom salt a spoonful at a time, stirring until each spoonful dissolves.
3. Keep going until no more will dissolve. That usually takes about 1 cup of Epsom salt.
4. Let it sit for a couple of minutes so the undissolved salt settles.
5. Slowly pour the clear liquid into a shallow bowl, leaving the undissolved salt behind.
6. Place the bowl in the refrigerator for about 3 hours, then check for needle-like crystals.

Comparison test: pour a second bowl from the same solution and leave it at room temperature. Compare crystal size and shape after 3 hours and again after a day.

Part 2: Grow one large alum crystal (1–2 weeks)

Alum naturally forms clear, eight-sided (octahedral) crystals. The University of Otago Department of Chemistry and the International Union of Crystallography both describe a seed-crystal method:

1. Make a saturated solution. Dissolve alum in hot water until no more will dissolve. Otago gives roughly 15 g per 25 mL as a guide, which varies with temperature.
2. Grow seed crystals. Pour some into a shallow dish and leave it overnight somewhere cool, dark, and undisturbed. Small crystals form as it cools and evaporates.
3. Choose your seed. With a magnifying glass, pick one crystal that is clear, sharp-edged, and single, not two stuck together.
4. Hang the seed. Tie it to nylon fishing line and hang it from a pencil laid across a clean jar.
5. Make fresh growing solution. Prepare a new warm saturated solution, let it cool to room temperature, and pour it into the jar so the seed hangs in the middle, not touching the sides or bottom.
6. Wait and record. Cover the jar loosely with paper to keep dust out. Measure and photograph your crystal every two days.

If small crystals start crowding the bottom or sides, move your seed to a fresh batch of solution. Otherwise they compete for the dissolved alum.

The science: supersaturation and nucleation

Hot water can hold more dissolved solid than cold water. When a saturated hot solution cools or evaporates, it briefly holds more material than it normally can. That’s called a supersaturated solution. As the Otago chemists put it, the solution then tends to get rid of the extra dissolved solid, and if conditions are right, it deposits that solid onto a seed crystal.

The first tiny crystals have to start somewhere, a process called nucleation. According to MIT’s Department of Chemistry, nucleation can be triggered by vibration or particles. If it happens too quickly, you get many small crystals instead of a few large ones. That’s why slow, undisturbed conditions grow bigger crystals, and why the refrigerator bowl in Part 1 gives a crowd of thin needles.

Safety notes

• Adults should handle hot water. Use warm, not boiling, water for Epsom salt.
• Don’t eat the crystals or drink the solutions. Label jars clearly and keep them away from food and pets.
• Wear safety glasses when stirring, and wash your hands after handling alum or Epsom salt.
• If you use superglue to attach a seed, use it in a ventilated area and keep it off your skin.
• Dispose of leftover solution down the drain with plenty of water, unless local rules say otherwise.

Check your prediction

Did the refrigerator bowl grow faster? Were those crystals smaller and more crowded? Use the words “supersaturated” and “nucleation” to explain what happened. If you can teach it to someone else, you’ve mastered it.

Want more experiments like this?

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