Crystal growing experiments

Alum or copper sulphate. Alum (from the spice aisle or a chemist) grows clear, sharp octahedral crystals and is forgiving, which is why I started there. Copper sulphate grows stunning blue crystals but needs more care. Sugar and salt crystals work too and are completely safe for growing with kids, though they grow more slowly and less dramatically than alum.

Days to weeks, depending on the crystal and how big you want it. A small seed crystal forms overnight as a hot saturated solution cools. Growing it into a large, well-formed single crystal means suspending that seed in fresh saturated solution and waiting patiently, sometimes for weeks, topping up the solution as it evaporates. Rushing it produces a cloudy clump of small crystals instead of one clean specimen.

The solution cooled too fast or had impurities. Rapid cooling makes the dissolved solid crash out as countless tiny crystals all at once, which looks like a cloudy mass. For clear, large crystals I cool the solution slowly, filter it before crystallising to remove specks that seed unwanted growth, and keep it undisturbed. Dust and vibration both trigger that messy rapid crystallisation.

Grow a seed, then feed it. I let a saturated solution form many small crystals, pick the single best-formed one, tie it to a thread, and suspend it in a fresh saturated solution. The dissolved material then deposits onto that one seed rather than forming new crystals. Keeping the solution exactly saturated, neither too strong nor too weak, is the patience-testing part that separates a showpiece from a blob.

Mostly, with sensible care depending on the chemical. Sugar, salt, and alum are low-hazard and fine around a kitchen with normal cleanliness. Copper sulphate and similar are more toxic, so I keep them away from food, wear gloves, label everything clearly, and never use kitchen utensils I will eat from again. Read the hazard information for any chemical before you start, and store solutions out of reach of children and pets.

⚠️ Some crystal-growing chemicals like copper sulphate are toxic if ingested. Keep them clearly labelled and away from food and children, use dedicated non-kitchen containers, and wear gloves when handling them.