Growing a Large Single Crystal

The seed-crystal method for growing one big, gem-like crystal from alum, copper sulfate, or other salts — plus how to pick a compound
Intermediate🕐60 min●Low hazardcrystallizationphysical-properties

Summary

Almost anyone can make a jar of tiny crystals; growing one large, clear, faceted crystal is a different craft. It rests on a single idea — supersaturation1 — plus patient control of temperature and cleanliness. This experiment is the general method that works for a whole shelf of salts, from beginner-friendly alum to the deep blue of copper sulfate. Master it once and every compound in the table below is open to you.

History

Large laboratory-grown crystals underpin whole technologies: quartz oscillators in every watch and radio, silicon boules for microchips, and the huge optical crystals in laser fusion research are all single crystals grown from solution or melt. The kitchen version — a thread, a jar, and a windowsill — uses exactly the same physics of nucleation2 and controlled growth that Alan Holden popularised in the classic Crystals and Crystal Growing.

Hazards & preparation

Note

Most crystal salts are low-hazard, but read the page for your chosen compound.

  • Copper sulfate and cobalt chloride are toxic if ingested and stain — wear gloves and keep them away from food areas.
  • You’ll heat water to dissolve the salt; handle hot solutions with care.
  • Disposal: heavy-metal solutions (copper, cobalt) must not go down the drain — see the Safety page. Alum, borax, and Epsom salt are benign.

Choosing a compound

The best beginner salts have a steep solubility curve — they dissolve far more in hot water than cold, so cooling a saturated solution makes it strongly supersaturated and eager to grow.

Compound Difficulty Best method Colour Time to ~1 cm
Borax Easy Cooling Clear 1–2 days
Potassium alum Easy Cooling Clear octahedra 1–2 weeks
Ferric ammonium sulfate Easy Cooling Pale violet 1–2 weeks
Epsom salt Easy Evaporation Clear needles 2–3 days
Copper sulfate Medium Cooling Deep blue 1–2 weeks
Cobalt chloride Medium Cooling Red-violet 1–2 weeks
Potassium nitrate Medium Cooling Clear 1–2 weeks

Start with alum or borax to learn the moves, then graduate to copper sulfate for a spectacular blue result.

Part A — Grow Seed Crystals

Materials: your chosen salt, distilled water, a clean jar, a pan for a hot water bath, coffee filters, nylon thread.

Steps:

  1. Heat distilled water and stir in the salt until no more dissolves, then add a small excess (5–10%) — you now have a saturated solution.
  2. Filter the hot solution through a coffee filter into a clean jar, leaving any undissolved solid behind.
  3. Cover loosely and let it cool slowly and sit overnight. A scatter of small crystals will form on the bottom.
  4. Pick out the single best one: clear, well-faceted, no cracks or twins. This is your seed.

Part B — Grow the Single Crystal

Steps:

  1. Make a fresh saturated solution the same way and filter it into a clean jar. Let it cool to room temperature so it’s mildly supersaturated.
  2. Tie the seed to a length of nylon thread, hang it from a pencil laid across the jar, and lower it in so it floats free — not touching the sides or bottom.
  3. Cover loosely (a paper towel) to slow evaporation and keep dust out.
  4. Put the jar somewhere with a stable temperature, out of draughts and direct sun, and leave it undisturbed.
  5. Refresh weekly: as the crystal grows it depletes the solution. Every few days to a week, lift the crystal out, make fresh saturated solution, cool it, and return the crystal. Trim off any stray small crystals that sprout on the thread.

What you should see

Over days to weeks the seed grows layer by layer into a large crystal with the compound’s characteristic shape — sharp octahedra for alum, blunt blue parallelepipeds for copper sulfate. Growth is slow and even when the solution is kept just mildly supersaturated; rush it and you get many small crystals instead.

Symptom Likely cause Fix
Many small crystals, not one big one Solution too supersaturated / cooled too fast Warm slightly to redissolve extras; keep supersaturation mild
Cloudy or milky crystal Grew too fast, or trapped solution/dust Slow the growth; filter solution; grow cooler
Crystals grow on thread/jar, not the seed Rival nucleation sites Use smooth nylon thread; remove stray crystals at each refresh
Seed dissolves Solution not yet saturated (under-supersaturated) Add a little more salt; cool the solution further
Crust on the surface Evaporation too fast Cover more; lower the temperature

The Science

Supersaturation

A saturated solution holds the maximum salt that will dissolve at a given temperature. Because most salts dissolve far more when hot, a hot saturated solution that is cooled suddenly holds more than it “should” — it is supersaturated, a metastable state storing the drive to crystallise. The excess salt comes out of solution, and if a seed is present it deposits there.

The steepness of the solubility3 curve is why some salts are easy: alum dissolves about 60× more at 100 °C than at 20 °C, so even gentle cooling produces strong supersaturation. Table salt barely changes with temperature, so it must be grown slowly by evaporation instead — which is why it’s one of the hardest.

Nucleation versus growth

Two processes compete. Nucleation starts brand-new crystals; growth adds material to existing ones. High supersaturation favours nucleation — you get a shower of tiny crystals. Low, steady supersaturation favours growth on the seed you provided, because the single existing surface soaks up the modest excess before new nuclei can form. The whole art of a big single crystal is keeping supersaturation in that gentle window.

Crystal habit

Each compound deposits atoms in a fixed lattice geometry, which shows up as its habit — the characteristic external shape (alum’s octahedra, Epsom salt’s needles). Impurities and growth rate can distort the habit, which is why clean solutions and slow growth give the most textbook-perfect forms.

Questions to Explore

  1. Why does one large crystal grow instead of many small ones when you keep supersaturation low? What are nucleation and growth competing for?

    Hint / answer

    Both draw on the dissolved excess. At low supersaturation the existing seed surface consumes the excess before new nuclei can form, so material piles onto the one crystal. High supersaturation instead spawns a crowd of new nuclei.

  2. Why is alum easy but table salt hard? Compare their solubility curves.

    Hint / answer

    Alum’s solubility rockets with temperature, so cooling gives big supersaturation and fast, controllable growth. Salt’s solubility barely changes with temperature, so cooling does almost nothing — you must evaporate water slowly, which is far more finicky.

  3. Why does refreshing the solution keep growth going? What has changed in the jar after a week?

    Hint / answer

    The growing crystal removes salt, so the solution drifts back toward merely saturated and the driving force fades. Fresh saturated solution restores the mild supersaturation the crystal needs to keep depositing.

Going further

  • Chase a perfect habit. Grow alum slowly and undisturbed and try to get flawless octahedra with mirror faces.
  • Two colours in one jar. Once confident, try a copper-sulfate crystal for the blue, then compare its blunt shape with alum’s sharp octahedra.
  • In the Crystals & Solutions track: see supersaturation released all at once in Hot Ice, and use solubility differences to separate a mixture in Fractional Crystallization.

Footnotes

  1. Supersaturation — A solution holding more dissolved substance than it normally could — an unstable state that crystallizes when disturbed.↩︎

  2. Nucleation — The first formation of a tiny stable seed from which a crystal or bubble grows.↩︎

  3. Solubility — The maximum amount of a substance that will dissolve in a given amount of solvent at a given temperature.↩︎