Growing Alum Crystals

Potassium alum is used in food and cosmetics and is not a dangerous substance, but it is not a sweet: do not eat it, wash your hands after handling, and keep it away from eyes. Work with an adult present if you are using near-boiling water.

Warm 200 ml of distilled water to roughly 60 °C. Distilled matters — dissolved minerals in tap water give extra nucleation sites and you get many small crystals instead of one big one.

Stir in potassium alum a spoonful at a time until a little refuses to dissolve. You have now made a saturated solution at that temperature, which is the whole basis of the method.

Weigh what you added and write it down with the temperature. Solubility rises steeply with temperature for alum, and that single fact is why cooling produces crystals at all.

Pour the warm solution through filter paper or a coffee filter into a clean jar. Every speck of dust left behind becomes a crystal you did not want.

Cover the jar and leave it undisturbed to cool to room temperature. As it cools the solution becomes supersaturated — holding more alum than it comfortably can — and small crystals form on the bottom.

Next morning, pour off the liquid and look at the crystals with a lens. Choose one with flat faces, clean edges and no cloudiness. A flawed seed grows into a large flawed crystal — the defect propagates.

Look at the seed: it is an octahedron, eight triangular faces meeting in points. Draw it. That habit comes from the cubic arrangement of the ions in the lattice and it is the visible output of invisible geometry.

Tie fine nylon thread round the seed — nylon rather than cotton, because cotton fibres seed extra crystals along the thread — and suspend it from a pencil so it hangs clear of the bottom and sides.

Prepare a new saturated solution and let it cool to room temperature before lowering the seed in. Putting a seed into warm solution dissolves it immediately, and this is the mistake almost everyone makes once.

Cover loosely to slow evaporation and keep dust out, then leave it somewhere with a steady temperature. Vibration, draughts and temperature swings all produce flaws — patience is a technique here.

Lift it out briefly, measure across the widest points, note the date and put it back. Plot size against time — growth is fast at first and slows as the solution is depleted.

Scrape out small crystals forming on the jar bottom. They compete for the same dissolved alum, and every one that grows is material your crystal did not get.

Take a second saturated solution and cool it in ice water. You get a mass of tiny crystals rather than one large one. That comparison is the experiment — same chemistry, different rate, entirely different result.

Blot it dry and coat it with clear nail varnish or keep it in a closed box. Alum crystals give up their water of crystallisation to dry air and go chalky and white over weeks if left out.

Habit is not decoration. Potassium alum crystallises in the cubic system and its usual growth habit is the octahedron. You did not shape it and no mould was involved: the ions stack in a repeating pattern, faces grow at rates set by that pattern, and the slowest-growing faces end up as the visible ones. The external form is a direct readout of internal structure — which is the founding idea of crystallography, established long before anyone could see atoms.

Saturation, supersaturation and rate. Solubility rises with temperature. A solution saturated hot becomes supersaturated when it cools — it holds more solute than it stably can — and the excess must come out. If it comes out fast, it comes out on many nucleation sites at once as a powder of tiny crystals. If it comes out slowly onto a single existing surface, that surface grows. Nucleation versus growth is the whole control you have, and it is why filtering, using distilled water and leaving the jar alone all matter.

The same principle at industrial scale. Controlled crystallisation is how sugar, salt, many pharmaceuticals and most semiconductors are purified and formed. A drug's crystal form can change how fast it dissolves in the body, so pharmaceutical companies spend serious effort on exactly the variables you are playing with in a jam jar.

Alum has a long history of its own. Before this was a school experiment, alum was a major industrial commodity — the essential mordant for fixing dye to cloth, and the subject of a large and strategically important trade in medieval and early modern Europe. Youblob already carries a blueprint on mordanting wool with alum; the mineral being grown here is the same substance seen from the crystallographer's side rather than the dyer's.