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Growing crystals. ALUM

We have three methods for growing crystals. This is the simplest method and can be used for growing alum crystals. It’s the material commonly found in commercial crystal growing kits and the crystals can be coloured using food dye. The chemical name for alum is potassium aluminium sulfate and it is a relatively cheap material with a number of uses including a mordant for dyes and for clearing drinking water.

The process of crystal growing involves creating a saturated solution (no more of the solid will dissolve in the water) at a higher temperature than your room. We use hot water out of a jug and add ~30 to 40 g for every 100 mL of water (5 to 10 flat teaspoons to half a cup of water).  Add the dye to the water before you add the alum.

The water doesn’t have to be boiling but the hotter it is the quicker things will dissolve and the more alum will dissolve. If you use cooler water then add less alum, otherwise it won’t all dissolve.

When no more alum will dissolve in the water pour some of the solution over a paper towel or similar on a shallow dish. We use filter paper and lab glassware but it’s not necessary. The paper will hold the crystals together and the shallow dish allows the water to evaporate quickly.

Leave the solution until all the water has evaporated and you should get some nice coloured alum crystals.


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