Naomi and some finished, fully grown Alum crystals..

Activity: Grow your own Crystal

On SCOPE’s Crystals episode, you saw Naomi produce her own sizeable crystals. Here’s how she did it:

Watch this clip

What you need

• ½ a cup of hot tap water
• 2 clean jars
• Kitchen paper
• Nylon fishing line
• An ice cream stick
• 3 tablespoons of Alum (Alum – or aluminium potassium sulphate – is used in pickling, but if your local supermarket doesn't have it, a chemist should be able to order it in for you)
  

What to do

1. Pour the hot water into the jar and add the alum a little at a time, stirring as you go. Keep adding Alum until no more will dissolve (avoid leaving Alum sitting on the bottom of the jar too). What you have created is called a super-saturated solution.
2. Now cover the jar with a sheet of kitchen paper and leave it overnight to do its thing.
3. After the time is up, if you look closely, you will see the beginnings of crystals on the bottom of the jar. Pour the solution from the first jar into a clean jar. The 'seed' crystals will be left behind.
4. Gently grab one of the biggest and best- formed – And tie one end of the fishing line around it (harder than it sounds). If all else fails glue with tiny dob of super-glue.
5. Tie the other end of the fishing line to the ice cream stick.
6. Hang the seed crystal into the jar of solution, adjusting the length so that the crystal is covered by the liquid, but not touching the sides or bottom of the jar. … And then leave it for several days and watch it grow.
   

What's happening?

Most of the earth's crystals may have formed by geological forces millions of years ago, but we've found a way of making crystals in just a few days.

The secret is to have a solution into which lots of alum has been dissolved. For most salts, warm water will dissolve more than cold water. Once it cools most, or all, of the alum will remain dissolved. However, give the Alum a place to re-crystallise, and it will. This is why the container was covered and why fishing line was used instead of cotton. Small dust particle or even the threads on cotton are enough to form the nucleus, or beginnings of a new crystal. As the water evaporates, there is less room for alum to be dissolved, and ideally, we want all of the alum to re-crystallise on the surface of the one ever growing crystal.

If crystals start growing on the sides or bottom of the jar, transfer the whole set up into a new clean jar so that your prize crystal won't have to compete for the alum.

The shape of Alum crystal is determined by the way the Alum molecules arrange themselves. The structure is said to be cubic, although the shape looks much like 2 pyramids stuck together at the bases. How exactly this is cubic is best explained here.