How to grow Alum crystals (Aluminium Potassium Sulphate)

Growing crystals is fun and very rewarding. It does take time and a lot of patience but the final results can be amazing.

The secret to quality crystals is working with clean equipment and good quality Alum or Aluminium Potassium Sulphate. The process involves using a saturated solution to firstly create a single seed crystal and then growing this seed crystal as large as you like.

Please note: As you will be working with hot water it is important that children are supervised at all times.

Ingredients needed to grow your own Alum crystals:

  • Water
  • Aluminium potassium sulphate 12 water
  • 2 clean jars
  • Fishing line
  • A pencil or stiff wire
  • Tablespoon
  • A cloth
  • Gloves

Instructions on how to Alum crystals:

Stage 1 – Producing a saturated solution.

  • Using gloves add alum to a jar containing water and stir until dissolved.
  • Keep adding more alum and mixing until no more will dissolve.
  • There should be some undissolved alum settled at the bottom of the jar.
  • Allow the jar to sit for an hour so that all undissolved solid settles to the bottom of the jar.
  • Carefully transfer only the liquid to a second clean jar and seal it for the next processes.

Stage 2 – Producing the seed crystal.

  • Pour some of the saturated solution into a clean jar.
  • Cover the jar with a cloth and place in a dark place (cupboard etc).
  • A few crystal should start forming at the bottom of the jar. (see notes)
  • Allow them  to continue to grow until they are large enough to handle easily.
  • Remove the crystals from the jar and keep dry for using in the next process.
  • Seed crystals that you wish to keep for later should be wrapped in tissue.

Stage 3 – Growing the large crystals.

  • Take the original jar of saturated alum solution.
  • Tie one of the seed crystals to a length of fishing line.
  • Tie the other end of the line to a pencil or some stiff wire.
  • Immerse the seed crystal into the jar making sure that it is not touching the sides or the base.
  • Place the pencil or wire across the top of the jar and fix in position with a bit of tape.
  • Cover the jar with a cloth and set aside to allow the crystal to grow.
  • Keep out of direct sunlight and away from heaters, radiators etc.
  • As the crystal grows the level of the saturated solution will drop.
  • you can top up the solution by making more of it (repeat stage 1) and adding carefully so as to avoid disturbing the growing crystal.
  • You can continue doing this for days, weeks, months or even years – it just depends on how big a crystal you want to grow.
  • When you are happy with the finished crystal remove it and store it somewhere dry.


Alum is the short name for aluminium potassium sulphate dodecahydrate (12 water)

For the cleanest crystals distilled water should be use instead of tap water .

It is very important that during all processes you keep the containers covered to prevent dust getting in which will spoil the crystals.

If during the seed growing process you notice that the complete surface of the base is covered in tiny crystals this means that the jar was not clean and you will need to start this process again.

Where to buy quality Alum


About mistralhowto

formulation chemist

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