Growing Alum Crystals for Fun

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We have been trying to grow crystals that we bought in a commercially-available crystal growing kit. So far the results have been disappointing. The image on the box does not really resemble the results we are seeing.

An Easy and More Reliable Crystal-Growing Experiment

My son and I tried growing crystals from a commercially-available crystal kit. The results were far from impressive. In fact, they were downright dismal. No way will the actual results resemble the images shown on the packaging. So we decided to try growing crystals the old-fashioned way.

Materials Needed to Grow your own Alum Crystals

  • Two small empty coffee jars or vertical-sided clear drinking glasses of the same size
  • Alum powder (available in the supermarket where pickling supplies are sold)
  • Mixing spoons & a Flat stick (Popsicle stick, or a pencil will do)
  • Nylon string (fishing line works best, crystals won't adhere to it)
  • A piece of cotton cloth or flat-bottom coffee filter

Method for Growing a Clear Alum Crystal (aluminum potassium sulfate)

Add one cup of boiling hot water to the first jar. Using a spoon, add the powdered alum (the chemical name is "aluminum potassium sulfate") one spoonful at a time, completely blending each until dissolved. Continue doing this until you reach a state of 'super-saturation.' You will see alum powder at the bottom of the hot water that will not dissolve into solution. You have 'super-saturated' the water's ability to hold any more solids at the hot temperature. Do not continue to add spoonfuls once you achieve super-saturation.

Cover the jar with the cotton cloth or flat-bottom coffee filter and clip it onto jar using cloths pins (or use a rubber band, just to keep the cloth top in place) and leave it undisturbed overnight. Evaporation can still occur through the porous coffee filter material, albeit a lot slower than without. This cooling and evaporation causes some alum to precipitate out of solution to the bottom of the jar where it will begin to form crystals. The coffee filter slows the rate of evaporation which creates larger crystals. We want at least one of these 'larger crystals.'

The next day, pour the room-temperature liquid solution of the first jar into the second jar, saving or capturing some or all of the minute 'seed crystals' that will have already formed on the bottom. You may need to use a pencil or spoon to 'break apart' these from the bottom of the jar.

This next part can be a bit difficult. Using any one of the larger crystals that precipitated to the bottom of the first jar, tie a loop of the nylon fishing line around the crystal. Attach the other end to a pencil or flat stick and suspend this crystal in the room-temperature solution of the second jar. Be sure this crystal on the string is fully immersed into the fluid, and also that it does not touch the sides or bottom of the jar.

Cover this second jar with the coffee filter as we did before, and wait. It may take several days but this 'seed crystal' will quite reliably grow .

If other crystals form on the sides of the jar, remove the hanging crystal and pour the cool liquid onto another clean jar or glass, and re-introduce the hanging crystal to it, and re-cover it with the coffee filter You do not want crystals in the same jar competing for the available dissolved alum. Your crystal on a string will grow larger if you do this.

Keeping the crystal growing chambers ('jars') in a room with constant temperature and out of direct contact with sunlight works best. Changes of temperature cause different rates of super-saturation and under-saturation. If the room and solution becomes warmer, the solution changes toward under-saturation (can withstand more alum dissolved) at a rate faster than the evaporation can maintain supersaturation. -The crystal will stop growing and may even slowly (but temporarily) dissolve back into solution until the temperature returns to stable, or cooling. You want the super-saturated solution to maintain steady temperature or even under slightly & progressively cooling temperatures such as what occurs in late summer or early fall, and you want this solution to evaporate slowly.

A Typical Alum Crystal

(this one is colored with chrome during growth, giving it the purple color)

alum crystal, colored with chrome ion when it was grown

(image source) (single crystal of a synthetic chrome alum with common octahedral shape)

When the seed crystal on the string reaches a pleasing size (perhaps 3/4 of an inch or a bit larger,) it can be removed from the solution and the string snipped away with fingernail clippers. Being nylon, this string may be nearly invisible in the clear crystal. Allow this crystal to sit upon a paper towel and gently dab and roll it around until completely dry. Leaving it 'wet' until it evaporates dry may cause minute mini-seed crystals to form on the surface, leaving your beautiful alum crystal with a smoky white haze all over. If you do this correctly, the resulting crystal will be clear or water-clear.

At least, this is how I remember it from chemistry in school. I am expecting to have better luck with this more reliable home crystal-growing experiment than with the commercially-bought one that clearly is never going to create the same effect as the picture on the label.

Grow Larger Alum Crystals

Alum crystals can be grown to much larger sizes if you wish. I have seen fist-sized crystals grown using methods similar to this, but the smaller ones (under 3/4-inch) look the best I think.


Chris Stonecipher
Posted on May 26, 2010
Kathleen Murphy
Posted on Feb 24, 2010