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How To Grow a Seed Crystal

A seed crystal is a small single crystal that you put in a saturated or supersaturated solution to grow a large crystal. Here's how to grow a seed crystal for any chemical that dissolves in water.

Halite

You can use a small single crystal, like this salt crystal, as a seed to grow a large crystal

 

Materials Needed to Grow a Seed Crystal

  • chemical you want to crystallize
  • distilled water (tap water is usually okay)
  • shallow dish (e.g., petri dish or a saucer)
  • heat source (stove, microwave, or hot plate)
  • nylon line (like for fishing)

Make a Solution

Ideally you would know the solubility of your chemical at different temperatures so that you could estimate how much of the chemical is needed to make a saturated solution. Also, this information is useful in figuring out what to expect when you cool your solution. For example, if the substance is much more soluble at a higher temperature than at a lower temperature, then you can expect crystals to form very quickly as you cool the solution (e.g., sugar crystals). If the solubility doesn't change much over your temperature range, you will have to rely more on evaporation to cause your crystals to grow (e.g., salt crystals). In the one case, you cool your solution to stimulate crystal growth. In the other case, you keep the solution warm to speed up evaporation. If you know your solubility, use that data to make a solution. Otherwise, here's what to do:

  • Heat about 1/4 cup (50 ml) of water in a glass container. A metal container may react with your chemical; a plastic container may melt. I boil water in the microwave in a Pyrex measuring cup. (Be careful not to superheat your water! It tends not to be a problem with microwaves that rotate the container, but be careful anyway.) For crystals that fall out of solution easily, you may only need water heated to coffee pot temperatures or even hot tap water. When in doubt, boil the water.
  • Stir in your chemical. Keep adding it until it stops dissolving and a little accumulates in the container. Give it a couple of minutes. Stir the solution again and add more solute (the stuff you are dissolving) if needed.
  • Pour some solution into a petri dish or saucer. Only pour the clear solution into the dish, not any of the undissolved material. You may wish to filter the solution through a coffee filter.
  • Crystals will form as the solution evaporates. You may remove a crystal before the solution fully evaporates, if desired. To do this, pour off the solution and carefully scrape off the crystal. Otherwise, you can wait until the solution has evaporated. Select the best crystal and carefully remove it from the dish.

Using Your Seed Crystal

Now that you have the seed crystal, it's time to use it to grow a big crystal:

  • Tie the crystal onto nylon fishing line with a simple knot. You want nylon and not 'normal' thread or string because they are porous, so will act as a wick for your solution, and because they are rough and will attract crystal growth away from your seed crystal. If the container you use to grow your crystals is completely clean and smooth and the line is nylon, your seed crystal should be the most likely surface for crystal growth.
  • You may need to scrape small grooves in your seed crystal so that it won't slip off of the nylon line. Nylon is not the easiest material to use to tie a knot.
  • Suspend your seed crystal in a saturated or supersaturated crystal solution so that it is completely covered. You don't want the crystal to touch the sides or bottom of the container. If your crystal solution isn't concentrated enough, your seed crystal will dissolve.
  • You made a saturated solution for your seed crystal, so you can use that procedure (except with more water and crystal chemical) to grow the 'real' crystal.
  • To supersaturate a solution, you make a saturated solution at a high temperature, then slowly cool it (with some exceptions, but I'll save that for another time). For example, if you dissolve as much sugar as possible in boiling water, the solution will be supersaturated by the time it gets to room temperature. A supersaturated solution will produce crystals quickly (often over the course of a couple of hours). A saturated solution may require days or weeks to produce a crystal.
  • Let your crystal grow in an undisturbed location. You may wish to cover the solution with a coffee filter or paper towel to keep dust or whatever from contaminating the solution.
  • Once you are happy with your crystal, remove it from the solution and allow it to dry.

 

How To Grow a Big Alum Crystal

Alum is found in the 'spices' section of the grocery store. With a bit of time and effort, you can grow a big alum crystal.

Alum Crystal

Alum crystals are probably the easiest crystals to grow. The chemical is non-toxic and the crystals grow quickly and reliably.

  1. Pour 1/2 cup of hot tap water into a clean jar.
  2. Slowly stir in alum, a little at a time, until it stops dissolving. Don't add the whole amount - just enough to saturate the water
  3. Loosely cover the jar with a coffee filter or paper towel (to keep dust out) and allow the jar to sit undisturbed overnight.
  4. The next day, pour the alum solution from the first jar into the clean jar. You will see small alum crystals at the bottom of the jar. These are 'seed' crystals that you will use to grow a big crystal
  5. Tie nylon fishing line around the largest, best-shaped crystal. Tie the other end to a flat object (e.g., popsicle stick, ruler, pencil, butter knife). You will hang the seed crystal by this flat object into the jar far enough so that it will be covered in liquid, but won't touch the bottom or sides of the jar. It may take a few tries to get the length just right
  6. When you have the right string length, hang the seed crystal in the jar with the alum solution. Cover it with the coffee filter and grow a crystal!
  7. Grow your crystal until you are satisfied with it. If you see crystals starting to grow on the sides or bottom of your jar, carefully remove your crystal, pour the liquid into the clean jar, and put the crystal in the new jar. Other crystals in the jar will compete with your crystal for alum, so it won't be able to get as big if you let these crystals grow.

Tips:

  1. You can use sewing thread or other string instead of nylon fishing line, but crystals will grow on the entire length of the submerged string. Crystals don't adhere to nylon, so if you use it, you can get bigger, better crystals.
  2. Alum is an ingredient used to make pickles. It makes them crispy.

What You Need:

  • 1/2 c hot tap water
  • 2-1/2 T alum
  • nylon fishing line
  • pencil, ruler, or knife
  • 2 clean jars
  • spoon
  • coffee filter/paper towel