A seed crystal is a small crystal that is used as a base to grow a large single crystal. Without a seed, crystals form slowly from random intermolecular interactions. When the seed is placed in a saturated or supersaturated solution it acts as a nucleation site. This decreases the time needed to grow a crystal and directs growth to a single region. Here’s how to grow a seed crystal and how to use one to get a large, perfect crystal.
Materials
The materials needed to grow a seed crystal depend on the type of crystal. Crystals grown from a melt (like many metal crystals) are different from those grown from solution (like sugar and alum crystals). For a seed crystal from a water-based (aqueous) solution, you need:
- Chemical to be crystallized (here are some recipes)
- Distilled water (if you don’t have it, tap water is usually fine)
- Shallow dish (like a saucer or petri dish)
- Heat source (stove, hot plate, microwave) – optional
- Nylon monofilament line (for some crystals) – optional
Methods
First prepare a saturated solution. Dissolve as much material (salt, sugar, whatever you’re using) as possible in small amount of water. The amount of water is not critical (1/4 cup or 50 milliliter is more than enough). A good way to ensure saturation is to stir the powder or granules in hot tap water or boiling water until a little undissolved solid remains. Give the solution a couple of minutes and then stir it again to see if it will accept more solute (the stuff you are dissolving).
You don’t want undissolved solute in the liquid used to grow the seed crystal. Either let the solid sink to the bottom or else filter the liquid through a filter paper or coffee filter.
Pour the hot liquid into a saucer or petri dish. Crystals form quickly as the liquid cools and evaporates. Select one or more of the best crystals and carefully scrape them off the dish. These are your seed crystals.
Use a Seed Crystal to Grow a Large Single Crystal
The basic premise is to dip a seed into a melt or supersaturated solution or to pass vapor across the crystal. For crystals grown from solution, the seed can be placed in a clean container of saturated or supersaturated solution or else suspended in the solution so all the crystal faces are exposed. Which method you use depends a lot on the shape of the crystal and whether having the crystal suspended from a line is desirable.
Prepare a supersaturated solution. Dissolve as much solute as possible in hot or boiling water and then let it cool to room temperature. Temperature affects solubility, so a solution saturated at high temperature becomes supersaturated at a lower temperature. Some crystals may start to grow in this solution. This is okay. You’re going to pour the supersaturated liquid into a new container to grow your large single crystal, leaving behind these other crystals.
The first method is to tie fine nylon monofilament fishing line around the seed crystal using a simple knot. The strand needs to be nylon monofilament because thread, string, or polyfilament line is porous. A porous line wicks liquid away from the crystal and up toward the attachment point. Also, monofilament is smooth rather than rough, so it won’t promote crystal growth. Pour supersaturated solution into a clean glass container. Suspend the crystal in the solution so it is completely submerged and doesn’t touch the bottom or sides of the container. You can tie the line to a pencil or butter knife placed across the top of the container.
The other method is to place the seed crystal on the bottom of a clean glass container. Choose a container free of scratches or any residue, as these defects can attract crystal growth. Carefully pour supersaturated solution into the container. You may need to reposition the seed crystal and knock off any air bubbles attached to it.
Whichever method you choose, monitor crystal growth and remove any competing crystals growing on the top or sides of the container. You can cover the container with a paper towel or coffee filter to keep out debris but still allow evaporation. If the liquid level gets low, you can add more supersaturated solution or you can move the crystal to a new container with fresh solution. Make sure fresh solution is the same temperature as the seed crystal. Once the crystal is large enough, remove it from the solution and let it dry.
Tips for Success
It’s critical the fresh solution be saturated or supersaturated or else the seed crystal will dissolve. Be mindful of the temperature. Remember, heat will make the solution less saturated, while cold will make it more saturated. Its best to avoid growing crystals in a window because the temperature may fluctuate. This also means you can use the refrigerator to stimulate crystal growth!
Uses for Seed Crystals
The semiconductor industry uses seed crystals to grow large single crystals (boules) of silicon using the Bridgman technique or Czochralski method. Seed crystals promote the growth of desirable type V crystals when tempering chocolate. Seeds are also used to grow crystals for crystallography and other purposes.
References
- Chocolate Alchemy. “Cocoa Butter Silk, A Primer.”
- Czochralski, J. (1918). “Ein neues Verfahren zur Messung der Kristallisationsgeschwindigkeit der Metalle” [A new method for the measurement of the crystallization rate of metals]. Zeitschrift für Physikalische Chemie 92 : 219–221.
- Nishinaga, Tatau (2015). Handbook of Crystal Growth: Fundamentals (2nd ed.). Amsterdam, the Netherlands: Elsevier B.V. ISBN 978-0-444-56369-9.