It’s easy to learn how to grow metal crystals. Unlike salt or sugar crystals, most metal crystals do not grow by dissolving them in a solvent and letting evaporation do the work. Instead, metal crystals grow from controlled cooling of molten metal or by precipitating the metal from a chemical reaction. Here’s a look at what you do and examples of popular metal crystals you can grow.
The Basics of Crystal Growth
All crystals grow in two stages, which are nucleation and crystal growth. Nucleation occurs when atoms or molecules aggregate into a cluster. Often, this starts around an impurity or even a scratch on a container surface. From there, crystal growth expands outward.
When you grow metal crystals from a melt, nucleation occurs as metallic bonding organizes atoms during the transition from a liquid (or gas) to a solid. When metal crystals grow via an electrochemical reaction, nucleation occurs on a substrate, which is usually a metal wire. The resulting crystal shape depends on how the atoms organize during nucleation, but also on rate at which they grow and the nature of the medium surrounding the nucleus.
How to Grow Metal Crystals From a Melt
Even if you’ve never grown any crystals at all, you can grow metal crystals from a melt. Basically, all you do is melt a reasonably pure sample of a metal and slowly cool it until it solidifies. Slow cooling is important because it gives the atoms a chance to achieve a structure rather than just freeze into some random order. For the best crystals, insulate the liquid metal by placing the container inside a nest of oven mitts, a warm oven, or a second container of hot liquid.
The crystal grows with liquid metal. Since metal is not transparent, you often get the best crystal by pouring off molten metal from a partially solidified sample. Knowing when to pour off some metal is a matter of trial and error. But, if you don’t get good results, you can always melt the metal and try again.
Metals typically have high melting points. So, choose examples with melting points low enough that you can achieve them using common everyday heat sources. Gallium melts if you drop it in warm water. Bismuth and lead readily melt on a stove top or gas grill. Aluminum melts using a gas torch. Keep metal toxicity in mind, too. For example, gallium and bismuth are reasonably safe to crystallize and handle, but proceed with caution with lead or cadmium.
Here are good metal crystals to grow from a melt:
- Bismuth Crystals: Like most other metals, bismuth starts out silver-colored. However, it readily oxidizes in air into a rainbow of colors. Melt chunks of bismuth in a tin, aluminum, steel, or iron container and slowly let it cool. The bottom of a food tin (e.g., tuna, canned cat food) or aluminum soda can works great, since you can pop the bismuth crystal out of the mold.
- Gallium Crystals: You can melt gallium in the palm of your hand or in a cup of hot water. A chunk of gallium in hot water usually cools slowly enough to get nice crystal structure.
How to Grow Metal Crystals Using Electrochemistry
The other way of growing metal crystals uses electrochemistry. Just about any metal grows using this method, but you need to refer to the metal activity series and the solubility rules. The metal activity series tells you which metals replace other metals in a single replacement reaction. The solubility rules tell you which metal compounds dissolve in water, which is the usual solvent for home and lab experiments.
In a nutshell, the process works like this: One metal salt dissolves in water. This metal is the one you crystallize. You have a sample of a second metal, higher on the activity series. This second metal replaces the first one in the solution, so the first metal precipitates out as a solid. Atoms of this solid undergo nucleation and crystal growth.
For example, a silver nitrate solution deposits silver crystals onto copper wire. Silver nitrate is soluble. You can substitute silver acetate (also soluble), but not silver iodide (insoluble). Silver is further down the activity series than copper. Platinum and gold are also below copper on the activity series, so if you can afford the expense of soluble platinum or gold compounds, you can grow crystals of these metals!
For some reactions, a battery or power source drives the process. In this case, there are two electrodes, with the metal salt solution completing the circuit.
Here are examples of metals crystals you grow using electrochemistry:
- Silver Crystals: You need a soluble silver compound and copper.
- Tin “Hedgehog”: Grow a spiky mass of metal crystals resembling a hedgehog using a soluble tin solution and either iron or zinc.
- Copper Crystals: Grow copper metal crystals using either electrolysis or else a single replacement reaction.
References
- Atkins, Peter W.; Julio de Paula (2006). Physical Chemistry (4th ed.). Weinheim: Wiley-VCH. ISBN 978-3-527-31546-8.
- Markov, Ivan (2016). Crystal Growth for Beginners: Fundamentals of Nucleation, Crystal Growth and Epitaxy (3rd ed.). Singapore: World Scientific. doi:10.1142/10127 ISBN 978-981-3143-85-2.
- Mersmann, A. (2001). Crystallization Technology Handbook (2nd ed.). CRC Press. ISBN 0-8247-0528-9.
- Pimpinelli, Alberto; Villain, Jacques (2010). Physics of Crystal Growth. Cambridge: Cambridge University Press.ISBN 9780511622526.