One lesson I wanted my students to learn was the relationship between temperature and crystal size in igneous rocks. As igneous rocks (rocks that form from magma or lava) form, they develop crystals. If you’ve looked closely at a stone countertop, you are familiar with the large multicolored chunks (crystals) that make up granite. If you’ve seen a piece of volcanic glass (obsidian) or regular glass (glass), you have seen crystals so small that they are indistinguishable from their neighbors. This range of crystal sizes is an effect of the temperature at which minerals in a liquid cool. This is the process I hoped my students could experience first hand.
Melting down rock in the classroom was out of the question. The obvious alternative was salt, and this is where the problems started. Every salt crystal lab on the internet starts with putting salt ions into solution by mixing the salt into water. A metaphor my friend Walter used echoed in my mind. Imagine the relatively large water molecules as basketballs in a box. While the box may fill with basketballs there is plenty of space between the balls. The salt, made of smaller molecules (golf balls, perhaps), fills the spaces in between. This is where the similarities between labs end.
In my first attempts, my wife was subjected to bowls of salt water with toilet paper rolls towering out of them like cooling towers on a nuclear power plant. The bowl/ tube creations littered our kitchen - the refrigerator, the window, the oven - for a day before I realized the technique wouldn’t work. In this process the crystals were forming as a result of evaporation. The cardboard drew the water from the bowl causing it to evaporate. With each drop of water that evaporated, the salt held within crystallized. As I mentally consulted Walter’s model, this process had little to do with the cooling mechanisms that form granite or obsidian
As is the custom in modern times, when faced with lack of information I consulted the all knowing internet. In my search I found resources that seemed great, but lacked critical details (But how do I get the seed crystal to form in the jar? And, what is a seed crystal?). Finally, I came across a description that fit Walter’s model and my understanding of igneous rocks. It encouraged heating the water (imagine spreading out those basketballs), adding in more salt (filling the new spaces with additional golf balls), putting in a string, and then cooling the water (shoving the basketballs back to their original spots). It was like Christmas morning when I reentered the kitchen after an hour or so to find that those water molecules had pushed the salt crystals out of solution and an into a coarse coat on the string. The next day I charged into the classroom and showed the students what I’d done, with one change - the crystal didn’t grow.
Once again I consulted the internet - the resource I found this time told me that Borax (sodium tetraborate, available in the laundry detergent aisle of your grocery store) grew larger crystals, more reliably and that they could be grown on a pipe cleaner cut into the shape of a snowflake (apropos, given the season). Once again I returned to my home lab/ kitchen to test it out. It worked perfectly, producing a beautiful white crystal boraxflake. Again, I eagerly set out to replicate my success at school. Again, I met with failure. After waiting the requisite two hours I was left with nothing but a soggy pipe cleaner, bent into the shape of a snowflake. Was something wrong with the water at school? Was I doing something wrong? Was I destined to fail in my epic quest to teach my students about crystals? With the countdown to Christmas break nearing, I dropped my pipe cleaner back into the water and went home to consult my last hope - Facebook.
The conversation that ensued provided as many questions as it did answers. Like each roadblock before it forced me to think about my model and clarify my understanding of how the world works. My setbacks provided more learning than my conquests, but the hope that these elements might crystalize drove me onward.
The next morning I got to school ready to try any or all of the advice I’d received, but first I checked the beaker I’d left the night before. To my surprise and relief I was rewarded with a fully formed crystal snowflake. Since that time I’ve run the lab with all of my classes. Like myself, they had successes and failures but I felt comfortable telling them, that if nothing else science is a learning experience and you don’t always get what you want. At least on the first try.
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