OK, that title isn't very creative, but I don't feel like a Madame Curie or Steven Universe quote here. And it reminds me of A Streetcar Named Desire, for some odd reason.
What is Bismuth?? Well it is a heavy metal, much like lead, but without toxic accumulation in the body. Therefore it is recommended as an alternative to lead and buckshot that is not toxic. I don't know why you would care if shooting someone, but I digress.
Bismuth is an element, number 83, meaning it is between Lead and Polonium (nasty stuff!). Bi is 86% the weight of lead by volume. It has 83 protons by definition. It is almost always found as isotope number 209, so it has a lot of neutrons.
We are primarily interested for four reasons:
1) It is relatively cheap and easy to buy online
2) It's melting point is only 521F, just a bit above the temperature to burn books (Fahrenheit 451). We can do this without a furnace. Even a kitchen stove can do this.
3) As it cools, it can oxidize on the surface into a rainbow of colors
4) It makes super cool hopper crystals (we'll save that for pt II)
Melting: well, it was super quick and easy with a Bunsen burner or a propane torch. I used both a crucible and a stainless steel container. It doesn't matter much.
Pouring and cooling: well it poured into my little graphite mold easily and cooled a bit slowly. Bismuth has relatively low thermal conductivity. Nice little bar:
Mistake: I have to admit I accidentally spilled some on my hand, which was quite painful for some time. I am lucky to make this mistake at 521F instead of with molten copper. Here is a good reminder to wear gloves and eye protection working with high temperatures. The upside (besides the learning I won't soon forget) is that I flung the metal out of my hand instinctively (like a dead frog's leg twitching) and made a beautiful piece of artwork that will show off the rainbow oxidation.
Why does it make the colors? Like all metals, it bonds with oxygen in the air at high temperatures (some do this more easily, like in the case of iron and rust). It makes Bi2O3 (same as the mineral bismite which I have found as yellow powder coating other minerals). This oxide is used to make "Dragon's Egg" fireworks.
Tiny variations in the thickness of the oxide layer are caused by the amount of time that spot is hot enough to bond with oxygen. In my experiments, rapidly cooled bismuth looks silver still. Quickly cooled silver has a gold colored tarnish. Then comes purple, and finally blues and greens for more slowly cooling areas.
Note that my hand splash is silver where thinnest, gold where some crystallization started, and blue/green with a bit of purple where a thick blob landed and cooled more slowly. The photographs just don't do it justice, you can get more of an effect in real life with better light and moving around.
In the mold, the top portion exposed to air cooled more quickly than the bottom, but the bottom was not exposed to oxygen (just the graphite mold).
Why does the thickness of the oxide matter? This is physics. These are interference patterns of light. Some wavelengths (colors) reflect back off the mirror-like metal and cancel each other out. Other colors reflect back pass through. The oxide is like a filter, in a way, where the thickness and angles determine the result. The phenomenon is known as iridescence and can be seen in many things such as soap bubbles and oils. But this one hangs around after cooling.
Thanks for reading,
Paul
p.s. It has come to my attention that some people would like to repeat these experiments without the hassle of finding the chemicals and/or having to buy too much, etc. I will try to keep an Ebay store alive (seller: cinnabarminerals) that offers low cost and related material for the purpose of enabling education. I will title offerings "Brave New Chemist Science Pack - (related Blog post name)." You can always comment and ask where to buy certain equipment too.
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