What is bismuth?

Its low melting point allows easy crystal formation – and can make spoons melt in hot tea

(Image source: Pixabay)

We know about bismuth’s unusual beauty because it also melts unusually easily. This metal element turns from solid to liquid at 271 degrees Celsius. That’s still more than enough to burn yourself, but much lower than other metals, which melt at thousands of degrees Celsius.

As it cools down, highly pure bismuth can form into stepped pyramids and other interesting and strange forms. This type of crystal is known as a ‘hopper’. The hopper crystal forms because as it cools, bismuth is strongly driven to rapidly form hard edges in preparation for making a cube-shaped crystal. However, the strong driving force never lets the faces grow. Instead it impatiently forms more edges, leaving metal spirals that merge into unusual structures. The bare metal can then also quickly react with oxygen in the air, forming a shiny, coloured surface.

This low melting point can be useful – and was also used in practical jokes. Mixing bismuth with other metals lowers their melting points, making easy-to-shape alloys. Today dentists use such alloys to make moulds of people’s mouths. Once people mixed bismuth with antimony (a metal-like element) to make type metal, which was used to print old books.

In the 1800s, Victorian pranksters made spoons from an alloy of eight parts bismuth, five parts lead and three parts tin. The alloy’s melting point was low enough for the spoons to vanish into a cup of hot tea. Given that today we know that lead can be poisonous, hopefully the victims didn’t finish their drinks!

(Image source: Pixabay)

How to make bismuth crystals

Thanks to its low melting point, you can in theory make pretty bismuth crystals at home. However, the temperatures and open flames involved still mean that great care is needed. Preferably, you should do it with an expert in a lab or a metalsmith’s forge.

To form the beautiful crystals, first put high-purity bismuth in an iron or steel container, for example a clean steel can. Then, heat the container to bismuth’s melting point, 271°C, using a gas flame, like a stove or a Bunsen burner, or an electrical hotplate. Turn off the heat, and when the bismuth freezes over, poke two holes in the solid surface. Pour the remaining liquid bismuth into a second iron or steel container, and leave to cool.

When it has cooled, break open the metal to find shiny, stepped bismuth crystals. And if you aren’t satisfied, you can re-melt the bismuth and try again!

This article was originally published in How It Works issue 131

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