Hopper Crystals

23 April 2018

Many of my favourite materials are metals. The weight, texture, colour and lustre makes each metal so unique. Off all the metals in the periodic table, Bismuth is pretty useless. It’s very brittle and places amongst the lowest thermal & electrical conductors, but it is the most naturally diamagnetic element and it can calm an upset stomach.

Interestingly, for a very long time, the only naturally occurring isotope of bismuth was considered the heaviest stable isotope. But back in 2003 it was discovered that bismuth is in fact radioactive, but has a half-life of 1.9×1019 years, which is over a billion times longer than the current age of the universe…

This is raw bismuth. It’s exactly what you’d expect of a metal – silvery grey, shiny, and lustrous. But this quintessential metal has a secret and beautiful alter ego.

If you heat a chunk of very pure bismuth to 270 °C / 520 F, it will melt into a shimmering pool of silverly liquid. This is quite low for a metal – you can do this on a gas hob at home (very carefully!).

Then comes the tricky part: Let it cool, very slowly and undisturbed. Using a sixth sense to detect when the moment is right, only developed by failing at doing this several times, pouring away the melt will reveal something beautiful…

This is a hopper crystal of bismuth I grew in my kitchen. Its interesting shape is due to the fact that, whilst cooling, the edge of the metal crystallises faster than the middle, and it doesn’t have the time to fill the inside in.

The beautiful iridescent colour arises from light interference in the tiny oxide layer that forms when the surface of the crystal touches air. Unfortunately, homegrown crystals like mine suffer from a bit of oxide scum which tarnishes the crystal slightly. However, if you have access to a glovebox or a lot of bismuth, you can get some incredible results.

The crystals I made were small (~10 mm) and it was very fun making them. If you want a decent sized crystal, it might be worth looking towards the professionals, or be prepared to invest in 5-10 kg of the stuff to make your own decent sized ones. There are some great vids on YouTube of kitchen chemists cooking up some huge crystals.

Below are some images of my crystals. Enjoy!

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