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During long, cold winters in medieval Europe, there was something weird going on with church

organs.

Across the continent, some of these instruments were getting what people at the time called

“tin leprosy” or “tin pest.”

The organs grew gray, sickly-looking circles that spread over time and made the pipes hard

and brittle. Eventually, many of them fell apart.

But tin pest isn't actually a sickness — or, as people once thought — the work of the

devil. It's actually just some pretty simple chemistry.

It all comes down to the fact that a lot of organ pipes, even now, are made with tin,

since it can produce bright, resonant tones. And it's pretty and shiny.

At least, sometimes.

The thing is, tin exists in two different forms, or phases, depending on the temperature.

It's a lot like how water can be liquid at room temperature and solid when it gets

cold.

Except, instead of going from liquid to solid, tin goes from one type of solid to another.

Above 13.2 degrees Celsius, tin is a white, shiny, strong metal. That's the form of

tin that people use to make organ pipes.

But below that temperature, tin becomes dull, gray, and easy to break. That's the form

those pipes were transforming into during those long, cold winters.

The reason these two forms are so different, even though they're both tin, is because

of the way the atoms arrange themselves in each phase.

In white tin, atoms are arranged in what's called a tetragonal, or rectangular, structure.

But in gray tin, atoms are farther apart, arranged in more of a cube shape that's

less sturdy.

These two forms are called allotropes. And you might be more familiar with this phenomenon

than you think.

A similar thing happens with carbon, which can either take the form of graphite, like

in your pencil, or a crystal-clear diamond, all depending on how its atoms are arranged.

When it comes to tin, though, the process of turning from white to gray doesn't happen

overnight.

In fact, it can take months or years of cold temperatures for it to noticeably transform.

This is because it doesn't happen as soon as temperatures drop to 13 degrees Celsius.

Just below the transition temperature, there's not enough difference between the energy of

white tin and the energy of gray tin to motivate a transformation — unless something acts

as a catalyst. But that energy difference increases as temperature drops.

Around -30 degrees Celsius, that energy difference becomes significant, and transformation can

happen much more easily.

But! If it gets much colder than that, another factor comes into play that makes a transformation

less likely.

See, white tin has to expand by about a quarter to transition to the gray form, and it takes

a lot of energy to get the process started.

Except, as the temperature drops, the molecules in tin vibrate slower, so they don't have

the energy to overcome the barrier that it takes to start the phase change.

That means that right around -30 degrees, there's sort of a sweet spot where the transition

is most likely to happen.

But even at an ideal temperature, tin takes a long time to transform. It's not like

water, which quickly turns into ice as soon as it's cold enough.

That's because both phases of tin are solid, so its atoms are locked up in tight lattices,

and it takes a long time for them to rearrange themselves.

That's why old tin organs weren't just collapsing constantly. It took a long cold

winter for those tin pipes to start transforming.

But once gray tin appears, it can spread really quickly, like a rash.

The expansion of the tin makes little cracks in the metal, and that creates more exposed

edges, where atoms are free to move around and rearrange themselves.

And before long, the atoms of the white tin reorganize into the structure of gray tin.

Luckily, now that we know what causes it, the tin pest is easy to prevent. Just adding

small amounts of other metals, like lead, can keep tin atoms from moving around and

changing form.

And since a lot of organ pipes are made of metal blends, many have been able to withstand

the test of time.

It turns out a lot of mysteries aren't so mysterious once you've figured out a little

bit of the science behind them. And you can build your science skills with the courses

on Brilliant.org.

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save 20% on an annual premium subscription. And as always, thanks for watching SciShow.

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