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During long, cold winters in medieval Europe, church organs grew gray, sickly-looking circles that spread over their pipes. People back then believed that this was the work of the devil, but as it turns out, it’s just some pretty simple chemistry.

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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 make 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 some times.  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 temperate, then get 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, grey, and easy to break.  That's the form the pipes were transforming into during these 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 grey 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 grey 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 the 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 grey tin to motivate a transformation, unless something acts as a catalyst, but that energy differences increases as the 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 grey 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.  This all means that right around negative 30 degrees, there's a sort of 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 grey tin appears, it can spread really quickly like a rash.  

The expansion of the tin makes little cracks in 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 re-organize into the structure of grey 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.

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