Possibly the oldest in the world, this pitch drop demonstration is also one of the slowest science experiments ever created and proof that good things come to those who wait.

A fluid solid

This pitch drop demonstration was made here in Edinburgh at the Royal Scottish Museum workshop in 1902. It was designed to show the fluidity of a ‘solid’ substance known as pitch. 

Pitch is a tar-like substance made either by boiling the sap of certain trees or by-product of crude oil distillation. Although it appears solid, by watching drops of pitch slowly form and drop from a funnel, scientists have measured and proved its unusual liquid properties. We don't know the origin of the pitch used in this demonstration.

The oldest pitch drop experiment on record was set up at Aberystwyth University, Wales, in 1914. Our experiment is 12 years older than that and may well be the oldest in the world.

Pitch: solid or liquid?

If you can smash a substance to pieces with a hammer, then it must be a solid. Right?

Surprisingly, this isn’t always the case. Although it is currently used for tarmacking and can be smashed by hammers, the debate over pitch’s molecular structure has a long history. As early as the 1880s, physicist Lord Kelvin was creating experiments at the University of Glasgow to show that pitch flowed like a liquid – albeit at very slow speeds.

In the late 19th and early 20th centuries, researchers at universities across the world were keen to further Kelvin’s research and the pitch drop experiment was born. Some pitch drop experiments involve pouring a sample of heated pitch into a sealed glass funnel. Once the pitch has had time to cool and settle, the stem of the funnel is cut and the long wait begins. Or, as in ours, lumps of pitch are placed in the funnel and gradually coalesce and flow.

The most famous pitch drop experiment was set up by physicist Thomas Parnell at the University of Queensland in 1927. During 88 years, the funnel has only dripped nine times. It currently holds the Guinness World Record for longest continually running laboratory experiment. In 2005 they were awarded the Ig Nobel Prize which celebrates unusual areas of research that “make people laugh, then think”.

Another pitch experiment was set up at Trinity College Dublin in 1944. While less famous than its Australian counterpart, the Trinity experiment was the first to capture a drop of pitch falling on camera.

These experiments have helped to show that pitch belongs to a group of substances known as 'glasses’. At room temperature, glasses are hard and seem solid. However, unlike water and other liquids, which freeze when cool enough, glasses are just extremely viscous. They appear solid because they steadily slow and stiffen when cool. However, the oldest window glass has not been around long enough to flow. These experiments show it would take billions of years to see flow in a glass window.

When will the next drop fall?

The simple answer is: we don’t know.

Since 1902, the experiment has dripped at least twice but it’s impossible to predict when the next drop will fall. One drop fell between 4 and 6 June 2016, shortly after we moved the demonstration from our Collection Centre to its new home at Chambers Street. However, this was triggered by the move onto display and would not have fallen so soon if we had left the demonstration undisturbed.

While other pitch drop experiments show movement roughly every ten years or so, our experiment has a narrower funnel which means the pitch moves even more slowly. Unfortunately, over the last century we haven't kept data like the experiments in universities. We don't know exactly when the previous drop fell, or indeed if there has been more than one drop before this which would help us predict the next one.

Even if a drop of pitch did fall, there’s no guarantee anyone would be around to see it. Take John Mainstone, who inherited the Queensland pitch drop experiment in 1961. Under his care, five drops fell but Professor Mainstone missed all of them. The pitch experiment makes no allowances for weekends (1979), conference coffee breaks (1988), or broken video cameras (2000)!

Eighty-seven years after the Queensland experiment was set up, another drop fell in April 2014. This drop was witnessed by three webcams and thousands of online fans. But not Professor Mainstone, who had died eight months previously without ever seeing the experiment in motion.

Our pitch demonstration (museum reference T.1903.36) is on view in the Enquire gallery at the National Museum of Scotland.