Adam and Eve coffee house

Up in the air with a Pure Over Brewer

The diffuser sitting on top of the Pure Over coffee brewer. The holes are to ensure that the water falls evenly and slowly onto the grounds below.

The Pure Over is a new type of coffee brewer that is designed to brew filter coffee without the need for disposable paper filters. The brewer, which is completely made of glass, is a perfect size for brewing one cup of coffee and, as promised, makes a lovely cup without the need for wasteful paper filters. Generally, for 1-cup filter coffees, the Pure Over has become my go-to brewing method, although it does have a few idiosyncrasies to it that are helpful to be aware of while brewing.

An advantage of this brewing device is that it provides a large number of opportunities for physics-watching, including a peculiar effect that connects brewing coffee to an air balloon crash into the garden of a London Coffee House. It concerns a feature of the Pure Over that is specific to this particular brewing device: the ‘diffuser’ that sits on top of it.

The glass diffuser has five small holes at the bottom of it which are designed to reduce the flow of the water onto the coffee bed so that it is slower and more gentle. In order to avoid the paper filters, the Pure Over features a filter made of holes in the glass at its base. This filter does surprisingly well at keeping the coffee grounds out of the final brew, but it works best if the coffee bed just above it is not continuously agitated. The idea of the diffuser is that the coffee grounds are more evenly exposed to the water, with the grounds closest to the filter being least disturbed and so the coffee is extracted properly.

As water is poured from a kettle through the diffuser, the water builds up in the diffuser forming a pool that slowly trickles through the holes. Initially this process proceeds steadily, the water is poured from the kettle into the diffuser and then gently flows through and lands on the coffee. At one point however, the pressure of the steam within the main body of the brewer builds until it is enough to push the glass diffuser up a bit, the steam escapes and the diffuser ‘clunks’ back onto its base on top of the pure over. Then, this happens again, and again, until there is a continuous rattle as the steam pressure builds, escapes and builds once more.

The ideal gas laws, such as that found by Jacques Charles, relate the volume and pressure of a gas to its temperature. The application of the laws helped to improve the design of steam engines such as this Aveling and Porter Steam Roller that has been preserved in central Kuala Lumpur, Malaysia.

The pressure of the steam builds until the force exerted upwards by the rising steam is greater than the weight of gravity pulling the diffuser down. Once enough gas escapes, the pressure is reduced and so the steam no longer keeps the diffuser aloft which consequently drops with a clunk. The motion could take our thoughts to pistons, steam engines and the way that this steam movement was once exploited to drive our industrial revolution. Or you could go one stage earlier, and think about the gas laws that were being developed shortly before. There’s Boyle’s Law which relates the pressure of a gas to its volume (at constant temperature). That would perhaps partially explain the behaviour of the pure over. But then there’s also Jacques Charles and his observation that the volume of a gas is proportional to its temperature (at constant pressure). This too has relevance for the pure over because as we pour more water in from the kettle, we warm the entire pure-over body and so the temperature of the gas inside will increase. Consequently, as the amount of hot water in the pure over increases, the temperature goes up, the volume of that gas would increase but is stopped by the diffuser acting as a lid. This leads to the pressure of the gas increasing (Boyle) until the force upwards is high enough, the diffuser lid rises upwards on the steam which escapes leading the pressure to once again drop and the diffuser top to go clunk and the whole cycle begins again.

Of course, we know that Boyle’s law is appropriate for constant temperature and Charles’s law is appropriate for constant pressure and so the laws are combined together with the Gay-Lussac/Amonton law into the ideal gas laws which explain all manner of things from cooling aerosols to steam engine pistons. And yet, they have another connection, which also links back to our pure over, which is the history of hot air balloons.

Charles discovered his law in around 1787, a few years after the first non-tethered hot air balloon ascent, in Paris, in June of 1783. The hot air balloon is a good example of the physics that we can see in the pure over. Although Charles must have suspected some of the physics of the hot air balloon in June, he initially decided to invent his own, hydrogen filled balloon which he used to ascend 500 m in December of 1783. Hydrogen achieves its lift because hydrogen is less dense than air at the same temperature. However, it is the hydrogen balloon that links back to coffee and coffee in London.

hot air balloon
The ideal gas laws also contribute to our understanding of the operation of hot air balloons. We are familiar with them now, but how would such an object have been perceived by observers at the time of the first flights?

The first balloon flight in England took place using a hydrogen, not a hot-air, balloon in 1785. The balloon was piloted by Vincenzo Lunardi who was accompanied by a cat, a dog and, for a short while, a pigeon (before it decided to fly away). But it was not this successful flight that connects back to coffee, it was his maiden flight on 13 May 1785. On that day, Lunardi took off from the Honourable Artillery Company grounds in Moorgate, flew for about 20 minutes and then crashed, or as they said at the time “fell with his burst balloon, and was but slightly injured”(1) into the gardens of the Adam and Eve Coffee House on the junction of Hampstead Road and, what is now, Euston Road. In the 1780s the Adam and Eve coffee house had a large garden that was the starting point for walks in the country (in the area now known as Somers Town)(2). Imagine the scene as, quietly appreciating your tea or coffee, a large flying balloon crashes into the garden behind you.

The Adam and Eve is no longer there, in fact, its original location now seems to be the underpass at that busy junction, and the closest coffee house is a branch of Beany Green. However there is one, last coffee connection and it brings us back to the pure over. The pressure of the steam under the diffuser needs to build until the upwards force of the steam can overcome the gravitational force down of the weight of the glass diffuser. In the same way Lunardi had to have enough lift from the hydrogen balloon to compensate for the weight of the balloon and its passengers. Lunardi had wanted to be accompanied by another human on the day of his successful flight. Unfortunately, the mass of two humans in a balloon was too much for the balloon to accommodate which is why, the human was replaced by the dog, the cat and the pigeon.

Which may go some way to illustrate how far the mind can travel while brewing a cup of coffee, particularly with a brew device as full of physics as the Pure Over.

1 London Coffee Houses, Bryant Lillywhite, George Allen and Unwin publishers, 1963

2 The London Encyclopaedia (3rd edition), Weinreb, Hibbert, Keay and Keay, MacMillan, 2008

Something in the air at Mace by Coffee Chemistry Signature, KL

3D hot chocolate art on an iced chocolate, Mace, Mace KL, dogs in a chocolate

Drinking an iced chocolate with friends.

Perhaps Mace by Coffee Chemistry Signature in Kuala Lumpur should really have a “cafe-art” review rather than a “cafe-physics” review. Indeed, it was because of its latte art that Mace, which operates from a light and airy building in Damansara Uptown, Kuala Lumpur, had been recommended to me. With a comfortable interior and friendly staff, Mace is an interesting place in which to spend some time. But it is certainly the artistic endeavours that are the striking thing about Mace. Nor is it just ‘latte art’. The cakes at Mace arrive at the table decorated into an artwork. It is interesting that every visit to Mace will provide a different creation to enjoy, providing a place that you could return to again and again.

Nonetheless, this is a cafe-physics review website and there is also plenty of science to be found in latte art. For example, one of our drinks arrived with a 3D latte art sculpture floating on its surface. This piece requires manipulation of the rigidity of the milk foam, a topic that has been covered previously on the Daily Grind. However this time, it may be worth looking a little deeper into our frothy coffee: What makes a bubble?

The answer may seem obvious, inside the bubble is “air” with the bubble surfaces being formed from the water and proteins in the milk∗. But it is the question of what air is, and the implications of that, that is today’s Daily Grind.

Tweetie pie with a cake at Mace, KL

Cakes can be shared with cartoon characters at Mace

It appears that it was Empedocles (492 – 432 BC) who first recognised that air was a substance†. A thing that existed all around us. But it took until the seventeenth century and the invention of the air pump by Otto von Guericke (1602 – 1686) before people recognised that air was heavy. Guericke was responsible for the spheres of Magdeburg demonstration about the strength of a vacuum. He had fashioned two hemispheres of copper. Each hemisphere fitted very closely to the other. He then used his air pump to pump the air out of the spheres (ie. make a vacuum) and tried to pull the two hemispheres apart. Accounts vary but it is said that teams of 8-15 horses tethered to each hemisphere were unable to pull the spheres apart because of the vacuum created within the spheres†.

It was von Guericke’s air pump, together with the work of Boyle on gases and Torricelli’s invention of the barometer that prompted Francesco Lana-Terzi, SJ (1631-1687) to design an ‘air ship’. The idea was simple: If air had a weight and it is possible to make something lighter than air (by making a space inside a copper sphere a vacuum), then it should be possible to make something lighter than air such that it would float, just as objects that are less dense than water float. What differentiates Lana-Terzi’s design from previous fantasies about flight (such as Daedalus and Icarus) was that Lana-Terzi based his ideas on solid principles of mathematics and physics. He calculated how heavy the air was and balanced that with the amount of air that he would have to pump out of four hollow spheres of copper in order that they could lift a gondola full of people.

latte art by Mace, Eiffel Tower and hot air balloon

Art on a cafe latte at Mace

Although there were practical problems with Lana-Terzi’s idea of an air-ship based on four hollow copper spheres, his ideas were correct and eventually led to the development of the hot air balloon. And it is with the hot air balloon that we return to coffee, to Mace and find a connection with a London cafe. The artwork on my cafe latte was not, ‘latte art’ in the sense to which we have recently become accustomed. It was however very much art on a latte, with a scene featuring the Eiffel Tower depicted in chocolate. Just to the right of the Eiffel Tower and suspended in the milky sky was a hot air balloon, floating away exactly as Lana-Terzi had envisaged. Back in 1783, on the corner of Euston Road with Tottenham Court Road, there used to be a pub/coffee house called the Adam and Eve. It was renowned for its cakes and cream and its large tea garden. As far as I can work out, the tea gardens extended to around what is now Brock St and the site of a Beany Green. It was here, in 1783 that the balloonist Vincenzo Lunardi (1759-1806) “fell with his burst balloon, and was but slightly injured”‡. Fortunately for Lunardi, and for ballooning in general, it was only a slight setback. Lunardi went on to make a number of balloon flights, including the UK’s first successful hot air balloon flight.

So next time you are in Kuala Lumpur, why not spend a while at Mace imagining floating in on Lana-Terzi’s air ship gondola while you enjoy a gorgeously frothy iced chocolate. Who knows, one day Lana-Terzi’s air ship gondola may even feature on their latte art, I’d love to see that picture!

Mace by Coffee Chemistry Signature is at Damansara Uptown, Kuala Lumpur.

∗ On Food and Cooking, The science and lore of the kitchen, H. McGee, Unwin paperbacks, 1984

† History and philosophy of science, LWH Hull, Longmans, Green and Co, 1959

‡ Quote from London Coffee Houses, Bryant Lillywhite, 1963