Conscience Kitchen, Notting Hill

Conscience Kitchen Restaurant and Coffee House on All Saints Road in Notting Hill.

It was 8am on an unseasonably warm morning in November. There were two cafes open on All Saints Road in Notting Hill, but Conscience Kitchen had an open door and comfy looking seating outside. Conscience Kitchen describes itself on its sandwich board as a “restaurant and coffee house”. At 8am in the morning, I wasn’t going to try the restaurant bit (though there were croissants available), but I did enjoy the coffee house bit. Seating had already been arranged outside. There were comfortable and cushioned seats immediately outside the cafe, a set of table and chairs on a converted parking space diagonally in front and a covered section in the parking space immediately outside the cafe. There were also plenty of seats in the spacious interior. The cushioned seats just by the window however offered a perfect spot to watch the world go by.

As it was a week day, plenty of people were either commuting to work or taking children to school. It is interesting how much you can discern about someone passing by when you listen to how their footsteps sound. Confident and clipped, shuffling or lethargic, or occasional combined with the whirling of the wheels of a scooter. A large number of characters passed by as I sat with my coffee. The coffee was an El Salvador single origin roasted by Round Hill coffee roasters. There was also a guest coffee on offer that day from a roaster in Amsterdam but as I didn’t realise this until I was paying I missed the opportunity to try both sorts.

Conscience Kitchen signed the lease on the shop in March 2020. What timing! Shortly after opening they had to close with the lock-downs and so the past eighteen months have been a series of adaptations as they renovated and grew their business. It does seem that their focus on good, organic food has attracted a loyal local following. At times during the second lockdown, the coffee house was turned into a produce store and while those days are far behind us (hopefully), that time did allow the locals to appreciate the care that Conscience Kitchen took over their ingredients. The pandemic times have also affected the seating arrangements with both the aforementioned parking space seating and the outdoor heaters a sign of our times. It was fairly warm that day and so I declined the offer of them turning the heating on for me, I had a hat and a coat after all. But this did give me a reason to look at the heater a bit more closely.

The heater at Conscience Kitchen. You can see the coiled element and the reflecting domed surface.

The heater consisted of a strange light bulb like fitting which led to a coil of what I had assumed was wire, enclosed in a tube and backed by a silvered domed surface. Investigating such heaters later, the ‘wire’ was more likely to be a weaved carbon fibre element. Regardless of what the heating element was made from, the mechanism of heating is the same. The power emitted by the element is the product of the electrical resistance of the element and the square of the current going through it. This relation, known as the Joule-Lenz law was discovered independently by Emil Lenz and James Prescott Joule in the 1840s. So why use weaved carbon fibre as a heating element? There are presumably a few reasons. Firstly, as a weave, a network of fibres, the heater will be more resilient if one of these, for any reason, breaks. If we had a single tungsten wire (as an extreme example), and it broke, the heater would no longer work. This makes the heater more long lasting. But there is a second, more physics based reason for using carbon fibre.

The power rating of the heater is defined as the energy emitted per unit time. When you subject a material to a given amount of energy, it is heated. The increase in temperature of the material is proportional to the amount of energy you put in, divided by the specific heat capacity of the substance which is material dependent. The specific heat capacity of woven carbon fibre is approximately twice that of copper and five times that of tungsten. This means that, for the same amount of energy the carbon fibre will heat up less than the metal wires. This provides the clue for the silvered dome. The heat from the heater is not really just coming directly from the electricity passing through the heating element. The second component is the infrared radiation emitted as a consequence of the temperature of the heating element. As the carbon fibre is not so hot as a metal element of the same power rating, the infrared radiation is at a different wavelength which turns out to be more efficient at keeping us feeling warm. The silvered dome was there to reflect the heat back towards the people on the terrace, further increasing the efficiency of this heater.

Looking further around, I noticed the hashtag on the Conscience Kitchen sandwich board: # Less is way more (unsure about the spacing!). Does this have an analogue in physics? Since the time of Joule and Lenz, physics has undergone increasing specialisation. In Joule’s time, physicists could investigate any number of topics which were also related to each other: heat, optics and electricity, or magnetism and fluid dynamics. Experiments with electricity informed our understanding of thermodynamics for example, while mathematics provided connections between magnetism and fluid dynamics via vortices. Researching one of these fields could, and did, lead to fruitful advances in other fields.

All about the coffee.

Since then, physics has become increasingly specialised and our research focus very narrow. In my field of magnetism, it is highly unlikely that I would get to investigate any aspect of fluid dynamics except for fun over the coffee table. It has been joked that, as individuals at least, we know ‘more and more about less and less’. This specialisation has however led to an enormous growth in our understanding of each of these sub-fields, and, correspondingly, a growth in the technological applications of the research. For example, dedicated research into a specific small detail of how electrical current travelled through layers of magnetic materials led to the sudden increase in the storage capacity of hard disks in the 1990s (and to a Nobel prize). The increased ability to store data has led to other fields being able to investigate highly data intensive areas and so produce advances in their subfields too. These are advances that could not have been made without specialisation.

Is this the ‘more’ of the “less is way more” equivalent for physics? Or is there perhaps a ‘way more’ about it?

The science historian LWH Hull described our situation as if the varying specialists were like people exploring the branches of neighbouring trees, “A man cannot understand other people’s problems by interrupting his own work to climb a few feet up their trees…”* Where then does this leave science? No physicist can any longer be a practitioner of the entire field of physics. Certainly no scientist can any longer understand ‘science’. And yet physics progresses because we work together in an inter-disciplinary way using our community to build a deeper understanding of the whole. This can only work because there is trust in other scientists and in the integrity of the work that they do. A trust that builds community which has consequences for our approach to society. Michael Polanyi took it further “Fairness in discussion has been defined as an attempt at objectivity, ie. a preference for truth even at the expense of losing in force of argument. Nobody can practise this unless he believes that truth exists.”**

“Less is way more”, but how “way more” do we want to take it?

Conscience Kitchen is at 23 All Saints Road, W11 1HE

*Quote from History and Philosophy of Science, LWH Hull, Longmans, Green and Co Ltd, 1959 – it is possible that it is not a verbatim quote as I only have my notes of this book with me at the moment and not the full text.

**In “Science, faith and society” by Michael Polanyi, Oxford University Press, 1946

Vacuum fillers

inverted Aeropress and coffee stain
The Aeropress on top of a mug, with a coffee spill. Clearly a badly performed inversion method brew.

The Aeropress is a lovely way to make a fairly quick cup of great coffee. Part filter, part immersion, it is a coffee brewer designed by an engineer. There are many ways to make coffee with an Aeropress but common to all is the ‘press’ towards the end where the plunger is pressed down onto the coffee pushing the liquid through the filter paper and into the mug. As you remove the Aeropress at the end, it can drip leading to coffee stains on the work surface. However there is a trick to prevent this. While watching James Hoffmann videos to improve my Aeropress technique, he mentioned that after pressing, he pulls the plunger back up a bit and this helps to prevent dripping. Genius. You can see his recommended Aeropress technique here.

The trick presumably works because the plunger has a rubber (or silicone) seal into the Aeropress base. This ensures that when you pull back the piston, there will be a slight vacuum created just behind the filter paper. As the air flows back into the space behind the filter it will primarily do so via the filter-end of the Aeropress (not the seal) and so any drips that were forming will be pushed back into the brewer. Rubber seals are not fantastically air-tight and will let air in eventually but for the few seconds that you need to take the Aeropress off of the mug and replace it upside down on the work surface this level of vacuum is sufficient.

So how air tight is a rubber seal? The seal in the Aeropress will not be very air tight at all. The seal created is purely through the rubber piston being pushed into the Aeropress body. However purpose-built rubber o-rings can support fairly respectable vacuums. Normal air pressure is 1 Bar or 1000 mBar. Using rubber o-rings that are clamped into place between separate parts of a vacuum chamber, it is perfectly possible to achieve vacuum levels of around 0.01 mBar**. For higher vacuum levels (or equivalently lower pressures), you would need to use a metal seal such as copper. As copper is slightly malleable, if you use it as a join between two parts of a vacuum chamber and clamp it together, you can create a very good (air-tight) seal. In this way you could pump the vacuum chamber to pressures of 10^-8 mBar or even lower. You would need this level of vacuum to make some of the components that are contained in your mobile phone or laptop, possibly even some of the components in the measuring scales you use to weigh the coffee. You would not need that sort of vacuum to make coffee itself.

How to brew a perfect cup? Would a bit of physics help with the clean-up?

The Aeropress is a fairly recent invention and yet similar problems, and solutions to Hoffmann’s trick would have been noticed in the past. And yet there is a common saying that “nature abhors a vacuum”, originally attributed to Aristotle. If we think that the explanation for the effect above seems sensible, how do we reconcile these two ideas? Descartes noted a similar problem in a wine keg. It is like the lid of a take-away coffee cup: for wine (or coffee) to flow out of a hole in a container, another hole is needed. Did the extra hole allow the wine to avoid the vacuum? Instead, Descartes explained it differently:

“When the wine in a cask does not flow from the bottom opening because the top is completely closed, it is improper to say, as they ordinarily do, that this takes place through ‘fear of a vacuum’. We are well aware that the wine has no mind to fear anything; and even if it did, I do not know for what reason it could be apprehensive of this vacuum…”*

The idea was that everything including space was absolutely filled with matter. So the extra hole in the wine keg allowed this extra matter to flow into the keg and the wine to flow out; if the Aeropress plunger is pulled back, matter would immediately flow back into the space created. The drops would be pushed back into the Aeropress and it would not drip. A very similar mechanism to the reason suggested for the behaviour above. It perhaps could cause us to question, what evidence do we have from our own daily lives about the existence of vacuums? How could we personally prove that they exist even as we rely on their existence for our consumer electronics?

Joseph Wright ‘of Derby’ An Experiment on a Bird in the Air Pump 1768 Oil on canvas, 183 × 244 cm Presented by Edward Tyrrell, 1863 NG725 https://www.nationalgallery.org.uk/paintings/NG725

There is a famous painting from the eighteenth century that demonstrates the creation of a vacuum in a home-setting. In “An experiment on a bird in the air pump” (pictured), Joseph Wright depicts the moment that air is taken out of a vacuum chamber containing a bird. The bird collapses in the vacuum as the audience looks on. We know the vacuum exists because the bird no longer has air to breathe. At the moment that we encounter the picture the scientist demonstrating could either let air back into the chamber and allow the bird to live or continue reducing the air pressure at which point the bird will die. What will he choose? The audience display a variety of reactions from the indifference of the couple on the left to the impotent horror of the girls on the right. Only two of the audience seem to be paying attention, even the experimentalist appears to be performing, and not participating in, the experiment. It could be argued that the painting speaks to us of the scientific method and the idea of being detached, outside of and observing the natural world. Imagining ourselves “independent observers” of a situation we are participating in. We are all detached and looking on, both controlling the life of the bird as well as claiming indifference to its fate.

As this was written, COP26 was continuing in Glasgow. We are at a specific point in time, just as with the “experiment on a bird”. Are we going to continue as we are or will we intervene and allow life to recover? Do we tell ourselves that we are indifferent observers or are we co-inhabitants of a common home? These are perhaps not considerations for a website about the physics of coffee. They may be considerations to have while enjoying, or certainly contemplating, a coffee. Whether or not you use a trick from vacuum science to help you clear up.

*Descartes, “The World”, ~1632

**Basic Vacuum Technology, 2nd Ed. Chambers, Fitch and Halliday, Institute of Physics publishing, 1998

A reason to add milk to your coffee

stirred cup of coffee with streak lines
It is astonishingly difficult to photograph the swirls of a stirred black coffee, still harder to capture the shape of the surface. This was an attempt with a strong light reflected on the surface.

You sit down to savour a well rounded, freshly roasted and just brewed pour over. Is there a good reason to add milk to it? Well, besides anything else, it may be a good test of an idea suggested by coffee-cup physics.

It’s about what happens as we stir our coffee. Many of us have contemplated our drink as we have stirred it either to cool it down or indeed to add milk or sugar. The surface of the coffee forms a depression at the centre, while at the walls of the mug, the surface forms a fairly steep slope. What is causing this shape and could it have any influence on how we appreciate our brew?

The shape of the stirred coffee surface is a consequence of the balance of forces acting on the surface. In addition to the force of gravity, there is the centripetal force on each bit of coffee swirling around the centre of the drink. These two forces have to balance at the surface of the water (assuming constant air pressure above the surface). If you make the further assumption that the coffee liquid rotates as one mass, so that the coffee at the edge of the mug rotates at the same angular velocity as the coffee at the centre, the centripetal force increases with increasing distance from the middle. This means that gravity dominates in the middle of the coffee whereas, towards the edge, the larger centripetal force is having a far greater influence. It is this that leads to the depression at the centre of the coffee and indeed the parabolic shape of the surface (click here for a mathematical derivation). The parabolae formed by such rotating liquids can be so perfect that liquid mirror telescopes have been developed to closely scrutinise specific parts of the sky. One problem with these liquid mirror telescopes is that the rotation of the liquid (often mercury) has to be perpendicular to the force of gravity. Which means that the telescopes are not able to move to different regions of the sky but instead only look ‘up’. Nonetheless, this does mean that they scan the same region of sky each night and so can be used very effectively to compare changes in that region of sky.

vortices, turbulence, coffee cup physics, coffee cup science
When a cup of water is first put onto a rotating platform, the liquid at the centre does not rotate at the same speed as the walls of the mug (that comes later). During these times, turbulent boundary layers appear at the walls of the mug which can be visualised with ink as has been done here.

Stirred coffee in a mug though is not a rigidly rotating liquid. Instead, the friction at the walls of the mug means that the coffee at the outer edge is slowed down and so the rotation is faster at the middle of the coffee than the edge. To form a parabola on the surface of a mug of coffee, it would be better to put the whole mug onto a record player played slowly. How does the shape of a stirred coffee differ from the surface of a coffee placed on a record player?

Initially, as the spoon is forcing all the liquid around together, the curvature will be approximately paraboloid. The interest comes once the spoon is removed and the friction between the coffee liquid and the sides of the mug becomes important. Towards the walls of the mug, the rotation will be slowed down which means that the centripetal force will decrease. Gravity will then dominate the combination of forces and the coffee surface will become flatter. As more of the coffee slows down, progressing from the edge of the cup towards the centre, the coffee surface will further flatten until the central depression is all that is left. As the friction slows more of the liquid down, so the depression at the centre of the coffee will also eventually disappear.

This is where the milk comes in. Assuming that you add cold milk to the centre of the rotating (hot) coffee, what should happen is that the milk (which is denser than the coffee because it is cold) will sink down towards the bottom-middle of the cup. As it sinks, so it will drag some of the swirling coffee down with it causing the coffee at the centre to accelerate and rotate faster around the centre of the cup*. The faster rotation will increase the centripetal force and so the central depression will become a bit more obvious again. This is the prediction anyway. So far, using chilled water and food dye, I have not been able to convince myself of the effect. But perhaps you will have more luck. Do let me know in the comments or over on social media, what results you get with this.

vortices in coffee
Vortices behind a spoon dragged through coffee. Experimental physics is a great excuse for playing with coffee.

Returning to the just stirred coffee, there may be one more thing to notice. At the interface between two moving fluids, a turbulent layer can form. We can see this when we first put a coffee on a record player (link here), or with the appearance of certain clouds (link here). This leads to a suggestion. As the coffee will be rotating faster at the centre of the cup than at the edge (owing to the resistance of the mug walls), the turbulence in the air over the centre of the cup will be greater than that at the sides. Fast moving fluids flow at lower pressure than slow moving fluids (Bernoulli’s equation). And although strictly speaking this is only valid for non-turbulent air flow, the principle can explain how planes fly and it may also have a consequence for our coffee.

As the air above the coffee at the centre of the mug will be moving faster than the air outside the mug, the air above the centre should be at an ever so slightly lower air pressure than that outside the mug. We know that water evaporates more quickly at lower atmospheric pressure. Consequently, more coffee aromatics will be evaporating from the centre of a just-stirred cup of coffee than from one you have left to sit still for a similar amount to time. To phrase this in a slightly different way, stirring your coffee should make it more aromatic and fragrant.

There are of course questions. Would the air pressure really decrease so significantly to affect the evaporation rate? How do you account for the fact that stirring coffee cools it relative to a coffee that is left to sit and wait? (Though why stirring a coffee should cool it is a whole other conversation). Nonetheless, it would appear to be a perfect excuse to brew and enjoy more coffee. Inhale deeply, stir contemplatively and, perhaps, add a little milk.

*In “Vortex flow in nature and technology”, HJ Lugt, John Wiley and Sons, 1983

The Dark Arts at Amar, Chelsea Green

Amar Cafe, Drinking coffee on Chelsea Green, Colombian Coffee
Amar Cafe on Chelsea Green. The small terrace area outside is on the spot of two car parking spaces.

Amar Cafe means to love coffee. Though this is in Spanish. In Bengali it apparently means “my cafe”. This could perhaps lead to a short meander onto a language inspired thinking trail about how a cafe that you love to frequent becomes “yours” in a certain sense. Though to return to the coffee, the cafe itself is a small space located on Chelsea Green. There are other branches of Amar cafe in telephone boxes around London and in Stratford upon Avon. A couple of parking spots just outside the cafe have been converted to an outside terrace complete with small olive trees at which you can enjoy your coffee in the fresh air of the Green. Although this is clearly a Covid-19 related temporary measure, it would be good if some of these outside places can remain on a more permanent basis. They do add to the character of the Green. Amar cafe specialises in Colombian coffees that they source themselves. All of the usual espresso based drinks are available as well as the option of a pour over. A small selection of pastries including empanadas are available for breakfast. There are about four tables outside and a couple of tables (along with a window bench) inside. As you enter the cafe, the bar is immediately in front of you. Outside, the cafe is painted a bright yellow colour which makes it stand out among the independent shops that are in this little quarter of Chelsea.

On the two occasions that we have visited, I have enjoyed a really well made pour over each time. Although I am not good at generating my own tasting notes, I would say that the coffee was sweet and syrupy, with a fruitiness and complexity that was very enjoyable. It was presented in a V60 jug together with a black, handle-less, porcelain cup.

V60 at Amar Cafe, Chelsea
Carafe of coffee and cup. The blackness of the coffee is similar to the blackness of the cup. On the carafe, condensation has formed on one side only. How did that happen?

Gazing into the filter coffee, there were patterns on the surface of the coffee that you could see reflected at different angles. But looking more deeply, it looked black, within a black porcelain cup. Where did the cup end and the coffee begin? How could you see black on black? Which was blacker?

A material appears black to us when it absorbs the majority of the visible light incident on it and doesn’t reflect or emit any visible light back. Until 2019, the world’s blackest material was “Ventablack”, but even this material only absorbed 99.96% of the light shining on it. Late in 2019, a new material was discovered that absorbs 99.995% of light shone onto it. And not just that, it absorbs 99.995% of light from the ultraviolet to the THz (between infrared and microwave). The material is truly black. But the inventors of this material were not trying to make a black material, they weren’t even really interested in optics. The invention came courtesy of a collaboration with an artist.

At the time, Diemet Strebe was the artist in residence at MIT. The scientists at MIT who would go on to discover this new black material, were interested in the electrical properties of carbon nano tubes (CNTs). CNTs are a layer of carbon atoms (arranged in the hexagonal structure familiar for layers of graphite) wrapped into a tube. Each tube may be just a few nanometers diameter but they can grow hundreds of micrometers long. Depending on how they are wound into a tube, CNTs can have a very low electrical resistance. But this electrical advantage is lost when you try to attach them to a metal like aluminium because the surface of aluminium always oxidises. Unlike aluminium, aluminium oxide is a brilliant electrical insulator. Which is great if you want to study effects in which the electrical current is blocked, but terrible if you want to utilise these fantastically conducting CNTs. This was the problem that the scientists at MIT wanted to solve. Their solution was to remove the oxide using salt water and then deposit the CNTs on top. (When phrased like this it sounds such a simple idea, “why did no one do this before”, but there are many experimental steps needed to be able to grow CNTs onto substrates such as aluminium and it has taken many years to get to this point). Once the CNTs were deposited, the authors found that not only did they have a good electrical conductor, the material was really black.

Coffee love. Some evidence of foam ripening at the surface of an oat milk latte.

What happened next is where the art comes in. Professor Brian Wardle who led the study was quoted as saying “Our group does not usually focus on optical properties of materials, but this work was going on at the same time as our art-science collaborations with Diemet. So art influenced science in this case.”

Thinking about how black the material was, the team decided to measure its optical absorption, which is when they discovered that they had broken the record previously held by Ventablack. And it was then that the art came back in. Strebe took a $2m natural yellow diamond and covered it with this ultra-black material. The result is striking (link). The composition, called “The Redemption of Vanity” could cause us to pause and ponder what we value as a society. What makes a sparkling diamond so valuable? How do we start to see objects by the fact that we can’t see them at all? If we extend this contemplation to our surroundings of Chelsea Green, we may wonder at this small little triangle of grass with its couple of benches. What does it reveal or hide? Does it help us to know that this is the last remnant of Chelsea Heath*, a bit of common land in which occupants of the surrounding manor houses down on (what is now) Cheyne Walk had the right to graze their livestock. Throughout this green, cows wandered up from the Thames as recently as the seventeenth century. A part of London that has disappeared, obscured by modern buildings yet held in memory by the street names and, the names of housing blocks.

As for why this material absorbs so much light, it is still an open question. It is known that the arrangement of the CNTs (including their alignment) can make a material coated in them very black. Even Ventablack is made from CNTs. It is a question that will probably continue to be discussed over many coffees. But is it a question that we would be asking again at all if it weren’t for the interaction in this case of art and science? Another point of contemplation that we can enjoy while looking into our coffee and just wondering ‘why’?

Amar Cafe is on Chelsea Green, 15 Cale Street, London, SW3 3QS and at several other telephone box locations.

*London Encyclopaedia, 3rd edition, Weinreb, Hibbert, Keay and Keay

Under pressure

What do you notice about this iced latte? The cup is rich with physics, but for this post, the important bit is the floating ice on top.

A coffee should be a time for relaxation, for reflection. As we come to the end of summer here in the northern hemisphere, we may want to enjoy one last iced coffee before we return to the warming coffees of winter. If on the other hand you are reading this from the southern hemisphere, the equatorial region, or some time after it was originally posted, you may be just starting to enjoy your iced coffees again. Either way, ice is remarkable and it is good to make some time to enjoy it. One of the things that makes it remarkable is what seems to be its very ordinariness: it floats.

Ice floats because the solid form of water is less dense than the liquid form. This is actually fairly unique to water. Most liquids get more dense as you cool them. As they transform into solids, they get denser still. This would mean that if you were to cool a liquid until it starts to solidify, the solid would sink, not float, on the liquid. If water were like most other liquids, all the ice in our iced-coffee would be at the bottom, not jiggling at the top. In addition to what would be an almost aesthetic problem for the coffee, this has consequences for life itself. When a lake or a pond freezes over, the fish and other aquatic life, can survive under the ice in the denser water. This odd property of water has helped life to evolve.

The reason for this strange behaviour lies in the way that water molecules bond together. Each water molecule can bond to a neighbour through a hydrogen bond. This optimises the structure to a layered form of well spaced hexagons (link here for an interactive model of water ice). Each corner of the hexagon is an oxygen atom. The size of the hexagon means that, if they weren’t arranged into a regular lattice, the water molecules could get closer together than they do in the solid phase. Which is another way of saying that the liquid can get more dense than the solid. Ice will float on water.

The layered structure of the ice crystals also means that each hexagonal face will tend to glide over the one below it or above it. It is this property of ice that means that we can determine the direction of glacial flow in centuries past. When fresh snow falls on top of a glacier, the density of the snow layer is about 50-70 Kg/m3. For comparison, the density of water is 1000 Kg/m3. Although each snow crystal is hexagonal, they have random orientation as they fall. As new snow falls, it pushes down on the old snow and compacts it until, about 80m down into the glacier, the density of the (now) ice is 830 Kg/m3. As the depth increases still further, the density increases to 917 Kg/m3 which is as dense as a glacier can be but is still much less than the density of water; a glacier would float. When the snow crystals are pressed down, the hexagonal layers of ice will glide past each other in the direction of push and the crystals will re-orienate. They will also grow as they merge with other crystals and as a result of the heat from the bedrock beneath them. This means that deep in the glacier, more of the crystals will be orientated in the direction of the push. Taking a vertical core of ice and looking at the orientation of the crystals in 0.5mm thick cross sections therefore reveals how they have been pushed as a function of depth. This in turn reveals which way the glacier has flowed in the past.

Sun-dog, Sun dog
A ‘rainbow’ of colour as seen in a ‘sun dog’ observed in central London. Note the order of the colours.

The structure of ice has one other surprise for those of us who are enjoying more coffee outside. Depending on the weather conditions, high up in the atmosphere, hexagonal ice crystals form. Because they are hexagons, they are, in effect, a section of a 60 degree prism. This means that light entering through one face, will be refracted twice to emerge from the crystal at 22 degrees relative to where it came from. If there are enough of these crystals high in the atmosphere, a bright circle will form around the Sun. For reasons that are probably obvious, it is known as the 22 degree halo. It seems fairly difficult to observe this halo. What is far more common to see are two bright regions of light at the 9 o’clock and 3 o’clock positions on the halo. In addition to being brighter than the rest of the light circle, these two regions often appear like a ‘rainbow’, but with the red on the inside of the halo and the blue on the outer edge. Known as “sun dogs” or parhelia, they too are a consequence of the ice crystals. As the ice crystals fall, they are more likely to fall flat so that each hexagonal face is horizontal. More of these ice crystals means that there is going to be more light refracted at the position horizontal to the Sun and so the light there is intense. They appear as separated colours for the same reason that the colours disperse with a prism: each wavelength of light has a very slightly different refractive index and so gets ‘bent’ by a slightly different amount. The ice crystals are bending the red a bit less than the blue.

This is a good time of year to keep an eye out for Sun dogs and haloes. And if we can do so while enjoying a well made iced coffee with the ice cubes floating at the surface, all the better.

Please do share any photographs you have of coffee with 22 degree haloes or sun dogs, either here or on Facebook or Twitter.

A return to Pritchard & Ure

A view from the terrace at Pritchard & Ure, overlooking the garden centre.

It is always great to realise that we have enough time to head across town to enjoy a coffee at Pritchard & Ure. If you haven’t yet tried it, Pritchard & Ure is a lovely spot in Camden Garden Centre (near Camden Road overground station). I first visited back in 2018 and ordinarily, I would not do a second cafe-physics review. But then 2020-21 have not been ordinary either and Pritchard & Ure too has changed. Back in 2018, a swaying pendulum prompted thoughts on how we knew that the Earth rotates. Since then, the world has moved in a different way.

In the case of Pritchard & Ure, this is reflected in a definite physical change to the cafe: a new terrace has been built overlooking a semi-outside section of the garden centre. This bit of the garden centre is sheltered from the rain by a permanent roof, almost like a permanent umbrella (see picture). The cafe on the other hand is protected from light rain and wind by a series of garden umbrellas. Apparently the indoor section of the cafe remains open if the weather becomes too awful (or presumably in autumn/winter). But in these times when it is good to be able to socialise outside, the new terrace offers a perfect place to do it. Accordingly, I took the opportunity to have an oat milk latte. While black coffee is normally a good test of the coffee in a cafe, I knew Pritchard & Ure served great coffee from my previous visit. Roasted by Workshop, the coffee is still offered in either a 6oz or 8oz size. But it’s been a while since I had enjoyed a properly made latte in a cafe and so why resist? We also enjoyed a spot of brunch, all while admiring the number of plants (and cacti) on view.

Can there be too much physics in one picture? Let me know what you see.

As before, obvious thought trains went in the direction of the science of plants and ecology. The large number of cacti just below our table was particularly suggestive of the changing conditions of our planet and the tendency for some areas of our world to be subject to more drought. The flowering plants too could prompt reflections on insects and how climate change is affecting them, including the possibility of mass extinctions. The past couple of weeks have seen Extinction Rebellion back in London as we prepare for COP26. One action that they took was an occupation of the Science Museum. The museum was targeted because Shell sponsor some of the exhibits including the “Our Future” exhibit about climate change. Extinction Rebellion have written an open letter to the Science Museum arguing, amongst other things that Shell gains “prestige and implied endorsement by the Science Museum group”. This is despite Shell’s own business plans not being “in line with limiting warming to 2C“. The museum disagrees with the principle of boycotting sponsorship by Shell on the grounds that such companies have the “capital, geography, people and logistics” needed in order to fight climate change. They also argue that some of these exhibits which help to inform the public about issues such as the science around climate change are only possible because of the financial muscle of companies such as Shell. It is a tough ethical cookie. One where we may have to try to read about the arguments and yet withhold judgement, knowing that most of us do not know enough, or have not thought deeply enough, to comment authoritatively.

The canal system built during the eighteenth and early nineteenth century required significant engineering expertise. This is a view from inside a loch on a canal within the M25 that surrounds London as the water fills through the gates, showing the loch gates and the walls of the canal.

A somewhat similar issue concerns the site of the garden centre itself. At the beginning of the 19th Century, the land belonged to William Agar (hence Agar Grove just north of the garden centre). Agar himself lived in Elm Lodge which was approximately where Barker Drive is now. He was involved in a dispute with the Regents Canal Company. He did not want the new canal to cut through his land. Finally, at the end of 1817 he relented and now, the canal cuts NW to SE just west of Pritchard & Ure. Was Agar a NIMBY (not in my backyard) or was his objection more complex? It’s another issue on which we have to suspend judgement. Though maybe this is easier to do as the case is over two hundred years old. Would we be so balanced if the Regents Canal were being built now and we wanted to react quickly on Twitter? What if the Regents Canal were HS2?

A more physics-based issue of balance could be seen in the umbrellas arranged over the terrace. They were supported not centrally but from the side, so the umbrella could be easily placed above the tables without the supports getting in the way. Immediately we could make connections to counterbalances and cranes. How is it physically possible that such a weight can be held by an outstretched (mechanical) arm? The weights of the flower pots standing on the umbrella bases may give us a clue.

There were many opportunities to think about issues of physics or balance on this terrace. It was a reminder of how good it is to go to a different cafe, put aside the smart phone, and just sit, enjoy a well made coffee and ponder about any subject that strikes your mind. Pritchard & Ure is a perfect place to do this, it remains a friendly space with good coffee (and food) at which you can enjoy thinking. And now, with the outside terrace, there is even more reason to go there as it is rare to find a cafe close-ish to central London with a large outdoor, and socially distanced, seating space.

Pritchard & Ure is at 2 Barker Drive, NW1 0JW

Activated Roasting

Brazil nut effect
Transforming green beans into the coffee we all recognise. Maillard reactions are behind some of the chemistry involved in coffee roasting. But how can we determine how fast a reaction will occur?

Coffee roasting is a complex process involving chemistry, physics and art. The experience and skill of the roaster turns the unpromising looking green beans into fragrant coffee beans that we can appreciate. Activated by the heat, many chemical processes occur as the aromatic volatiles are formed, compounds in the bean are transformed and the bean changes colour to that deep brown appearance with the smell that we associate with coffee. One of these processes are the Maillard reactions.

Maillard reactions transform “reducing sugars” such as glucose and fructose into the browning melanoidins (via a couple of intermediary steps). They are responsible not just for the colour and aroma of coffee, but also for the crust of a freshly baked loaf of bread, the transformation of a steak or just browned (not caramelised) onions and all manner of culinary processes. In coffee, the Maillard reactions usually start to become noticeable above 140C. At higher temperatures you also get caramelisation. But even at room temperature, or at body temperature, some Maillard reactions occur, just very slowly. Maillard reactions have even been implicated in the formation of certain cataracts. What is it that determines how fast the Maillard reactions occur?

The rate at which a chemical reaction takes place is determined by an energy known as the activation energy. The activation energy is the energy that the molecules would need to overcome in order to react together. It may be the result of having to overcome a repulsion between the molecules getting close together, or it may describe an energy needed to transfer electrons from one chemical to the next. Molecules can gain this energy from heat which means that at higher temperatures, more molecules have the energy for the reactions to occur. We could rephrase this to say that the rate of the reaction is greater at higher temperatures. This is expressed mathematically with the Arrhenius relation. In a fantastic illustration of the connectedness of things, this same Arrhenius relation can be used to describe many other phenomena such as how fast water evaporates from a coffee cup, how quickly milk goes off and even how long semiconducting devices will last before failing.

The Arrhenius equation also describes how quickly steam will evaporate from a coffee cup. As you can see above the cup here at Carbon Kopi

Although the reactions are faster at higher temperatures, there is no defined temperature below which they stop. Instead, the rate just decreases to such a point that the reactions happen rarely. Perhaps you could observe some of the chemical changes of roasting coffee at room temperature if you waited long enough. But before that point, other reactions with lower activation energies would occur or fungal growth may happen that would turn the beans rancid. Best to follow the roasting recipes.

Yet for coffee there is an additional complication before the Maillard reactions can happen. Unlike the situation where all the chemicals are together and able to react, the chemicals in the coffee bean exist within a structure. The molecules are not necessarily in the same place as each other; they need to move across the bean, including perhaps through the cell walls. And as the bean is heated, there are structural transitions that make it easier (in some cases) and harder (in others) for the chemicals to meet each other in order to react. What exactly happens when coffee is roasted?

To track what was going on Loong-Tak Lim and colleagues at the University of Guelph looked at how parameters such as the lightness of the roast or the weight of the bean varied as a function of roasting time. They roasted a lot of (small batch) coffee. Impressively, they also managed to put a thermometer right into the middle of a green coffee bean to track the temperature of the interior of the bean rather than the atmosphere in the roaster. The unfortunate detail was that they had to glue the thermometer in place.

Roasting coffee at four temperatures (220, 230, 240 and 250C), they showed how the degree of roast (indicated by the lightness of the bean) varied with roasting time and temperature. Unsurprisingly, a higher roast temperature produced a darker roast more quickly. But there were surprises too.

When they plotted the lightness of the roast as a function of time, they saw not one reaction with one activation energy but two. The two regions were quite distinct indicating that something chemically significant happened to the roasting process at around the point indicated by a “medium” roast. The activation energy of the first stage was 59.7 kJ/mol while the second stage had an activation energy of 170.2 kJ/mol. Whereas the first stage was over pretty quickly, the higher activation energy of the second stage meant that it happened far more slowly.

Don’t they look great? Roasting coffee connects to a vast range of concepts in physics and chemistry. Perhaps now is just a time to appreciate them.

The same sort of two step process was seen when they looked at how much mass the bean was losing as it was roasted. A lot of mass was lost early in the roast but as the roast degree went on, so the reaction slowed.

What caused the rapid slowing down of the second stage? One of the suggestions was that it was associated with the moisture loss as the green beans dried. A second suggestion was that a structural transition in the bean (of which there are many at these temperatures) hindered the reaction dynamics. This highlights a difference between coffee roasting in the lab and in the cafe. In the lab, the beans were rapidly heated to a set temperature at which they were held until the end of the experimental roasting time. In contrast, to produce great tasting coffee, many roasters will tweak the temperature-time profile of the roast so that a lot of the drying occurs before the Maillard reactions are allowed to ramp up. In a sense, the science is behind the experience here. To find out what is going on most parameters have to be kept constant while only one or two are varied. It doesn’t make great coffee but, hopefully, it is the start of a journey to understanding what is really happening as the beans seem to magically transform into something we can drink.

Meanwhile for those of us who neither roast nor experiment with the coffee but rather just enjoy the results of other people’s work, we can admire the connections that are being illustrated through working out what exactly happens as we roast the coffee. From the vastly disparate subjects covered by the Arrhenius equation, to the fact that the structural transitions that affect the coffee roast also occur in ceramics and magnetic materials. You will often hear it said that “everything is connected”. For coffee at least, this is yet another case where that appears to be true.

A short (lived) black

coffee at Story
A black coffee with bubbles on top. The colours on a bubble are the result of light interference. But sometimes the top of the bubble could appear black. What is happening there?

The long black can be distinguished from the Americano by the order in which the espresso and the water are added to the cup. This in turn will affect the type of bubbles on the surface of the coffee. As a guess, the long black (espresso last) will have many more but smaller bubbles than the Americano (water last) which will probably have larger, but fewer bubbles. Perhaps this guess is wrong, this could be an excuse to get out and drink more coffee.

We are used to the coffee being black and the bubbles on the surface reflecting a rainbow of shimmering colours that change with the light and with time before they finally burst. We know the physics of the colours on the bubbles: they are the result of the interference of reflections from the outer and inner surface of the bubble cancelling out certain colours and adding to others dependent on the bubble skin’s thickness. But what about black bubbles? Or, if not entirely black, perhaps the cap of the bubble can, for a short while, appear black just before the bubble bursts?

It is easier to take a short break from coffee and look for this effect in soap films. Like the bubbles on coffee, soap bubbles are caused by the surfactant in the soap solution having a hydrophilic (water loving) and hydrophobic (water hating) end. The hydrophilic end of the surfactant can point into the water (coffee) leaving the hydrophobic end to form a surface. When this is agitated with air, the hydrophilic ends remain contacted with water resulting in bubbles which are thin layers of water surrounded by these surfactant molecules. In coffee the surfactant is not soap but is formed by the lipids and fatty acids. These bubbles are therefore slightly weaker than the soap based bubbles and so while they will form on a coffee, it is not easy to make a film of a coffee bubble in the same way as you can dip a wire loop into a soap solution and come out with a soap film.

However, we can use the stability of the soap film to investigate the colours in the coffee bubbles and watch the colours evolve with time. At this point, I would strongly encourage anyone reading to grab a solution of soap and a wire loop and start playing with soap films.

Soap film in a wire loop held by a crocodile clip.
A soap film in a wire loop showing reflected horizontal coloured bands that are the result of light interference.

Holding the wire loop so that the soap film is vertical with a light source shining at it, we can watch as the film changes from being uniformly transparent to having bands of colour form and move down the film. We watch as there is a red/green band and another red/green band and then on top of the bands there appears a white, or at least pale blue, almost white, band and above that a layer that doesn’t reflect the light at all. If we view the soap film against a dark background looking only at the reflected light, this top portion of the film appears black. Rotating the loop we can see that the bands effectively stay in the same position because it is gravity pulling on this soap film that is causing the film to be thicker at the bottom than at the top. And we recognise that the coloured bands are revealing that thickness change to us by the fact that they are changing throughout the film. If we are careful as we rotate the wire, we could even see vortex like motions as the layers settle into their new position relative to the frame including at the very top where there are swirls and patches of fluid that mix the black layer with the coloured bands. What is going on there?

In fact, this black layer is one of the thinnest things that they human eye can see, and it occurs because of a subtle piece of physics. All waves have a number of properties defined by the position of the peaks and troughs on the wave. The wavelength is the distance between two equivalent points on the wave. The amplitude is the height of the peak (or trough). And the phase is the position of the wave relative to the peak (or trough). When light is reflected at a surface of a material that has a refractive index greater than that which the light is travelling through (eg. air into water, soap, or glass), the reflected wave has a 180 degree phase shift relative to the incident wave. Each peak becomes a trough, each trough becomes a peak. When light is already travelling through water, soap or glass and gets reflected at the surface of the material that is effectively air, there is no phase shift and the light is reflected back with the same phase as the incident wave (a peak remains a peak and a trough a trough).

At the top of the soap film, the layer is so thin that the light reflected from the first surface (180 degree shift) overlays that reflected from the back surface (no phase shift) so that peak and trough cancel each other out and we see no light reflected whatsoever for any visible wavelength; the surface looks black.

As bubbles ‘ripen’ or age, they will become thinner at the top of the bubble. It is at this point that you may be lucky enough to see a region of the bubble from which no light is reflected, this is the black film.

Which leads to some immediate questions. When we look carefully at the soap film, the boundary between the upper white band and the black film is quite sharp, it is not gradual as we may expect if the soap film were completely wedge shaped with gravity. It suggests that the top of the film is very thin and then suddenly gets thicker at the point where we start to see the colour bands. Moreover, the black film does not appear to mix with the thicker film just beneath it. As we watch, just before the soap film bursts, we get turbulence between the black layer and the thicker film, but the turbulent patterns appear like two fluids next to each other, not the same fluid in a continuum. And then, one final question. If we can’t measure the thickness of the black film with light (because it is all reflected as black) how can we know how thick this film is? If we rely on the light interference method, all we can say is how much thinner it is than the wavelength of light.

In fact, careful experiments have revealed two types of black film, which to us experimenting at the kitchen table would be indistinguishable. There is the common black film and the Newton black film. The Newton black film is effectively two layers of surfactant molecules only and is about 5nm thick (which is 5 millionths of a millimetre). The common black film is thicker, but is still much less than 100 nm thick. Investigating how these films behave is still an active area of research.

One last observation may prompt us to play for a bit longer with the soap films. Johann Gottlob Leidenfrost (1715-94) noted that if you put a sharp object such as a needle through the region of the soap film that showed the coloured bands, the film could self-heal and wouldn’t necessarily burst. If however you pierced the black region of the film, the film always burst entirely.

It seems that we could play endlessly with soap films, perhaps while watching the bubbles in our coffee. However you enjoy your coffee, have fun experimenting.

A couple more soap films showing reflected coloured interference bands. At the top, the film has become so thin that no light is reflected (clearly seen in the image on the right, where the lamp in the top left should be a circular reflection but is not reflected in the region above the coloured bands). In the image on the left, you can see what looks like turbulence or mixing just above the uppermost band.

To err is human…

Press Room coffee Twickenham
A smaller V60. For one cup you would use less coffee, but the errors on the measurement will always be there.

Preparing a good V60 requires 30g of coffee (for 500 ml of water)*. This can be measured using a set of kitchen scales, but a first estimate can also be obtained, if you are using whole coffee beans, by timing the passage of the grind through the grinder. Using an Ascaso burr grinder, my coffee used to come through at an approximate rate of 1g/s, so that, after 30 seconds, I’d have the perfect amount of coffee. Recently however this has changed, depending on the bean, sometimes 30g is 40 seconds, sometimes just less than 30 seconds.

Clearly there is an error on my estimate of the rate of coffee grinds going through the grinder. This may be influenced by factors such as the hardness of the bean (itself influenced by the degree of roast), the temperature of the kitchen, the cleanliness of the grinder and, the small detail that the ‘seconds’ measured here refers to my counting to 30 in my head. Nonetheless, the error is significant enough that I need to confirm the measurement with the kitchen scales. But are the scales free of error?

Clearly in asking the question, we know the answer will be ‘no’. Errors could be introduced by improper zero-ing of the scales (which is correct-able), or differences in the day to day temperature of the kitchen (not so correct-able). The scales will also have a tolerance on them meaning that the measured mass is, for example, only correct to +/- 5 % Depending on your scales, they may also only display the mass to the nearest gramme. This means that 29.6g of coffee would be the same, according to the scales, as 30.4g of coffee. Which in turn means that we should be using 493 – 507 ml of water rather than our expected 500 ml (the measurement of which also contains an intrinsic error of course).

Turkish coffee
A Turkish coffee provides a brilliant illustration of the type of particle distribution with depth that Jean Perrin used to measure Avogadro’s constant. For more information see here.

The point of all of this is that errors are an inescapable aspect of experimental science. They can also be an incredibly helpful part. Back in 1910, Jean Perrin used a phenomenon that you can see in your coffee cup in order to measure Avogadro’s constant (the number of molecules in a mole of material). Although he used varnish suspended in water rather than coffee, he was able to experimentally verify a theory that liquids were made up of molecules, by the fact that his value for Avogadro’s constant was, within error, the same as that found by other, independent, techniques. Errors also give us an indication of how confident we can be in our determination of a value. For example, if the mass of my coffee is 30 +/- 0.4 g, I am more confident that the value is approximately 30 g than if the error was +/- 10 g. In the latter case, I would get new scales.

But errors can also help us in more subtle ways. Experimental results can be fairly easily faked, but it turns out that the random error on that data is far harder to invent. A simple example of this was seen in the case of Jan Hendrik Schön and the scientific fraud that was discovered in 2002. Schön had shown fantastic experimental results in the field of organic electronics (electronic devices made of carbon based materials). The problem came when it was shown that some these results, despite being on different materials, were the same right down to the “random” noise on the data. Two data sets were identical even to the point of the errors on them, despite their being measurements of two different things.

A more recent case is a little more subtle but crucial for our understanding of how to treat Covid-19. A large study of Covid-19 patients apparently showed that the drug “Ivermectin” reduced mortality rates enormously and improved patient outcomes. Recently it has been shown that there are serious problems with some of the data in the paper, including the fact that some of the patient records have been duplicated and the paper has now been withdrawn due to “ethical considerations”. A good summary of the problems can be found in this Guardian article. However, some of the more worrying problems were a little deeper in the maths behind the data. There were sets of data where supposedly random variables were identical across several patients which suggested “that ranges of cells or even entire rows of data have been copied and pasted“. There were also cases where 82% of a supposedly random variable ended in the digits 2-5. The likelihood of this occurring for random variables can be calculated (it is not very high). Indeed, analysis of the paper showed that it was likely that these values too were either copy and pasted or “invented” because humans are not terribly good at generating properly random numbers.

A gratuitous image of some interesting physics in a V60. If anyone would like to hire a physicist for a cafe, in a 21st century (physics) recreation of de Moivre’s antics at Old Slaughters, you know how to contact me…

Interestingly, a further problem both for the Ivermectin study and for the Schön data comes when you look at the standard deviation of the data. Standard deviation is a measure of how variable is the measured outcome (e.g. duration of time a patient spent in hospital). For the ivermectin study, analysis of the standard deviations quoted on the patient data indicated a peculiar distribution of the length of hospital stay, which, in itself would probably just be a puzzle but in combination with the other problems in the paper becomes a suggestion of scientific fraud. In Schön’s data on the other hand, it was calculated that the precision given in the papers would have required thousands of measurements. In the field in which Schön worked this would have been a physical impossibility and so again, suggestive of fraud. In both cases, it is by looking at the smaller errors that we find a bigger error.

This last detail would have been appreciated by Abraham de Moivre, (1667-1754). As a mathematician, de Moivre was known for his work with probability distribution, which is the mathematics behind the standard deviation of a data set. He was also a well known regular (the ‘resident’ mathematician) at Old Slaughters Coffee House on St Martin’s Lane in London[1]. It is recorded that between 1750 and 1754, de Moivre earned “a pittance” at Old Slaughters providing solutions to games of chance to people who came along for the coffee. I wonder if there are any opportunities in contemporary London cafes for a resident physicist? I may be able to recommend one.

*You can find recipes suggesting this dosage here or here. Some recipes recommend a slightly stronger coffee amount, personally, I prefer a slightly weaker dosage. You will need to experiment to find your preferred value.

[1] “London Coffee Houses”, Bryant Lillywhite, 1963

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

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