pour over

Coffee, oils and Clapham Common

A pour over at Röst Stätte coffee in Berlin. How does the preparation method affect the oils visible on the surface of a coffee?

Have you been making more coffee at home through the Covid-Lockdown times? Each morning, I have taken time to brew a coffee (or several depending on the way I feel that day) and then sit down and notice what is going on in the mug. The way the steam swirls upwards in turbulent patterns, the white mists on the surface of the coffee and the peculiar effects they have on reflected (and refracted) light from the coffee’s surface. And, the oils that appear on the surface of black coffee.

The appearance of these oils is very dependent on the way that you make your coffee. A cafetiere/French press is an immersion method of brewing coffee with no subsequent fine filtration of the grounds. It is therefore quite likely that the oils present in the roasted coffee bean will make their way to the surface of your coffee. If you brew by a pour over method on the other hand, it is thought that the paper filter should take out the oils as you brew. However, even when using a paper filter on a V60, a thin layer of oil can sometimes be seen on the surface of my coffee, visible in the sunlight on those mornings. How much oil is making it through the filter?

Regardless of whether you view the oils as important for the flavour or detrimental to it, there is something quite remarkable about oil patches on the surface of water: they can be a single molecule thick. Of course, you can pile more oil on the surface of the water and then you see the interference effects with light as you see on the surface of polluted rain puddles next to roads, but if there is a large enough area of water, the oil will spread out until it is one molecular layer thick.

How can we know that it is just one molecular layer thick? In one of those experiments that it is probably better to know about rather than to rush out to repeat, a clue came in the 1760s when Benjamin Franklin put a teaspoon of oleic acid (found in olive oil) on the surface of Mount Pond on Clapham Common. As he watched, the surface of the pond, which had been active with many capillary waves blowing over it, was calmed as the oil dispersed across the surface. First the oil remained in a small patch but it then grew, and grew until it reached the other side of the pond.

Mount Pond on Clapham Common. Probable site of Franklin’s 1760s experiment stilling the waves with a teaspoon of oil.

Franklin had been expecting the calming effect of the oil on the water waves, in fact he had been looking for it. On his journey to the UK from the USA he had been watching the wakes behind the ships in the fleet that were accompanying his ship on the journey. Two of the ships showed remarkably calm wakes, a fact that he had remarked upon to the ship’s captain. The captain had responded quite flippantly that it was probable that the cooks had emptied their greasy water over the sides of the ship. Mariners knew that oil and greasy cooking water, calmed the waves around the ships. We can learn a lot by talking to each other and listening to their experience.

The mariners knew that oil calmed the water but why? How? If we think about the oil as a surface layer over the water, it becomes possible to imagine an answer to this. Without the oil, when the wind blows over the water it will act to exaggerate the small perturbations on the surface of the water (caused by water flow, falling raindrops etc) which can then grow into waves. With a layer of oil on the surface, when the wind blows, if the oil is thick, it will act to blow the oil into a thinner layer covering the water surface. If the oil is thin already, it would take a lot of energy to stretch the oil surface to accommodate a growing wave. Either way, rather than exaggerate an existing perturbation on the surface of the water, the wind over an oily surface will tend to drag out the oil film, which will have the effect of calming any perturbations rather than encouraging them.

But how realistic was it that Franklin’s teaspoon of oil could have covered Clapham Common pond? About one hundred years after Franklin, Agnes Pockels and Lord Rayleigh were studying the effect of oil on the surface tension of water. As they did so, they calculated the thickness of thinnest oil layer that they could disperse over the surface of the water bath they were studying. Pockels calculated this thickness as 1.3 nm, Rayleigh at 1.6 nm, either way, a layer that is 10 000 times thinner than a grain in the smallest espresso grind coffee.

And one molecular layer thick.

It was while experimenting with surface tension that Agnes Pockels and Lord Rayleigh (separately) calculated that a layer of oil on water was just over 1 nm thick.

So to return to Clapham Common pond. 1 teaspoon is 5 cubic cm. If the oil formed a layer 1.5 nm thick over the surface of the pond, it would disperse over an area just slightly over 3000 square meters. It is perfectly possible for one teaspoon of oil to disperse over the surface of Mount Pond in Clapham Common. But what is possible is not necessarily advisable so let’s reverse the question and ask how much oil is on the surface of the coffee? Assuming that what is on our coffee is genuinely one molecular layer thick, or about 1.5 nm*. My cup has a radius of 4cm, meaning that the volume of oil on the surface is 0.0075 cubic millimetres. One metric teaspoon of olive oil is 5 cubic centimetres or 4.55 g. If we use the ratio of the volumes to calculate the ratio of the mass, we find that the oil we can see on the surface has a mass of about 7 micro grammes. A tiny amount, but a value consistent with studies suggesting that a small amount of cafestol (associated with the lipids in the coffee) gets through to the brew even in pour overs.

There is plenty to notice in a coffee, what do you see in yours?

*It is of course possible that the oils are actually thicker than this, but the paper filter does result in an oil film that is far from continuous across the coffee surface, suggesting that the oil is already stretched as far as it could be.

Notes from Berlin

What a cinnamon bun! Refinery coffee, Berlin
What a cinnamon bun! Coffee and bun at Refinery Coffee, Albrecht Strasse, Berlin

Cinnamon buns, doughnuts and plenty of coffee.

There is a very vibrant speciality coffee scene in Berlin with plenty of excellent cafes offering an interesting variety of coffees and pour overs. A city break of just a couple of days is nowhere near enough to even start to scratch the surface of the city. Coupled to that, we arrived during the Berlin coffee festival so many cafes were participating in public cupping and tasting events. So much to explore. But if you are rushing around, can you really stop and notice things?

How can you experience a place when you travel? Carl Jung pondered this very point when thinking about Rome, he wrote:

“I have travelled a great deal in my life, and I should very much have liked to go to Rome, but I felt that I was not really up to the impression the city would have made upon me…. I always wonder about people who go to Rome as they might go, for example, to Paris or to London. Certainly Rome as well as these other cities can be enjoyed aesthetically but if you are affected to the depths of your being at every step by the spirit that broods there, if a remnant of a wall here and a column there gaze upon you with a face instantly recognised, then it becomes another matter entirely.”*

We may not all have the sensitivity of Jung towards visiting a place but it can nonetheless be illuminating to reflect on the sentiment. This is particularly true of a city like Berlin where the remnants of walls are an ever present reminder of the dangers of ideologies, as well as the ease with which they can seize us.

pour over, Roststatte, Berlin
Pour over at Roststatte, spoilt for choice for coffee in Berlin.

How do you visit a cafe so that you can appreciate the space beyond the aesthetic? We visited several cafes including Brammibal’s Donuts, Common Ground, Oslo Kaffeebar, the Refinery and Roststatte. We also attempted a visit to The Barn (Mitte) but it was sadly too crowded on our visit. Each cafe revealed something unique and each was memorable for its own reasons. The lovely pour-over at Roststatte, the long black with character at the Refinery, the vegan doughnuts during a heavy rain shower at Brammibals. And yet we know how many cafes we missed (as you can see in this guide here or here).

And yet, what stood out as something to stop you in your tracks? What can you sit and dwell with as you savour your coffee? In hindsight, it is interesting that the connections at Oslo Kaffeebar were both very much connected with nature. It was not the wood lining of the cafe and the plentiful wooden furniture around the cafe but the spiders web style tiles on the table and something we saw at the window.

tiled table, Oslo Kaffeebar, Nordbahnhof, Berlin
The spider-web tiled table at Oslo Kaffeebar, near the Nordbahnhof in Berlin

The tiles on the table at the Oslo Kaffeebar were a regular array of spider’s webs. Each identifiable immediately as a web and striking for its regularity. The surprising uses of spider’s silk have featured on Bean Thinking before in a cafe that sadly no longer exists, but it was the regularity of the webs that prompted thoughts about the effect of different drugs, sadly including caffeine, on the behaviour of spiders. But it was a visitor to the outside of the cafe that struck us. A bird, silhouetted against the light, was perched on the (vertical) brick wall outside the cafe. What was it doing there? After it flew off, it was back, again in the same awkward perch but then it darted into the corner that the window made with the brick wall exterior to the cafe, could there be a nest there? The decline of bird species in our world as industrial scale farming has replaced hedgerows with monotonous fields of crops is well documented. But there is more to the bird-human interaction than that. Some bird species have adapted to the way we have traditionally built our houses, the problem being that modern building methods and renovations can threaten their ability to share our space. Other bird species have evolved to adapt to the way humans want to interact with birds with Great Tits for example apparently evolving longer beaks to make it easier for them to access the food put in bird feeders. What do these considerations reveal about evolution and our place in the world?

Oslo Kaffebar, Berlin
View from inside the Oslo Kaffeebar. To what extent does our culture influence our architecture, decoration and even our science?

On the other side of the Tiergarten, the pink tiling of Brammibal’s Donuts contrasted with the teal tiling that had been ubiquitous on the U-bahn line 5. The teal tiling somehow highlighted how even strictly utilitarian architecture nonetheless evokes an emotional response. In addition to considering how this challenges our understanding of architecture as representative purely of form, it can prompt a question: is a utilitarian philosophy consistent with an environment that allows science, (and the pursuit of knowledge for curiosity’s sake) to flourish**? (a question with repercussions for our own, consumerist and atheistic society). To what extent is our scientific development dependent on the prevalent attitudes of our culture? To be somewhat hyperbolic about it, is it possible to continue to do science, as we have traditionally understood it, in a consumerist society that demands constantly new entertainment (itself a form of consumerism)? Do we not replace ‘science’ with ‘technology’ and replace those questions that ask about our place in a world of reality and truth with questions that ask how we can better manipulate our world (where truth and reality as such no longer matter)? And what, in turn, does that do to our understanding of humanity’s place in the universe and so back to our cultural outlook?

We are then left with a couple of questions for ourselves. When travelling, can we allow the space to affect us with, as Jung says, “the spirit that broods there”, or do we take ourselves, imposing our own lens on another space? Can we open ourselves to encounter and is it not urgent, lest walls arise in our minds as well as our countries? I do not have any answers to such questions, but the cafes of Berlin, of London, and of many other places around the world would be a great place to ponder them.

*C.G. Jung “Memories, Dreams, Reflections” Fontana Press, 1961 and reprint editions.

**The question really is, if we consider that the best thing for society is to maximise the happiness of the maximum number, this could tend to promote the sort of science that produces results, technology or devices quickly. This short-term investment in science is contrary to the ideal of funding science for the sake of knowledge and arguably against the idea of being able to investigate the world as it is as opposed to merely developing the technologies that we can use. Is this true? Does it matter?

Cracking pour overs

cracks in a wheat field
Cracks in the soil in a field after a dry spell. But there are many connections between coffee and soil.

Summer this year has so far been quite hot and dry. Perhaps you have seen the grass dying back. Or maybe you have noticed the cracks forming in the soil in your local parks and fields. Such cracking is the result of the very dry weather and hopefully you won’t find it in your coffee, but there is another effect concerning soil compaction that connects to brewing a morning coffee as well as farming it.

It’s about the rain. As each raindrop falls to the Earth, it makes an impact with the soil underneath. While a light drizzle is not going to have that much of an impact, a larger raindrop of diameter say, 5mm, is going to hit the earth at about 9m/s – and that could cause quite a stir. Each impact will shake off smaller sized particles of soil which dislodge and get stuck in the pores between the larger soil particles. So the smaller particles start to ‘clog’ the pores between the soil particles and reduce the ability of water to penetrate into the soil. And although it seems a small effect, the result of this clogging of the pores by the smaller soil particles is to reduce the water permeability of the soil by 200-2000 times*: a soil crust is formed.

lilies on water, rain on a pond, droplets
The impact of a drop? Each rain drop can have a significant effect on the soil surface

This crust not only reduces the amount of water that gets through to the roots (by reducing the soil’s permeability), it also acts as a barrier for seedlings coming up: while many seeds can get through quite strong layers, even Sorghum struggles to push through if it needs pressures of 13-18 Bar to break through this crust*. So even without any farming machinery causing further soil compaction, just the rain is going to affect how additional water goes through the soil and how plants can grow out of it.

We are getting to the coffee bit.

The crust strength is influenced by the number of small (clay-type) particles in the soil. Clay particles are less than 2 microns in diameter which is smaller than the grind size you would find in even a Turkish coffee grind. But if we were to grind very brittle coffee beans (that shattered into many smaller particles as well as the grind size we want), or we were to use a blade grinder leading to a large distribution of grind size in our freshly ground coffee, we may expect to see some effects like this while brewing.

optical microscope image in water
Two coffee grinds compared under a microscope. How does the uniformity of particle size in a grind affect the clogging of a pour over? Magnfied 5x

If we think about a pour over brew (as opposed to an espresso or an immersion type), the initial pouring of water over the grind bed will dislodge any smaller particles in the grind and clog the grind in the same way as the rain on the soil. So if we were grinding way too fine, or using a blade grinder, or had a preference for darkly roasted (more brittle) coffee beans, it is possible that our pour-overs would tend to ‘clog’ more than if we were using a uniform medium grind of more lightly roasted beans. Has anyone experimented with this?

But the second soil connection we may notice as we prepare our pour over is that after our initial pour, as we let the coffee ‘bloom’ and the CO2 bubbles out, we receive a lovely aroma. A wonderful coffee smell as the grind bed continues to out-gas. This may remind us of petrichor, which is that great, and distinctive, smell of rain. And it turns out that petrichor is formed by the rain hitting the soil surface and producing air bubbles as it falls. The air bubbles then burst releasing aerosols from the soil which are so familiar to us as the scent of rain. A similar process to the blooming of the coffee grind. But just as with the coffee grind, as the water continues to fall and particularly if the pour over clogs to leave us with a water layer on the surface of the coffee (or soil), the aroma will reduce (or at least change) as the mechanism producing the smell changes.

bloom on a v60
Blooming petrichor, or should it be coffichor?

On a farm or in a garden, the effect of this soil compaction can be reduced by practises such as mulching. In addition to reducing the impact of individual rain drops on the soil surface, the mulch reduces evaporation of the water from the surface and changes the albedo of the soil. All things that may help coffee farmers to grow healthier coffee plants. In our pour overs, it is probably not a good idea to add any form of mulch! But this does mean that we can experiment more with the grind!

There are many more connections between your coffee and the earth around us, what will you notice?

*Soil Physics, WA Jury and R Horton, Wiley and Sons Publishing, 2004

Time standing still at VCR, Kuala Lumpur

VCR chalkboard

A trip down memory lane via a new cafe. VCR in Bangsar, KL

One of the first science-based talks I gave was about how VCR tapes worked. Depending on how you viewed it (and whether you had to listen), this was either an achievement given that I was at school and didn’t really understand magnetism nor magnetoresistive devices, or a thing to be suffered through (for much the same reasons). So when I learned that a new café called VCR had opened in Bangsar in Kuala Lumpur, it prompted a series of fond (and a few embarrassing) memories.

Moving on, it is clear that this second branch of VCR (the first is in Pudu, in the main part of KL), aims to provoke such memories of times past. From the name of the wifi to the pulleys behind the counter and the wooden screen at the back of the café, various details around the café pull your memory in different directions. However the coffee is very much in the present. With three types of coffee available to try as a pour over as well as the standard espresso based drinks, this café has a lot to offer. The coffee is roasted by VCR themselves in their Pudu branch. There is also an extensive food menu with an interesting Chawan mushi as well as an intricate avocado toast (topped with pomegranate seeds, toasted quinoa and feta).

coffee at VCR Bangsar

Coffee and pour over jug. But is the number 68 or 89?

The friendly baristas were happy to advise on which coffee to match with which brewing device (though there seemed a marked preference for V60s on the days I visited). In total I tried 4 pour-overs, one with the Kalita Wave and the others by V60. These coffees were all excellent but very different. A couple were fruity, one was sweet and full bodied, one reminded me a bit of the local fruit durian, not I hasten to add because of its taste, but because the aroma from the cup was so different from the flavour of the drink. It was a great privilege to be able to try these different coffees consecutively and to really experience the variety of flavours in coffee. Great care was taken while making the pour over before it was brought over to the table, together with a jug of water, it also seemed to me that the baristas kept a discreet eye on me afterwards to ensure I enjoyed the coffee. So it was a good experience to have had the opportunity both to enjoy one of those pour overs and to observe the people and the surroundings of VCR when I had to wait for 1 hour for someone with no phone and no book. If you get the opportunity to do this I would very much recommend it. Find a comfortable café, order a coffee and then sit, without distractions, and watch what your mind notices and where it wanders for an hour.

An obvious place for a mind to wander would be to the mechanism of tape recording (and why mini-disks are the superior recording medium for the elegance of the physics involved). However, in an hour a mind wanders far further than the name. Supporting the cakes (and a display case for the 2nd place award of the brewers cup), was a table with a concertina type decoration around its edge. Was this a nod to the Kalita Wave brewing device? This is a significant difference between the V60 and the Kalita Wave: the ridges (or wave pattern) on the filter of the latter. How does coffee flow past these ridges? Does this difference in flow dynamics make a difference to the taste of the coffee?

variables grind size, pour rate, pour vorticity

It seems that there would be a lot of physics to observe in the fluid flow in a Kalita Wave filter.

A few weeks previously a friend had made a (lovely) coffee with her Kalita Wave. It was interesting to note the different dose of coffee she used and the way the grinds built up in the ridges (compared with my ‘normal’ V60). Why do the grinds end up in the ridges? Why is there a layer of dust on the blades of a fan? Why do some corners of a building collect more dust or leaves than others? Are these questions related and does it change the flavour of the coffee in the Kalita?

In fact, there are many subtleties in understanding how fluids move around solid objects. One of these is that at the interface of the fluid with the solid, the fluid does not flow at all, there is a stationary layer. Known as a boundary layer or Prandtl boundary layer (after the person who first suggested their existence, Ludwig Prandtl), realising these layers existed revolutionised the field of aerodynamics. The problem had been how to model the drag experienced by a solid object in a fluid flow. Although perhaps only of academic interest in terms of the flow of coffee around a Kalita filter or a spoon, by the end of the nineteenth century and particularly, with the invention of airplanes, how to calculate fluid (i.e air) flow around a solid (i.e. wing) object became very important for practical reasons.

vortices, turbulence, coffee cup physics, coffee cup science

Another cool consequence of boundary layers:
Vortices created at the walls of a mug when the whole cup of coffee is placed on a rotating object (such as a record player).

Prandtl introduced the concept of a boundary layer in 1904. The idea allowed physicists to treat the main body of the moving fluid separately to the layer, very close to the solid, that was dominated by friction with the solid. This meant that the Navier-Stokes equations (that are used to describe fluid flow and ordinarily do not have an analytical solution) are simplified for this boundary layer and can be quantitatively solved. Although simple, by the 1920s Prandtl’s layer (and consequently the solvable equations) were being used to quantitatively predict the skin friction drag produced by airplanes and airships.

The boundary layer allows us to understand how vortices form behind cylinders or around the corners of buildings. I suspect a mix of the boundary layer, turbulence caused by the coffee going over many of the ridges and the brick like stacking/jamming of the coffee grains would combine to explain the difference in the grind shape around the Kalita Wave and the V60 filters. What this does to the flavour of the coffee and whether better brewing would involve more agitation, I will leave to Kalita Wave coffee lovers to investigate. And when you do, I would love to hear of your results, either here on Facebook or Twitter.


Waiting for the drop at Kurasu, Kyoto (Singapore)

Kurasu Kyoto Singapore, coffee Raffles City

The sign towards the entrance at Kurasu Kyoto, Singapore

Kurasu Kyoto, in Singapore, was recommended to me as a great place to experience pour-over coffee. Although they will serve espresso based drinks too, it is the pour over coffee for which they are famous. The Singapore branch is at the front of a shared working space in an office block. Entering from the street, you have to go up one level before the smell of the coffee will guide you to the café.

Ordinarily, coffee chains would not be featured on Bean Thinking. However, despite it’s name, this is a ‘chain’ of only two outlets, the original branch in Kyoto, Japan and this one in Singapore. The menu featured several coffees with their differing tasting notes together with a few other drinks. Coffee is shipped from Japan weekly as well as being locally roasted in Singapore. It is very much a place to enjoy your coffee while sitting on the comfortable chairs before getting back to work (or perhaps, a place to meet potential colleagues over a refreshing cup of coffee). And it is highly likely you will enjoy your coffee which is prepared for you as you wait.

coffee machine, V60 Kalita

The bar and some of the coffee equipment in the cafe space at Kurasu Kyoto Singapore

There is no hint of automation here. Each cup of coffee is prepared carefully and individually by the barista behind the bar. V60 or Kalita, it was somewhat mesmerising to watch the pour over being prepared, rhythmically, carefully, by hand. Indeed, automation seems almost alien to this place where the act of making coffee is truly artful. Once prepared, the coffee is brought to your table in a simple ceramic mug for you to taste for yourself and see how your tasting notes compare.

As I was watching, two thoughts occurred to me, the first of a directly scientific nature, the second more about our society. Firstly watching the barista slowly prepare the pour over, it is difficult not to be reminded of the pitch drop experiment.

You may remember the story from 2013 and then again in 2014. Two experiments that had been set up in 1944 and 1927 respectively finally showed results. The experiments were (indeed are, they are still going) very similar and concerned watching pitch (which is a derivative of tar) drop from a funnel. Pitch is used to waterproof boats and appears to us almost solid at room temperature although it is actually a liquid but with an extremely high viscosity. To put this into perspective, at room temperature coffee has a viscosity similar to water at about 0.001 Pa s, liquid honey has a viscosity of about 10 Pa s, but this tar has a viscosity of 20 000 000 Pa s. The experiments involved pouring this tar into a funnel and then waiting, and waiting, for it to drip. Both experiments seem to drip only approximately once a decade but until 2013 (and 2014 for the other experiment), the actual drop had never been seen. Both experiments are now building their droplets again and we await the next drop in the 2020s.

Imagine waiting that long for a drip coffee.

coffee Kurasu Kyoto Singapore

Apparent simplicity. The coffee at Kurasu Kyoto Singapore

But then a second thought, there is currently a lot of angst, particularly about automation and our environmental and/or political situations, as if they are something from outside ourselves being imposed upon us. To some extent it is true that we are not in control over many things happening around us. But in our feeling of powerlessness, are we resigning more than we ought to of our responsibility for the power that we do have? It was something that deeply concerned Romano Guardini in his essay “Power and Responsibility”¹. To use the example of automation and the pour over. Guardini argues that people become poorer as they become more distant from the results of their work (e.g. by automating the pour over coffee with a machine). And that the better the machine, the “fewer the possibilities for personal creativeness”¹ that the barista would have. For Guardini, this has consequences for the human being for both barista and customer. The barista clearly loses the element of their creativity when preparing a pour over with a machine but the customer too is affected by the loss of a personal contact, possible only through individually created things. Rather than celebrating each other as individuals we become consumers with tastes “dictated by mass production”¹ and people who produce only what the “machine allows”. To respond to the challenges of our contemporary society involves discovering where we each have responsibility and exercising it, no matter how small or large that responsibility seems (to us) to be.

Which is somehow resonant with the interview that one of the Kyoto based baristas at Kurasu Kyoto gave that was recently circulated by Perfect Daily Grind. Asked what was her preferred brewing method, she replied it was the V60 because of the control that the individual barista could gain over the flavour of the cup merely by tweaking some of the details of the pour. A knowledgable art rather than a technology. And it is precisely this knowledgable art that you can see carefully and excellently practised in the Singapore branch.

Kurasu Kyoto (Singapore) is at 331 North Bridge Road, Odeon Towers, #02-01

“Power and Responsibility” in “The End of the Modern World”, Romano Guardini. ISI books, (2001)


Now you see it now you don’t at Bond St Coffee, Brighton

Outside Bond St Coffee Brighton

Bond St on Bond St, Brighton

A couple of weeks back, I tried the lovely Bond St. Coffee in Brighton on the recommendation of @paullovestea from Twitter. It was a Saturday with good weather and it turns out that this particular café is (understandably) very popular and so, sadly, to begin with we could only sit outside. That said, it was a lovely spring day (sunny but a bit chilly) and so it was quite pleasant to watch the world go by (or at least Bond St) while savouring a well made pour-over coffee. All around the café, the street decoration hinted at times past. Across the road what was obviously a pub in times gone by has turned into an oddities store. Air vents to a space underneath the window seating area in Bond Street café itself suggested an old storage space. A seat in the window appeared to have been re-cycled from an old bus seat.

But it was the pour-overs at Bond St. Coffee that had been particularly recommended and they certainly lived up to expectations. I had a Kenyan coffee roasted by the Horsham Roasters. The V60 arrived at our bench seat/table in a metal jug together with a drinking glass. The angle of the Sun caught the oils on the surface of the coffee, reminding me of Agnes Pockels and her pioneering experiments on surface tension. Pouring the coffee into the glass I thought about the different thermal conductivities of glass as compared to metal and how I had put both down on the wooden bench. How was heat being transferred through these three materials? And then, as I placed the metal jug back on the bench I noticed the reflections from the side of the jug and thought, just why is it that you can see through the colourless glass but the metal is grey and opaque?

Metal jug and transparent glass

Metal jug, glass cup. V60 presentation at Bond St Coffee

On one level, this question has a simple answer. Light is an electromagnetic wave and a material is opaque if something in the material absorbs or scatters the incoming light. In a metal, the electrons (that carry the electric currents associated with the metal’s high electrical conductivity) can absorb the light and re-emit it leading to highly reflective surfaces. In glass there are no “free” electrons and few absorbing centres ready to absorb the light and so it is transmitted through the glass.

Only this is not a complete answer. For a start we haven’t said what we mean by ‘glass’. The glass in the photo is indeed transparent but some glasses can be more opaque. More fundamentally though, there is a problem with the word ‘opaque’. For us humans, ‘visible’ light is limited to light having wavelengths from about 400nm (blue) to about 780nm (red). ‘Light’ though can have wavelengths well below 400 nm (deep into the UV and through the X-ray) and well above 780 nm (through infra-red and to microwaves and beyond). We can see the spread of wavelengths of light visible to us each time we see a rainbow or sun dog. Other animals see different ranges of ‘visible’ light, for example, bumble-bees can see into the ultra-violet. So, our statement that glass is transparent while metal is opaque is partly a consequence of the fact that we ‘see’ in the part of the spectrum of light for which this is true.

Sun-dog, Sun dog

Sun dogs reveal the spectrum of visible light through refraction of the light through ice crystals.

For example if, like the bumble-bee, we could see in the UV, some glass may appear quite different from the way it does to us now. Even though the glass in the photo lacks the free electrons that are in the metallic jug, there are electrons in the atoms that make up the glass that can absorb the incident light if that light has the right energy. There are also different types of bonds between the atoms in the glass that can also absorb light at particular energies. The energy of light is related to its frequency (effectively its colour*). Consequently, if the energy (frequency/ wavelength) of the light happens to be at the absorption energy of an atom or an electron in the glass, the glass will absorb the light and it will start to appear more opaque to light of that colour. Many silicate glasses absorb light in the UV and infra-red regions of the electromagnetic spectrum while remaining highly transparent in the visible region. High purity silica glass starts to absorb light in the UV at wavelengths less than approx 160nm†. Ordinary window glass starts to absorb light in the nearer UV†. In fact, window glass can start to absorb light below wavelengths of up to ~ 300 nm, the edge of what is visible to a bumble bee: The world must appear very different to the bumble bee. At the other end of the scale, chalcogenide based glasses absorb light in (our) visible range but are transparent in the infra-red.

Looking at how materials absorb light, that is, the ‘absorption spectrum’, enables us to investigate what is in a material. It is in many ways similar to a ‘fingerprint’ for the material. From drugs discovery to archaeology, environmental analysis to quality control, measuring how a material absorbs light (over a wider range of frequencies than we can see) can tell us a great deal about what is in that material.

Perhaps you could conclude that whether something is opaque or crystal clear depends partly on how you look at it.


Bond St Cafe is on Bond St, Brighton, BN1 1RD

*I could add a pedantic note here about how the colour that we see is not necessarily directly related to the frequency of the light. However, it would be fair to say that a given frequency of light has a given ‘colour’ so blue light has a certain frequency, red light a different frequency. Whether something that appears red does so because it is reflecting light at the frequency of red light is a different question.

†”Optical properties of Glass”, I Fanderlik, was published by Elsevier in 1983.