Biscuit Crystals

biscuits gone wrong, crystals in the oven

Expanding biscuits are a 2D example of a close packed crystal lattice.

Blaise Pascal once wrote of the benefits of contemplating the vast, “infinite sphere”, of Nature before considering the opposite infinity, that of the minute¹. And although the subject of today’s Daily Grind involves neither infinitesimally small nor infinitely large, a consideration of biscuits and coffee can, I think lead to what Pascal described as “wonder” at the science of the very small and the fairly large.

The problem was that my biscuits went wrong. Fiddling about with the recipe had resulted in the biscuit dough expanding along the tray as the biscuits cooked. Each dough ball collapsed into a squashed mass of biscuit, each expanding until it was stopped by the tray-wall or the other biscuits in the tray. When the biscuits came out of the oven they were no longer biscuits in the plural but one big biscuit stretched across the tray. However looking at them more closely, it was clear that each biscuit had retained some of its identity and the super-biscuit was not really just one big biscuit but instead a 2D crystal of biscuits. The biscuits had formed a hexagonal lattice. For roughly circular elements (such as biscuits), this is the most efficient way to fill a space, as you may notice if you try to efficiently cut pie-circles out of pastry.

salt crystals

Salt crystals. Note the shape and the edges seem cuboid.

Of course, what we see in 2D has analogues in 3D (how do oranges stack in a box?) and what happens on the length scale of biscuits and oranges happens on smaller length scales too from coffee beans to atoms. Each atom stacking up like oranges in a box (or indeed coffee beans), to form regular, repeating structures known as crystal structures. To be described as a crystal, there has to be an atomic arrangement that repeats in a regular pattern. For oranges in a box, this could be what is known as “body centred cubic”, where the repeating unit is made up of 8 oranges that occupy the corners of a cube with one in the centre. Other repeating units could be hexagonal or tetragonal. It turns out that, in 3D, there are 14 possible such repeating units. Each of the crystals that you find in nature, from salt to sugar to chocolate and diamond can be described by one of these 14 basic crystal types. The type of crystal then determines the shape of the macroscopic object. Salt flakes that we sprinkle on our lunch for example are often cubic because of the underlying cubic structure on the atomic scale. Snowflakes have 6-fold symmetry because of the underlying hexagonal structure of ice.

It is possible to grow your own salt and sugar crystals. My initial experiments have not yet worked out well, but, if and when they do, expect a video (sped up of course!). In the meantime, perhaps we could take Pascal’s advice and wonder at the very (though not infinitesimally) small and biscuits. And if you’re wondering about where coffee comes into this? How better to contemplate your biscuit crystals than with a steaming mug of freshly brewed coffee?

¹Blaise Pascal, Pensées, XV 199

Counting the caloric at Jaz & Jul’s Chocolate House, Chapel Market

Jaz Jules chocolate house

Jaz and Jul’s, The Chocolate House on Chapel Market

The London coffee houses of the seventeenth and eighteenth centuries have entered history as Penny Universities, places of debate and centres of news. Together with the (scientifically based) Grecian, there was Jonathan’s in Exchange Alley (origin of the stock exchange) and Lloyd’s on Tower Street (associated with insurance). But along side these coffee houses there were the chocolate houses, Whites and Ozinda’s on St James’ St and the Cocoa Tree in Pall Mall. White’s in particular developed such a reputation that it features in Hogarth’s The Rake’s Progress (which can be seen at Sir John Soane’s museum).

So it is an interesting bit of history repeating to find Jaz & Jul’s, a chocolate house on Chapel Market. The interior here is very far from Hogarth’s rendering of White’s. Here, light fittings hang from the ceiling like drops of chocolate about to melt into the café while photographs of cocoa plants and farms adorn the walls. Moreover the emphasis on social responsibility, including in sourcing, mean that this establishment is worlds away from the debauched shenanigans at White’s. Their coffee is roasted and supplied by Monmouth while the cakes are hand made and, needless to say, very chocolatey. The light and fluffy chocolate-Pimms cake arrived with my coffee presented on a plate and matching cup that reminded me of a mint-chocolate-chip ice cream.

Interior of Jaz and Jules Chapel Market

The chocolate counter at Jaz and Jul’s

The side of the counter was tiled to resemble a bar of chocolate, which immediately reminded me of the physics and chemistry of chocolate crystallisation. However, the physics connection of this cafe-physics review is a bit more lateral than that. Soon after I had enjoyed my incredibly chocolatey cake at Jaz & Jul’s, a study was released which showed that Britons were significantly under-reporting their daily calorie intake. Could it be that the obesity epidemic is a result of us eating too much rather than merely exercising too little? Apparently, rather than consume the (recommended) levels of 2500 kcal for men and 2000 kcal for women, many people were eating up to 3000 calories per day. Everything in moderation of course and there was plenty of room in my own calorie count for that great piece of cake (honestly). But the word ‘calorie’ turns out to have a connection with chocolate in a more unexpected way.

Calorie comes from the Latin, calor, meaning heat which in turn hints at how we used to think about heat itself. While we now think of heat as energy, which is why it doesn’t even strike us to equate the ‘energy’ in the chocolate cake with the number of kilo-calories in it, this is not how heat was always viewed. In fact, in the eighteenth century, about the time of the old chocolate houses, heat was thought of as a type of fluid, caloric. Caloric was thought to be able to flow in and out of all substances. When something got hot it was because the caloric flowed into it, when something got cold, it was because the caloric had leaked out. Caloric theory was in many ways very successful in understanding heat and heat processes. For example, the theory easily explained thermal expansion, if a fluid had to flow into something in order for that thing to warm up, then surely, the fluid has to occupy some space, the object must expand to hold it!

Mint choc chip cutlery

Coffee with the Chocolate-Pimms cake.

One area that was tricky for caloric theory though was the fact that friction could cause something to heat up. Such heat generation is crucial for our extraction of chocolate. Once harvested from the plant and cleaned, the cocoa bean is first roasted then shelled to leave the cocoa ‘nibs’. These nibs are then ground more finely. As they are being ground, the friction caused by grinding is enough to cause sufficient heat to melt the cocoa butter in the nibs which is then extracted and retained for later use*. How could you explain this heating if you thought of heat as a fluid? The traditional explanation was that as the two objects rubbed against each other (in this case, nib and stone grinder), the caloric fluid would be squeezed out, it would appear as if heat had been generated.

Benjamin Thompson, Count Rumford (1753-1814), disagreed with this explanation of heat. In the course of a colourful career he had been involved in manufacturing cannons in Bavaria. Rumford had noticed that a lot of heat was generated each time a cannon shaft was bored out. The heat produced continued as long as the grinding continued. If the heat were due to the cannon leaking caloric, surely there would be a point at which the cannon stopped getting any hotter. Yet this did not happen. Rumford suggested (correctly) that instead what was happening was that the energy generated by the boring was being transferred into the metal of the cannon, causing microscopic motion.

Although the heat as motion/energy idea eventually caught on, caloric in some ways still survives in the name that we give to our food energy intake. And so we can return to the cake, could it be that spending time thinking about the caloric in the cake can justify the calories consumed eating it? Sadly the jury is out on whether thinking counts as calorie counting exercise. It seems that the brain’s energy consumption is already so great (at 20% of our resting metabolic rate), that intense thinking does not add too much to the energy consumed by the brain. So we’ll need another excuse and I don’t think we have to look far. The coffee and chocolate at Jaz & Jul’s is delicious enough to justify a significant chunk of your daily calorie count, just based on considerations of taste. Everything in moderation!


Jaz and Jul’s is at 1 Chapel Market, N1 9EZ

*”Chocolate: A Global History”, by Sarah Moss and Alexander Badenoch, published by Reaktion Books, 2009


All in a Glaze at Straits Times Kopitiam

Straits Times KopiTiam, outsideDespite the name, “Straits Times Kopitiam” is in Aldgate, London. Kopitiam translates to “coffee shop” and they can be found throughout Singapore and Malaysia. It’s not just coffee though, Kopitiams traditionally sell a wide range of fantastic noodles & rice dishes which will be taken for breakfast or lunch while chatting with friends. A couple of large communal tables in this Aldgate kopitiam provide the space for sitting down and chatting with your neighbours, while the selection of food on offer is very western, with sandwiches and cakes replacing some of the more typical Asian dishes. Although this is a bit of a shame, it is perhaps understandable. Hopefully, as time goes on we will get more great SE Asian food here and it will become more economically viable to sell it. Still, the Straits Times Kopitiam is, just as its SE Asian namesakes, a place where you can sit and enjoy a drink in very friendly surroundings. As we were sitting there, various people came in to chat about the crayon drawings on the wall at the back while messages were suspended on bits of string around the sides of the cafe. Such touches add to its friendly, almost intimate, social atmosphere. You may not know who roasts the coffee here but where else could you get a Teh halia (a type of tea with ginger), a hot chocolate with chilli or, a Milo Dinosaur (if you like iced drinks it is probably better for you to try it rather than have me describe it)?

enamel mug, teh halia, Straits Times kopitiam

An enamel mug containing Teh halia

A feature common both to the Straits Times Kopitiam and many such places in SE Asia though is the cups, enamelware cups with a navy rim. Such cups were apparently ubiquitous until fairly recently when they faded from fashion, though they are now back in a type of retro-chic, Singapore style. The great advantage of metal cups was that they were cheap and easy to produce. So in a world before plastic, a metal mug was the way to go. That said, metal crockery and cookware can tend to rust, or look a bit ugly and so the cheap metal cups were often enamelled to produce the type of mugs that are now used in Straits Times Kopitiam.

The story of how the mugs are enamelled leads us to the story behind the Giant’s Causeway in Northern Ireland. How? Well, enamelling uses the fusing of a glass onto the metal by heating it to approximately 800 C. Glass can be quite transparent and colourless so what gives it the white and blue colouring of the mugs used in the Straits Times Kopitiam? The blue is fairly easy, it is caused by the addition of metal oxides such as cobalt, but the white? That is harder to pin down and although some investigation suggested that some chemical additives could be used to make the enamel more opaque, it was also suggested that the opacity of the white enamel is due to the crystallisation of the glass.

Giants Causeway, lava crystals

A view of the Giant’s Causeway, taken by Jim “code poet” and shared under Creative Commons attribution license. Original  file location:

When a substance is heated above its melting point and then cooled, crystals can grow in it, the size and shape of which depend on the heat treatment of the material, for example how fast you cool it from its melting point. For enamel the material is glass but this crystallisation process can also be seen in chocolate. The heat treatment makes the difference between the formation of long crystals that give chocolate a hard, crisp sheen and smaller crystals that make the chocolate more dull, which is why the technique of tempering chocolate is so tricky. James Keir (1735-1820) wrote a study in the Royal Society’s journal Philosophical Transactions that described how glass crystallised as it was cooled. In particular he noted that cooling the glass slowly from its molten state produced a white and opaque type of glass that was at the time being called Reaumur’s porcelain. By studying the shape, form and cause of these crystals in glass, Keir made a sudden extrapolation: could the regular shapes of the Giant’s Causeway or the pillars of Staffa be caused by crystallisation of molten lava? This is indeed the explanation that we now have of the Giant’s Causeway*. Rather than being put into place step by step by a road building giant, the slow cooling of molten lava produced the crystal structures that are now visible on the shoreline.

Concerned that it would be thought that the had overstated the connection from glass to molten lava and geological features, Keir wrote at the end of his paper:

No just objection can be drawn against this analogy from the magnitude of the former [lava] compared with the minuteness of the latter [glass]: for the difference of size between the small vitreous crystals and the stupendous basaltic columns, which support mountains, islands, and provinces, is no more than is proportionate to the difference usually observed between the little works of art and the magnificent operations of nature“.

Does any more need to be said?

Straits Times Kopitiam is at 66A Whitechapel High Street,

“On the crystallizations Observed on Glass. By James Keir, Esquire, of Stourbridge. Communicated by G. Fordyce, M.D.F.R.S” Phil. Trans. R. Soc. Lond., 1776, 66, 530-542

* Note added 21 Oct 2015. It seems that we’re still debating how the Giant’s Causeway was formed. A recent article (here) suggests it is about how things crack as they cool rather than about how crystals form. However, the link to cooling glass and cooling ceramics remains. Keir was certainly on to something.