invisible ink – Bean Thinking

Coffee, hot chocolate and cake at Second Shot in Marylebone,
Second Shot — Coffee, hot chocolate and cake at Second Shot in Marylebone

First impressions count, and the first impression we got of Second Shot‘s second branch north of Marylebone Station, was of a very friendly, local spot type of cafe. A small crowd were sitting around a table discussing a topic in an animated way. Various others were popping in or out, chatting with the barista, one person was sitting at a table with a laptop. And despite the fact that the weather was turning and the sky was becoming an ominous grey, the cafe itself seemed bright and open, with plenty of light coloured wood to complement the large windows.

We decided on a long black, hot chocolate and nut free brownie. Not wanting to spoil the initial taste of the coffee, I waited for the long black to arrive so that I could try that first before trying a bit of the brownie. The coffee was very well made, from Square Mile, and a perfect complement to the squidgy but moreish brownie.

On the walls, plants were hanging in pots with their leaves trailing down, while the light was reflected off a series of drawings sketched using coffee as an ink (they were for sale). It is surprisingly easy to make ink out of many different household (and not quite so household) items. Coffee is one base ingredient and, not surprisingly, makes a brown ink. But to bind the ink and to make it more viscous, gum arabic is frequently added to home-made ink recipes. The gum arabic is needed particularly if you are going to write with the ink with a fountain pen as it makes the ink viscous enough that it can flow through the nib.

coffee art, pictures using coffee ink, sketch, Second Shot — Hanging plants and coffee art, the walls at Second Shot, Marylebone.

Just as with the gum arabic, often ingredients are added to a product that are crucial to it, but that we do not realise they are there. Another example is the seaweed extract that is added to some plant based milks in order that they produce a better milk froth. But what if you don’t add all the ingredients at the same time, what happens if you add only one ingredient of the ink rather than all of those that are necessary?

One type of black ink that has been used for centuries is made from oak galls. It is even thought that ink based on galls was used to write the 4th century Codex Sinaiticus (the oldest complete copy of the Christian New Testament that we know of). The galls form on oak trees when the larvae of different types of wasp secrete a chemical that produces abnormal growth in the tree. The gall is the protective casing around the wasp larva, a type of home for the wasp larvae as they mature. Once the wasp has matured, it burrows through the growth which has turned from green to brown and it leaves a tiny little hole where it exited the gall. Galls can be seen on many oak trees during the summer and autumn and I’ve even seen them on the oak trees of various central London parks.

After the galls have been crushed and soaked, iron sulphate is added to the gall solution to produce a deep black ink*. Again, gum arabic is added to bind the ink to the paper and to increase its viscosity. But it is this ink that was also a popular ‘invisible ink’ first described over 2000 years ago. Philo of Byzantium** (who lived from 280-220 BCE) describes making an ink with the oak galls only. Forget about the iron sulphate for the moment. When it dries, the ink is nearly invisible on the paper, it can easily be missed and so can be sent to a collaborator as an invisible message. When the collaborator washes the paper with a solution of iron sulphate, the black ink appears on the paper and the message is revealed. Although the recipe has been known for two millennia, it has been used as an invisible ink even as recently as Mary, Queen of Scots and the American Revolution**.

coffe ink example — Coffee ink made for writing with a fountain pen (including recipe). Perhaps a yellow-ish paper was not the best medium to use to showcase the ink. An inadvertently (almost) invisible ink.

This idea, of making the invisible visible, by adding a key ingredient seems to form a nice metaphor for the societal aspect of the work of Second Shot. On their website they describe this aspect as:

“We’re changing perceptions on homelessness by being a destination that serves some of London’s best coffee, alongside a unique community atmosphere, amazing food, and just so happens to be changing lives.

We employ people who have been affected by homelessness, train them up and transition them on to long term employment elsewhere, helping them on their individual journey taking them from where they are, to where they deserve to be.“

Key ingredients of training, and accompaniment on individual journeys that combine to change our perceptions but that are not realised by us as we consume great coffee at this friendly cafe: Making the invisible visible, but doing it without us even realising that we have received a hidden message.

Second Shot’s second branch is at 49 Church St. NW8 8ES

*”Make Ink: A forager’s guide to natural ink making”, J Logan, Abrams New York, 2018

** “Prisoners, Lovers, & Spies: The story of Invisible Ink from Herodotus to al-Qaeda”, K Macrakis, Yale University Press, 2014

Breathing underwater at the London Particular

table and inside of the LP — Inside the London Particular

Tucked out of the way in New Cross, the London Particular has always been just that little bit far away to travel to, but always so tempting, a siren calling towards New Cross. The reviews of the food and the place were intriguing, while the coffee is roasted by HR Higgins, a roaster with a café that always seems closed when I get the opportunity to pass by (which is usually Sundays). So it was with some relief that I finally managed to get to the “LP” a couple of weeks ago. Towards the end of a row of shops, the space outside the café has plenty of seats where you can enjoy a spot of lunch and/or a coffee on a warm day. Inside feels more cosy. A bar on the left of the entrance forms a corridor with the wall that you walk through to get to a room with communal table at the back. In addition to the communal table, there are a series of individual high chairs along the wall. At the back of the café is a window with an old device sitting on it. “An old digital multi-meter” I said before being corrected by my sometime companion in these reviews, it has a dial, it must be an “analogue multi-meter” then! It did seem to be able to measure current and resistance and it did have a dial to indicate the value measured. Quite why it was sitting, unconnected, on the windowsill is anyone’s guess.

AMM, LP, NC — An Analogue multi-meter. But why was this sitting on the windowsill at the back of the cafe?

The lunch menu is good. Enough items there to provide choice, few enough that each can be done well. Significantly, the true London Particular, the pea soup, was not on the menu on the day we were there. We had a light bite of lunch, a black coffee and shared the jug of mint infused tap water that was placed on our section of the table. At the other end of the table, another customer was enjoying her lunch. So although communal, the table gave us enough room to be private and have our own conversation. A mirror along the wall above the table reflected the blackboard menu between the table and the bar. Thinking about mirror writing reminded me of Dr Florence Hensey and his letters of lemon juice ink. Back in the eighteenth century he had operated as a spy out of coffee houses on the Strand and in St Martin’s Lane¹. Spying on England for France, his letters, written in lemon juice (invisible ink) passed without detection before the frequency of correspondence drew suspicions. Times move on. Spies would surely no longer write in lemon juice or even mirror writing to avoid detection.

Lunch on a week day was a very good time to experience this café. It must get quite crowded at weekends or brunch times. So it was good to be able to sit back and contemplate our surroundings from the back of the café. In the foreground of our view though was the water jug. With fresh mint leaves stacked inside, it was evident that air had become trapped under some of the leaves forming tiny bubbles. How had the air got stuck there? Was it merely that the leaf was blocking the air bubble from rising through the water? Could there be slightly more to it?

Coffee and mint water in New Cross — Coffee and mint water at the LP

There is a popular expression “like water off a duck’s back”. Perhaps it arose because the duck’s back is often thought one of the most waterproof surfaces we know. But what makes the duck so waterproof? Why does water just form drops and then fall off the back of the duck? It is not because the feathers are oily. We sometimes ‘wax’ our waterproofs with a grease to make them resistant to getting wet and so perhaps we have thought that the duck’s back was just a bit greasy? And yet a study done back in 1944 showed that mere oil could not account for the waterproofing of the duck’s back.

Before delving into why the duck’s back is such a waterproof surface, it’s helpful to know how to quantify ‘waterproof-ness’ in the first place. To measure how waterproof something is, we use what is known as the contact angle, which is the angle that the drop makes with the surface on which it is sitting. Surfaces that are not waterproof (technically we call them “wettable” or hydrophilic), have very low contact angles, the ‘droplets’ of water on the surface are flattened. Waterproof surfaces on the other hand (imaginatively called hydrophobic), have contact angles which are much greater than 90º (it may be helpful here to have a look at the cartoon illustrating this point). Droplets that formed on a duck’s back had contact angles much greater than 90º, indeed, they formed almost spherical drops of water. What could be going on?

artemisdraws cartoon, contact angle, wettability — How ‘wettable’ a surface is can be defined by the contact angle that the drop makes with the surface. Image thanks to artemisworks.

The answer is in the details of the feather. The feather is not a flat surface but a material that has irregular protrusions and structure at the micro and nano-scale (one thousand and one million times smaller than mm scale respectively). These protrusions trap air within the feather and so effectively suspend the drop above the feather surface. The droplet does not have a flat surface on which to spread out. The structure means that the contact angles of the drops of water on a feather can be even higher than 150º; the droplets are held up almost as if they are spheres of water.

mint infused water at the LP New Cross — A breath of fresh air under water. Air bubbles trapped under mint leaves.

Another creature that uses the irregular protrusions on the hairs on its legs for waterproofing is the spider. The hairs on the legs of a spider mean that, just as the duck’s back, the spider’s legs are extremely waterproof. But it also means that air is trapped under the droplets. Consequently, if a spider finds itself submerged under water, the air under the droplets forms little bubbles similar to those under the mint leaf in the London Particular. And this allows a drowning spider the air it needs to breathe. Nanostructure helping the duck to dive and the spider to survive. And the mint water to be particularly refreshing on a warm day in a very pleasant place for a spot of lunch and a coffee.

The London Particular can be found at 399 New Cross Road, SE14 6LA

¹London Coffee Houses, Bryant Lillywhite, Pub 1963