How a fountain pen works

We all know what a fountain pen is. It’s a tube filled with ink and a nib that delivers that ink onto paper. Simple.

Hang on, can it really be that simple? No. For proper function of a fountain pen there has to be a close balance between ink flow and a flow of air in the opposite direction. We can start to explore the how and why of that by considering the question of why ink does not flow out of a pen until the writing starts.

Before getting into the nitty-gritty of fountain pen magic, let’s consider a simpler system: a ballpoint. Yep, a standard ballpoint pen like the Bic Cristals that my wife prefers over my pens!

Why does ink not flow out? Part 1: ballpoint pens

Ballpoints pens have a ball in the end that transfers ink from the reservoir to the page by rolling. The ink reservoir in a Bic Cristal is a simple tube open to the air at the top, so why does the ink not flow out of the pen by gravity and, similarly, why does the ink not flow out of the back of that tube when the pen is held tip up? Some ballpoints have pressurised ink reservoirs (e.g. the Fisher Space Pen) so how do they not leak everywhere?

 You probably already know the answers. What stops the ink from flowing out of a ballpoint pen is just the viscosity of that ink. Ballpoint ink is thick and sticky oil-based stuff and so it does not flow readily though the tight space between the ball and its socket. In fact, it is so thick and sticky that it almost doesn’t flow at all and that’s why it doesn’t leak out the back of a Bic Cristal ink tube unless you leave it in a very hot car. (I cannot resist putting in a link to the “pitch drop experiment”) The viscosity of the ink allows the pen to hold ink in all orientations, and its stickiness allows the ball to deposit ink onto the rolling ball and then the page.

[Rollerball and gell pens are closely related to ballpoint pens, but they often (always?) use water-based inks that are less viscous then the oil-based inks of ballpoints. I am not going to discuss them beyond saying that if you are intent on using water-based ink then why not use a fountain pen?]

Why does ink not flow out? Part 2: fountain pens

Fountain pen inks are almost entirely made of water and are neither sticky nor viscous and so the ink regulating task of a fountain pen is entirely different to what happens with a ballpoint pen. Fountain pen ink would certainly run out the back of the unsealed ink reservoir of a ballpoint pen. So how does a fountain pen stop its runny ink from running out all over the place?

It is often said that capillary action and surface tension prevent ink from leaking from a fountain pen. Without explaining either of those (related) things, I will say that they are not the whole story. A simple experiment will prove that. Put a full ink cartridge into a fountain pen and hold the pen nib downwards. Now melt a hole in the back of the cartridge with a hot pin. Make sure that you do that while wearing white clothes because that will help you see what comes next: the ink will now dripping freely from the pen! Perhaps you don’t really need to do the experiment as it is safer to leave it as a thought-experiment (a gedankenexperiment for Pelikan, Lamy, and Kaweko enthusiasts!). And, of course, you can just take my word for what would happen.

When the back of the ink cartridge has a hole air can easily enter (and leave, if it has a mind to) and so the air pressure at the top of the ink in the cartridge will immediately equalise with the ambient air pressure. As the ink flows out of the pen onto your white trousers more air will enter the cartridge to replace the volume of the ink that flow out. If you were now to put your finger onto the end of the cartridge to block the hole the outflow of ink would slow and then stop as a slight negative pressure develops at the top of the cartridge to counterbalance the weight of the ink column.

You can see the pen maker Ian Schon demonstrate a real version of that experiment using a plastic pipette in place of a pen here. If you watch closely you might notice that there is a little ink left inside the tip of the pipette at the end. That little bit of ink is left behind because of the surface tension of the ink and the capillary action of the thin tube at the tip of the pipette. Doing the experiment with a fountain pen will equivalently result in a little ink being retained in the feed and nib and that is totally because of surface tension and capillarity!

How does ink come out when you write?

So, when a fountain pen is full and held nib-downwards the ink is held back by both capillary action and a pressure gradient between the inside of the cartridge and the air outside. When you move a fountain pen nib across paper the ink in the nib tip is left behind on the paper largely because of capillary action. You can think of it as the ink being dragged out of the nib by the paper and you will not be far from the truth. As that ink is dragged out of the pen a slight negative pressure will develop inside the pen (unless you have put a hole in the cartridge!). If nothing else happens, pretty soon that negative pressure will prevent ink from flowing out onto the paper: the nib will “dry up” and the fun stops. Of course that doesn’t regularly happen with a good fountain pen like those made by Platypus Pens. Instead, once the negative pressure reaches a sufficient level a bubble of air will enter the pen by way of a channel in the feed.

Too much air access into the reservoir will lead to ink leaking, and too little will lead to drying out of the nib. The outflow of ink and inflow of air have to be well balanced for reliable writing performance of a fountain pen, and regulation of that is the most important function of the feed. And that is the topic of the next blog post.