How does a fountain pen work?Updated 10 months ago

In the most basic terms, a fountain pen is a shell surrounding some sort of ink reservoir that is connected to a feed and a nib. The feed, nib, and reservoir work together to regulate the ink’s flow so that you make intentional marks and not just messy, beautiful pools of ink. 

The Mechanism Behind Fountain Pens

Our beloved pens come in all sorts of shapes and sizes, and they use a variety of filling mechanisms. I’ll talk more about filling mechanisms below, but all of them have this thing in common:

A fountain pen is a controlled leak. 

Unless you’re pointing the pen straight up in the air, the ink in the pen wants to get out, and it wants to take the path of least resistance to do that. The task of the rest of the pen is to create a situation where that ink reliably escapes where, when, and how we want it to.

I think it’s probably well-known that nibs use capillary action to move ink from the reservoir to the page. Capillary action happens when the forces of adhesion and cohesion work together to move a liquid. As Lakshminarayanan Mahadevan puts it: 

The pores [in the paper] draw in the fluid via capillary – surface tension – forces, while the viscosity resists this motion," he said. "The moving pen drags along the fluid, and again viscous forces resist this. Together they shape the blot, if one hesitates, and the line when one's thoughts flow from the mind to the machine that records them – the pen.³

So, the paper pulls, the ink’s viscosity resists that pull, the nib guides the ink to the page, and the feed…feeds the ink to the tip of the nib. This is a really elegant way to think about the interaction between the pen and the paper. It’s a “competition for the ink between pen and paper.”⁴ 

Surely capillary action is working here, but if that were all that was going on you would run out of ink really quickly. You’d use the ink that was near the tip of the nib, and maybe the ink in the feed, but capillary action is a fairly weak force and it won’t do the whole job. We can’t forget about another interaction: The Glug!

Think about a milk jug. When you take the cap off of the jug and turn it upside down some of the milk in the jug falls out, but not all of it. It goes “glug!” and then some more milk falls out. That glug is happening because as the milk falls out of the jug, it creates a vacuum in the jug. A sort of bubble at the opening of the jug. When it pops, air is pulled in to fill the space, the pressures are equalized inside and outside the jug, more milk falls out, and the process is repeated. It’s an air-exchange operation.⁶ You get the glug because there is only one hole, and the milk and air have to take turns.

Glug. 

If you were to poke a hole in the jug, then all of the milk would fall out at once because the air would replace the milk at the same rate the milk falls out, and you wouldn't have a glug.

Your ink reservoir works like that milk jug, but on a much smaller scale. As your feed runs lower on ink the reservoir glugs more ink into the feed. When your pen is working well, though, you’re not going to notice this glug. You’re just going to keep writing with a smooth flow of ink instead of having ink falling out onto the page in a “blurp.” And that’s because the feed isn’t just guiding ink down the nib. It’s also regulating the air-exchange between the nib/feed and the ink reservoir. 

Different feeds work differently, but the key is that they have different channels for the air and the ink so that about as much air enters the ink reservoir as ink leaves it. The exchange isn’t perfectly smooth, it’s an unsteady flow, but you wouldn’t want it to be perfectly smooth because then all of the ink would fall out. So the pen’s feed is responsible for regulating the rate at which air is exchanged with ink in the reservoir. When the feed is working well, it flattens the glug so much that you aren’t going to notice it, and that’s really clever.

All of this hidden feed-action is made more impressive when you think about what we’re asking the feed to do. It needs to work pretty much the same way with liquids that contain different dyes, have different pH levels, contain different pigments, contain shimmers, are at different temperatures, AND are contained in different ink reservoirs. The amount of ink in the reservoir makes a big difference to how this unsteady flow operates (because there’s more pressure pressing down when there’s more ink, as in an eyedropper vs a cartridge). The feeds in our pens need to try and regulate the ink flow when the pen is full just as well as when it’s nearly empty, and that’s pretty impressive. Our fountain pen feeds, nibs, and ink systems are keeping all of these forces in balance while we write, and that’s pretty special. 

 So, that’s how a fountain pen operates. It is more complicated than we usually think it is, but the core concept is that the fountain pen is always a controlled leak. The cool part is just how that leak is controlled.