The function of the fountain pen feed is to transport the ink from the reservoir to the nib and to control its flow. Simple, isn’t it? Most of us tacitly expect the feed to perform this task, without us being aware of what this it entails.
Nevertheless, many impertinent demands, of which we are oblivious, impact on and make the life of a feed very difficult.
1.) Supply ink and adjust its flow as it is affected by writing speed, intermittence, and style of writing as well as the hygroscopy (blotting strength) of the paper.
2.) Compensation of the various characteristics of the ink, such as different colours and rheology. For more, see my writing on Ink.
3.) Compensation for the effects of varying temperature, air pressure, gravity, and angle of writing
4.) Initial supply of ink through a dry feed or even worse, through a virgin feed
5.) Transporting ink through a dried and/or blocked feed
6.) Compliment the sucking of ink in relevance to the various filling systems (piston, vacuum)
Are you slightly more impressed? These independent criteria may compensate or amplify each other in their effect on the ink flow. All this is done by a static control system, meaning, there are no moving parts, there is no mini-computer inside, which registers these variations and adjusts the flow by altering the orifice of a valve.
The static control system is developed and set by the fountain pen manufacturer, that’s it. For the rest of its natural life, the feed has to cope with all the changing conditions it is challenged with. (Except some “tuning” performed by enthusiasts.) What a job, furthermore, what an excellent achievement of the genius who has designed it. (While self-praise is the truest form of praise, plagiarism is its sincerest … by me)
The feed is self-regulating, meaning, there are no knobs and buttons for the user to control it (could be a good idea) or inbuilt control mechanisms, which could vary the flow rate of ink in response to a change of conditions.
The flow rate of ink is not controlled directly. It is the inflow of air into the ink reservoir, which allows ink to escape from it, or not. The air inlet either passes air or not. It is a yes/no regulation, air bubble in or not. I will explain this comment in the chapter Bubbles and Bottles.
The mechanical dimensions of the air inlet into the reservoir keep the air bubbles at a size small enough and occurring at a frequency so that the writer does not experience any variation in ink flow.
What determines the timing? The amount of ink as it runs down the feed to the nib is moved by forces acting on the ink such as: capillary, surface action, hydrodynamic, air-pressure, gravity, the suction of the paper, writing speed, and – and – and. Details you find under Surface Tension and Capillaries.
Only briefly here: the ink flow is initiated when the ink is pulled down by the weight of its fluid column (level in the reservoir down to the nib). That’s why fountain pens only write nib down. The pulling of the capillarity in the feed and nib supports this flow, but the drag of gravity is essential, elementary.
As the ink flows out, the air volume above the fluid in the (solid) reservoir develops a vacuum, and the ink flow reduces. When the pulling and sucking forces are equal, the flow stops entirely. (Depending on design and material, bladders or sacks may work differently.)
The only way to start the flow again is to allow air to enter into the reservoir, the vacuum reduces, and ink can run out.
Detailed information you find in the chapter Application to the Feed.
Insiders could just gobble on in their own time. Others I will endeavour to introduce gradually to the individual systems and modes of control carried out by this small black component of plastic. It holds the secrets of the function of a good fountain pen. After you have read the chapter on the feed, I guarantee, your relationship with your fountain pen will have changed, forever.
Early fountain pen feeds had no air vents or overflow slits but only a capillary canal (or two or three) for transporting the ink from the ink reservoir to the nib. The only means for controlling the flow was by determining the cross-section of the capillary during the production of the feed and later, during writing, through the variation of the cross-section due to the widening or closing of the tines.
I wonder what would have been messier, using an inkwell and a dip-nib with a holder, or such a prehistoric fountain pen. I guess it could explain the reason for the expression “fountain” pen.
How does it do all that?
Please read on Bubbles and Bottles
Above all: Enjoy!
11 March 2016