Tim's Thoughts

Does Pen Size Matter? 

In a few past Thoughts, I dissected the roles played by ink, clips, and feeds in the functioning of fountain pens. Now, it’s time to consider why pens are the sizes they are. 

Pens are, first, sized to be sold, to fit the hands and writing styles of their owners. A pen can’t be too big or too petite for a writer to grip comfortably and hold with balance. There are certainly exceptions at both ends of the size range, including some giant eyedroppers and truly tiny Peter Pan pens of the 1930s, but these are curiosities that have major issues in functioning, filling, and use. Thus, while a pen’s outer appearance certainly matters for a pen’s success, its size must enable the pen’s ability to function. 

For the vast majority of pens, what really matters in pen size is its capacity to take in ink, hold a reasonable amount for future use, and push out air, which is why with all the differences in exterior dimensions, most pens are strikingly similar inside. The need to take in ink and let a small amount out via the nib is obvious; less obvious is the equally important need to replace the ink with air to maintain internal air pressure. By 1915, fountain pen technology had mastered the internal fit of a latex sac, sealed to a nipple on the gripping section to create a sealed ink reservoir that connected directly to the slits on the pen’s feed for ink and air transport. These slits in the feed and nib, usually approximately .003” wide, were standardized to enable ink, which is both more viscous and thicker than water, to flow in a capillary manner from the sac to paper. So this was done, known, and hasn’t changed significantly since, although in modern pens, fins and channels have been added to larger feeds and often larger nibs to enable the use of broader, ink-thirsty nibs and even more recently, pigmented inks.  Within fifteen years, the entire range of pens with sacs had been produced. Ignoring the smallest sacs because of their obvious volume limitations, the largest sacs in the largest pens (Sheaffer Oversize Balances, Parker Senior Duofolds, Waterman 54) proved to not be as efficient as smaller sacs, because the lever (or button) and pressure bar that filled the sacs could not generate enough vacuum to fill them. 

Physically, while a latex sac could not be larger than the space it was allowed inside a barrel, its real limit was the diameter of the sac nipple. Thus, inside barrel diameter, usually around 3/32” wider than the sac nipple, and generally no larger than 11/32”, became a size criterion during the latex sac era. Pens’ barrels fit tightly over gripping sections to create a smooth line and pleasing grip, while also providing a limit for the lever or button to fill the sac inside. It all worked together quite smoothly, and this smooth functioning became durable. 

By the mid-1930s, with the public tiring of lever and button filled pens, Sheaffer and Parker figured out how to insert a reliably functioning mechanical (Sheaffer) or pneumatic (Parker) vacuum mechanism into their barrels without using latex sacs. The mechanism’s fit had to match the inside of the barrel, including an air-tight seal. Their design genius created mechanisms that were almost all the same size inside the pen (the largest Vacumatics have larger mechanisms in large barrels, but they’re not often seen), compensating for internal barrel variation rather than having a mechanism size for each pen size. These pens, filled directly into the barrel, also featured the enticing ability for a user to see ink sloshing inside the pen. By the 1960s, when cartridges and cartridge converters took over pen manufacture, the entire internal diameter constraint disappeared, because the cartridge or converter inside had a hole in its end that fit over the pen’s sac nipple; it needed no other mechanical complexity, relying on capillary action and a minor interior vacuum to work. With this change, marketable exterior dimensions became more important than interior. Finally, most modern fountain pens use cartridge/converters and pistons, with generally the same dimensions as earlier pens. 

In Europe, incidentally, piston-driven pens have predominated for decades. The pistons are mechanical means of creating a vacuum, pulling ink in against a sealed membrane. As long as the cork or o-ring seal against the barrel is air-tight, the pen will fill and hold its ink without leaking. Again, the match of the internal barrel’s diameter with the moving piston’s outer diameter, including a thin layer of lubrication between them, is the key to piston pens’ functioning. 

All in all, getting a pen to work correctly is theoretically pretty simple: ink needs a sealed reservoir to stay in a pen and a functioning feed to let ink out and air in while in use. Most of the issues that get a pen sent to me for repair are in restoring this relationship, their necessity made obvious by a leak. My first thought when examining a leaking pen is always to consider whether the pen has a sac, since the failure is usually in the sac. If there is no sac, finding the failure is a bit more challenging, but it’s always there to be found!