For what could be a simple protective tube (and in some cases, like the Waterman 12, is a simple protective tube), there is complexity to the caps of pens that takes rather more explaining than one would expect. The various sub-components of the cap include:
Outer Cap: A simple protective tube. Frequently the same material as the barrel, but sometimes made of metal. The main role of the outer cap, apart from being decorative, is to protect the point from blows.
Inner Cap: This is one of the first complexities that appears in caps. The inner cap is an extra seal, designed to press against the front of the section, leaving the point and feed exposed to the minimum volume of air possible. Once a very little ink evaporates, this tiny volume of air is humidified and there is no further evaporation (theoretically– caps are not spaceships, and their air-tightness is imperfect). In hooded pens like the Parker “51″, the inner cap presses the sides of the hood rather than the face of the section, but the function is the same.
Vents: This is a seemingly counter-productive element in the cap– a hole or two. These exist to prevent the cap from filling with ink when it is removed. Were they not there, or were they somehow occluded (as I relate in my blog), it would be possible for a partial vacuum to develop inside the cap as the inner volume increases during removal, and ink would be drawn out of the point. With vents in place, the interior of the cap communicates with the outside air almost immediately once the inner cap comes out of contact with the section, and no problem develops.
In vintage pens, the vents are usually quite obvious, and give a sense of the depth of the inner cap as they are usually almost immediately beneath it. In later pens, they are frequently concealed; if you’re concerned to find them, blow into the mouth of the cap and you should find them easily enough. Some pens, like Sheaffers of various ages, have no vents at all, and instead rely upon a very small swept volume between capped and open and also upon the buffering capacity of the feed– any ink pulled forward in these is treated as priming rather than excess.
Clip: The clip should almost have its own page. Early pens did not come with a clip built in, but for those who wanted some means of attaching the pen to their pocket a “relief clip” could be slipped onto the cap (see the Waterman 12 for an example).
Other clip styles include:
- Rivetted clip, attached to either the face or top of the cap (e.g. Waterman 52 and Waterman Stalwart)
- “Z” clip, in which an internal tongue is trapped under the blind cap (e.g. the Pencil-Quill).
- Tabbed clips, held in place by little tabs passed through slots in the cap and then folded. (e.g, Scripto 100, a very good example of why it’s a bad system)
- Ring clip, with the top of the clip held on by a jewel or derby (e.g. Parker Duofold).
- Spring clips, with a pivot and an actual mechanism inside (e.g. Sheaffer Legacy).
Derbies, Jewels and Tassies: I’m lumping these together as they all fall into a broad category of things found on the top of a cap. Jewels and tassies tend to be found together, as the latter is a decorative metal ring into which a jewel is set, and a jewel is just about any flattish decoration atop a pen. A jewel can be found in absence of a tassie, since the ring on a ring clip doesn’t technically count as one. Both are sometimes found at the tail of the barrel as well, to give the pen some design symmetry
A derby is… something taller than a jewel, and having no associated tassie. They can take the shape of just about any form of headwear, finishing the top of the cap and frequently also serving as the thing that keeps a ring clip in place.
Bands: While decorative, cap bands also perform a preventative function. They are mean to keep the mouth end of the cap from splitting if the pen is thrust into it too aggressively, or at least to arrest the cracks from this sort of injury before they get out of hand.
What’s left? In extremely old pens, the cap is just a simple tube which slips over the pen. Screw-on caps appeared in pretty short order, and have never disappeared– in most of these, the threads are cut directly on the inside of the outer cap. After about 1940, one started to see a return of slip-caps, but there was some kind of internal mechanism which serve to grasp the barrel. This is frequently a set of sprung bands which grasp part of the pen’s barrel (the “clutch”), or a recess into which some sprung part of the barrel can lock; for very successful examples, look to the Parker “51″ and the Lamy 2000. In newer pens, a “click” cap mechanism sees the inner cap made of a deformable plastic which grabs part of the section, and unlike the metal mechanisms these are given to rapid wear.