The functions of a fountain pen cap are:
1. To protect the nib and feed against damage.
2. To seal off the nib and feed from the outside air, so that when removed, they will write immediately the nib touches the paper.
3. To hold the pen in an upright position by hooking the clip onto a pocket.
Not much thought appears to have been given to the early caps; they were just something to protect the nib. Made of hard rubber like the pen barrels, they were shaped mainly with rounded tops on hand operated lathes. For example, on a 1912 Waterman Safety it is not easy to see the purpose of a vent hole which appears at the top of the cap. While it would allow the air to escape when the cap was fitted, it also allowed air to enter and so dry off the nib point or, conversely, if not carried upright, the cap would fill with ink.
Slip-on caps were prone to cracking, consequently many of the early models had a gold band reinforcement. Common in the UK are Mabie Todd or FDW hallmarks (Francis Waterman's registered London mark) with early bands normally in 18ct.
Screw threaded caps replaced push-on, and some pens even had a thread on the barrel end on which to screw the cap when not in use.
Over the years, the cap form has been influenced by materials, availability, fashion and manufacturing techniques. Hard rubber continued to be used after the end of the 1914-18 war as some rubber plantations in the Far East were actually owned or controlled by the pen manufacturers. But when Walter Sheaffer introduced the Jade Green pen made from a new UNBREAKABLE plastic material, this changed the whole industry, as it allowed a variety of different shapes to be fabricated.
As other plastic materials were developed, caps were formed from rolled sheets or injection moulded, as well as machined from blocks. In fact, not only the way a cap is made, but the colour and pattern can profoundly affect its strength. (Note: Always look very carefully at web and lizard caps for spiral unwinding or cracks.)
Back to Walter. As he already had the patent on his famous lever, he was a prime influence on design. In fact, the 20s are remembered for colour and function rather than styling, as form was mainly regular cylindrical shapes. In 1929 however, he introduced the Balance range of Sheaffer pens designed on the bull-nosed racing cars, signaling the start of the 30's streamline era, soon copied by the other makers who re-shaped their pens to follow the trends. 40's shapes were influenced by the war, and in particular aviation e.g. the Parker 51 took its shape from a jet (without wings) and the Eversharp Skyline was rocket shaped. New materials, and particularly non-corrodible metal components became commonplace (e.g. the 51 cap) and the cap internal design and cap assembly more adapted to mechanisation.
The war had a great effect on cap design, not only on materials, but as military regulations stated that, when carried, pens should not protrude above the pocket line, allowing pocket flaps to be closed. Clips were thus positioned right at the top of the cap.
The cap components, functions and problems
The cap more often than not is simply a cylinder, to which are attached a clip, an inner cap or spring clutch, and decal. It is normally threaded internally. The main problem with caps are: cracks in the lip (all models), cracks at the top (always look carefully at Conway Stewart herringbones), deformation (shrinking) near the top (Stephens), dings in metal caps, threads stripped, clips loose or broken, bands missing.
Nothing will excite or depress a collector more than if you tell him the cap is cracked. Sadly, it is often impossible to repair a badly cracked cap, but minor cracks can be repaired and new lips can be fitted without much difficulty - but not cheaply.
The first and most important thing to know before attacking a crack is - what material is the cap made from? After which you can choose the correct glue or solvent. If it is a cellulose nitrate cap, then solvents for cellulosic paint are excellent (try nail varnish because it has dissolved cellulosic polymer in it and can reinforce). Hard rubber and caesin demand two-pack resin and so on. It is always best to ensure the crack is tightly squeezed during hardening, and this is best achieved by fitting a band. It may not be original but it can eliminate further crack development. You can buy bands or collect them from old pens.
These have adorned caps from the latter part of the last century, and the early ones were tapered to tighten on to the hard rubber. Later, grooves were cut around the cap and single or multiple rings or bands fitted over the grooves and bumped into the slots with a sort of power tool. This has 8 segments which puts pressure on the band in 8 places simultaneously. Replacement bands can be fitted and are best made in 14ct gold. This is a specialist activity and normally costs about £15 per band. If you have a band that is loose, you can 'glue' with superglue or a couple of dabs of solvent. Be careful! Not too much.
These often have to be removed before you can fit a side clip. (This is the type of clip secured by rivets or a metal back plate.) There is a reliable tool for removing inner caps. Although it is expensive it does work well, but even so, take great care and avoid very thin lipped caps. Not all inner caps are fixed. On some of the 1930s models like Duofolds, the inner cap is part of a screw-in top used with a ring-style clip which is easy to service. Removing these is often like removing a section and requires soaking - and patience.
If visible, keep them clear. Use a pin to remove clogged dust, as the hole serves to reduce condensation and also reduce the suction effect when removing the cap.
These are caps which do not engage the barrel due to wear of the cap and/or barrel. If replacement parts are unobtainable there are one or two possibilities worth trying before the major task of rebuilding the inside of the cap with resin and re-threading. First, try shortening the section. This can be at the nib end or the barrel end (reducing the step). An easier way is to reduce the inner cap length. Both these ploys allow the pen to enter on to new threads and very often only one or two threads are necessary to get a good hold. IT IS CRITICAL to ensure the nib has enough clearance, so you may have to adjust the nib position in the section a little. Sometimes it may be simply due to dirt, so clean the threads on the cap and barrel well and wipe with a little WD40.
While manufacturers have followed trends in pen styling, they have been reluctant to change clips and decals to any great degree. For instance, today the arrow appears on Parker clips; Sheaffer retain the white dot; Montblanc the white star depicting the snow-capped summit of the mountain. The new Eversharp Skyline is almost identical to the original. Pelikan clips still resemble the bill of the pelican, and italian pens such as Omas stay with traditional design. The exception would appear to be Waterman, who seem more responsive to culture change, particularly after the French influence when manufacture moved to Nantes.
Early clips simply pushed onto the cap. Swan eyedroppers, which were slim and fragile, had a special clip which you fixed in your pocket. It was, in fact, a metal cage and the pen was pushed in and out.
In the 1920s Watermans, Mabie Todd, Sheaffer, Conway Stewart and others fixed clips with rivets or a back plate on the side of the cap. Many of these pens are missing clips and it is difficult to find replacements. They are not easy to repair, but providing the inner cap can be removed then it is possible providing you can make the correct pins or back plates in soft metal. (18ct gold is excellent but rather expensive, so maleable silver is good. Silver wire, or beat out an old spoon and cut to form a back plate).
Called washer clips in America, it became very popular in the 20's and 30's to fit the new style cap which had the loose detachable inner cap integral with the top which, when screwed up, anchored the clip. Parker used them in varying sizes on the Lucky Curve and Duofold models. Other manufacturers throughout the world copied this style using the screw top as a clip screw anchor, sometimes with a fixed innercap. The clip is fragile on the ring and can break, but it is easily repaired.
1931 saw the introduction of the roller clip, designed to assist easy access and prevent wear on the pocket. It first appeared on Wahl Eversharp pens.
In 1932 a new style clip appeared on the Parker Vacumatic pen in the shape of an arrow. It had a blue enamelled diamond shape at the head which guaranteed the pen (just as the Sheaffer white dot did) for the lifetime of the purchaser. The way of fastening these clips was constant from 1932 to the 1970's, with a brass screw into which a decorative plastic 'tassie' was also screwed. On later pens such as 51s, the screw enters a detachable clutch system. Later 51s have a non-removable clutch. They present little difficulty to replace. Tassies are removable with a rubber friction pad. Tap washers fixed on a stick are very effective.
1940's Waterman and 50's Conway Stewart models have the clip fixed in with a nut, accessible without removing the blind cap. Corrosion can be a problem, but most nuts can be removed with soaking and patience. A tool to loosen the nut can be made easily from an Allen bolt fixed to a rod. Swan and Blackbird clips (1940/50) are a simple push fit and be easily be removed by levering out with a screwdriver or thin bar.
In conclusion, in spite of all variations, I have always found the clip to be the weak spot on any pen. Many are easily repaired, but always remember that it is often possible to buy replacement caps.