Sunday, March 15, 2015

Taper cap construction notes

It can be difficult to understand the inner workings of something without cutting it open, and pens are no exception. But sometimes one is fortunate enough to be spared the need to cut -- as with this damaged Waterman taper cap, which nicely illustrates how these caps were originally made and how they can be told apart from recently-made reproductions.

Click on the picture to see the larger version, and you'll see that the interior of the cap has an evenly textured surface. There are no machining marks, nor is it polished smooth inside. This is because the cap was made by wrapping a steel mandrel or core with a thin layer of latex "dough", which was then vulcanized, still mounted on the mandrel. When the mandrel was withdrawn, the textured surface that we see was left behind where the hard rubber had been in contact [more on other traces left by this method here]. Since a smooth finish was needed on the cap's exterior, the cap was made slightly oversize and turned down to final dimensions on a lathe.

The mouth of the cap was the other area finished in this way. The rough surface left by the mandrel would not offer an optimally precise mating surface for mounting the cap on either the section or the end of the barrel, with the roughness also being prone to leave scratches. The localized smoothing is more clearly visible in raking light.

It wasn't only taper caps that were made in this way. This was the norm for all hard rubber cap manufacture, since molding to rough shape minimized the need to remove (and waste) material. Look inside slip-on caps and in nearly every case you'll see that same characteristic roughness left behind from where the hard rubber was vulcanized on the mandrel. This is not usually visible inside screw-on caps which were machined both outside and in, for the interior dimensions and finish had to be much more precise to accommodate both the threads and the inner cap.

That texturing is also absent from newly-made replacement caps, which to date have all been made from solid hard rubber rod stock. It isn't so difficult to make other types of cap in this way, but taper caps with their thin walls and narrow tapering profile are a bit of a challenge. Most reproductions are a bit heavier and thicker-walled than an original, and if you look inside, you will often see a series of telltale steps where the interior was hollowed out by drilling using a series of progressively-sized drill bits -- a construction method never used in the past. Buyer beware, for there are a lot of newly-made replacement taper caps out there, and even experienced dealers have been taken in. There are several regular pen show sellers who have not been sufficiently forthcoming about identifying such caps, sellers who most collectors still think of as honest and trustworthy.

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