HOW TO TIGHTEN A FOUNTAIN PEN CAP.Vulcanite: “hard black vulcanized rubber.” Vulcanize: “harden (rubber or rubberlike material) by treating it with sulfur at a high temperature.” I found this cigarette card (c. 1908–1919) while browsing in the New York Public Library Digital Gallery.
The annoyance of a loose cap to your fountain pen can very easily be remedied. Hold it over a flame for a few seconds, thus softening the vulcanite, and then squeeze the cap to a slightly oval shape as shown in a somewhat exaggerated form in Section 2. The cap will now fit the pen and remain in position.
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[Definitions from the New Oxford American Dictionary.]