This black hard rubber taper cap came from a pen produced about 1905. After nearly a century of oxidation, the hard rubber was a drab olive/brown color. Now, it is the original black color.
Color Restoration of Black Hard Rubber Pens
$60 with metal filigree, $50 without filigree.
Black hard rubber (also called ebonite) was widely used for pen caps and barrels until plastic became more popular during the 1920's and 1930's. The material oxidizes on the surface making it appear dull, even a drab brown or olive color.
Do not soak these pens in bleach (it can corrode metal parts), 409 household cleaner, or even in water, as you may damage rather than restore your hard rubber pen.
Many pen people refer to what I do as 'blackening' a pen. Some people think that this means that I 'paint' over the oxidation. I don't do this because it doesn’t restore the surface. Instead, I use a proprietary chemical solution that reacts with the oxidized hydrocarbons on the surface and returns the hard rubber to its original jet black color.
"I couldn't believe how different my Waterman's filigree pen looked after you blackened it. The hard rubber looked almost olive green under the silver filigree before I sent it to you. Now it's black as coal, and one of the best looking pens in my collection."
Labron W. Miller II