This is another historical material for fountain pens: the oldest. This material was first invented by a very well know inventor: Charles Goodyear.
Its composition is rubber vulcanized with sulfur and this was the first material used to make fountain pens. Unfortunately not many makers are left and most of them have little knowledge of the process used to make the colors of those early pens.
In six years of experience working with ebonite we noticed that the quality of this material is variable, mainly due to its principal use. Today it is no longer used for fine objects but used in technical devices with no consideration for aesthetic requirements. In fact the old beautiful mottled and ripple ebonites have disappeared forever.
Colors today are opaque. Colorants used in the past such as iron oxide are proving to be cancerous and therefore are no longer in use. Pens made in ebonite become opaque because of the tendency of sulfur to rise to the surface. If exposed to sunlight the color tends to fade.
Very dark ventilated place.
Use a microfibre fabric (like those for eyeglasses) to polish. Afterwards a silicon sponge can be used.