Hard rubber dominated fountain pen manufacture up until the mid-1920s, and remained in wide use into the 1930s. Yet because of its vulnerability to light, most old hard rubber is now found in a more or less faded state. One of the first questions new collectors ask is if this fading can be reversed. Strictly speaking, the answer is no: the process of deterioration is not reversible. Yet there are a number of things that can be done to restore the original appearance of a faded hard rubber pen.
The most obvious process is polishing. When hard rubber fades, what is affected is the exposed surface. By abrasive removal of that surface layer, the original, darker color beneath is revealed. The problem with polishing, however, is that removal of the outer layer also removes or weakens imprints and chasing, and can affect the fit of caps and barrels. When taken to extremes, it can round edges and alter profiles. In general, polishing is a more viable option where the hard rubber is smooth and without imprints, and where the fading is only slight.
When pen collectors refer to "reblackening", however, that does not normally including polishing. Currently, reblackening methods fall into two categories. The first involves adding pigment to the faded surface. The second involves chemical alteration of the faded surface.
Adding pigment to the surface can further be divided into two approaches -- paint and dye -- though in fact there is always some overlap. Paint, of course, is pigment in a binder applied on top of the surface, while dye is pigment absorbed into the surface. Paint would tend to be the more reversible, with dye better at retaining the visibility of the original surface. Since even the best dyes have limited penetration, however, all blackening agents of the first type can be worn off at least partially by polishing -- an important consideration for pens that are to be used regularly, and for maintenance of all pens so treated.
Chemical alteration of the surface comprises a multitude of agents, some quite caustic. What most have in common is deep penetration of the faded surface, so that subsequent polishing will not reveal a lighter, faded layer beneath. This comes at a cost, however, since these chemical agents typically leave the surface more or less dulled, with a pebbly or alligatored texture. This is why those who advocate such methods call for oiling or waxing of the surface to cover up this residual roughness. And though it is not sufficiently appreciated, virtually all chemical blackeners work by stripping off the faded surface. The removal of material is at least as great as that caused by mechanical abrasion, and is easily measurable with calipers.
As a whole, adding pigment to the surface is the preferred method. It is largely, if not completely, reversible, and it leaves the original surface essentially intact. While wear or inadvertent polishing may leave lighter patches in the reblackened surface, repeat recoloration is quick and simple. While some chemical blackeners are known to attack metals, and others' long-term effect on hard rubber has yet to be established, hard rubber has been painted and dyed since very early in its history with no reported ill effects recorded to date. Among pen collectors, blackening pens with dye has been done since the 1980s, and most likely earlier.
How to reblacken faded hard rubber is one question; whether it should be done is another. There are those who strongly believe that reblackening, by any method, should not be done.While the opponents of reblackening have been very vocal, the great majority of advanced collectors take a more nuanced view. Most would prefer to have all their black hard rubber pens like new with perfect original color, but are pragmatically accepting of sensitively restored examples of items that are difficult to find in any condition. In general, if badly done, reblackening will reduce a pen's value, yet if done well, can enhance a pen's beauty and desirability. This is particularly the case with overlays, where a major part of the original aesthetic was the contrast between the silver or gold and the black -- an effect quite lost when the black is light brown instead. It is much less the case with early pens, all of hard rubber, where the strength of the imprints and the chasing is paramount. There, reblackening is more likely to devalue, as collectors of such pens tend to be especially attuned to originality and historic signficance.
Finally, some notes are in order regarding the nature and prevention of fading. Although many refer to the fading of hard rubber as "oxidation", that is a misnomer. The damage done to hard rubber by exposure to light is done by the breaking of chemical bonds, the crosslinks that make the molecular structure of intact hard rubber so impervious to chemical action. Yet light-damaged hard rubber often appears largely unfaded until it is exposed to water, at which point it can instantly fade as essential components of its de-crosslinked surface are washed away.