Hard rubber, also known as ebonite or vulcanite, was the main material for fountain pen manufacture up until the mid-1920s. It remained in wide use for caps and barrels into the 1930s and for decades more for feeds and sections. In recent years there has been a resurgence in the use of hard rubber for pen manufacture; new hard rubber is not that different from old, so the observations here apply equally to both.
Newly-made hard rubber is highly resistant to a wide range of harsh chemicals. Black hard rubber parts were often originally shaped using steam or boiling water, neither of which affected its color or finish. Yet because of its vulnerability to light, old hard rubber is often 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. Nonetheless, there are a number of things that can be done to restore something of 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 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. Reblackening methods fall roughly into two categories. The first involves staining or pigmenting, while the second involves chemical alteration of the faded surface. Note that some methods and practices cross this dividing line, combining elements from both categories.
Staining is one of the simplest but least satisfactory approaches, at least as usually done. Rubbing oil onto faded hard rubber may darken it somewhat, depending largely upon how faded and porous the surface is. In most cases not much is accomplished, while the surface is left unpleasantly slick and oily. This also applies to using mineral oil as a finish after other reblackening methods.
Adding pigment is considerably more effective. There are two main approaches here, paint and dye, though there is almost always a degree of overlap. Paint 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 some or all of the faded surface. The removal of material can be as great as that caused by mechanical abrasion, and can be seen by measurement before and after treatment.
A word is in order here about so-called deoxidizing products. Although many refer to the fading of hard rubber as oxidation, that is misleading. 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. This damage is often not immediately visually apparent, and in fact has been shown to continue for up to two weeks after light exposure. When exposed to water, however, the damaged surface can instantly fade as parts of its de-crosslinked surface are washed away. Atmospheric humidity can also react with a light-damaged surface, combining with free sulfur liberated by de-crosslinking. While the discolored surface can be removed or altered, the molecular damage cannot be repaired or reversed. Despite the clever name, "deoxidizers" do not undo surface deterioration: rather, they facilitate partial removal of the faded surface, while impregnating the remainder of the faded layer with a penetrating stain. This can readily be ascertained by examination of a treated surface under bright light, or by exposing it to water. Visually, the surface is still brown-orange, though the stain makes it appear nearly black under normal indoor lighting. Contact with water will soften the surface, which will then slough off if rubbed with a paper towel or rag. This is not the case with harsher chemical surface stripping methods which remove all the deteriorated surface -- not that such harsh methods are desirable, in that they remove so much material and tend to leave a far rougher surface.
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. Hard rubber should be stored away from light and humidity. Lower temperatures will also slow down any deterioration. Even with surface deterioration, hard rubber remains highly durable and under normal conditions does not undergo the sort of progressive deterioration seen with more modern semisynthetic materials such as celluloid and cellulose acetate. Ventilation of storage areas is desirable, though not as essential as for cellulosic plastics. Old and well preserved hard rubber does not outgas to a significant degree, but the free sulfur in deteriorated surfaces can react with atmospheric humidity to produce acidic vapors.