Why toothpaste is a bad choice for pen polishing

Popular recommendations can lead you astray. One example is the use of toothpaste to polish pens, and especially nibs and metal trim. Experts on antique silver warn against using toothpaste, but the websites I've seen don't explain why. So here's the scoop.
The abrasives used for toothpaste aren't carefully graded for particle size, unlike abrasives specifically intended for polishing precious metals and plastics. In large part that's because they are specifically chosen to be hard enough to scour off gunk sticking to your teeth, without being so hard as to scour off the hard outer layer of your teeth themselves. On a gold nib or gold plated trim, though, it's a different story. The toothpaste abrasive is much harder than those surfaces, and because the grit size is uneven, will remove an excessive amount of material while still leaving the surface covered with fine scratches.

Overall, you are best off using a jeweler's cloth -- the Sunshine brand being one of the most popular and widely distributed.

Wax on pens

I'm not sure how it all started, but in the past ten years or so it seems a significant number of enthusiasts have begun waxing their pens. While this may make some sense for pens with finishes that cannot take much polishing, or which are especially vulnerable to environmental exposure, there are many cases where wax is decidedly not a good idea.

Heading the list are pens made of celluloid or cellulose acetate. Both materials give off acid gases as they age, gases which can accelerate the aging process if not allowed to escape (more on celluloid storage here). Coating a celluloid or cellulose acetate pen in wax will prevent escape of those acids, trapping them within the material. The harder waxes favored for use on pens, such as carnauba, are also the least permeable, despite recent claims of breathability on pen forums (see Greener Donhowe, O. Fennema, "Water vapor and oxygen permeability of wax films", Journal of the American Oil Chemists’ Society, Sept 1993, vol. 70, issue 9, pp. 867-873).

Wax on hard rubber is a more mixed case. Protection from light and in particular UV will be minimal, so the main benefit would be to exclude moisture. Water will have no effect on pristine hard rubber, but exposure to light will eventually break down the crosslinks formed during vulcanization. This surface damage is often invisible until the rubber is exposed to water, at which point it will fade instantaneously as de-crosslinked material is washed away. Humid air can have a similar effect, though without the loose surface compounds being washed away. Instead, one ends up with an acidic crust, thanks to the free sulfur in the damaged surface. Waxing hard rubber that has latent surface damage (that is, exposure to light, but no exposure to water) would help prevent both fading and formation and release of acids.

Wax does change the look and feel of a surface, however, and waxes do inevitably break down, losing their ability to protect and turning cloudy or yellow. If the dulled wax is difficult to remove, this can be a serious problem. What seems wonderful for the first few years can become a nightmare after ten or twenty. For while polishing off old wax residue may be no big deal on a wooden table or a steel knife blade, it's quite another matter to get it off something like the delicate and irregular surface of Japanese maki-e lacquer work.

As important as knowing when and when not to wax, is knowing what wax to use. For a knockabout pen of no historic value it doesn't really matter. For something rare and significant, though, you'll want to approach wax selection like a conservator. Commercial products for household or automotive use would be disqualified immediately. You'll want to avoid waxes such as beeswax and carnauba, which are either acidic or prone to become acidic as they age. You'll also want to avoid polyethylene waxes, which under some circumstances can be almost impossible to remove (see Danal L. Moffett, "Wax coatings on ethnographic metal objects: justifications for allowing a tradition to wane", Journal of the American Institute of Conservation, vol. 35, no. 1, 1996, pp. 1-7, where removal required nothing less than immersion in hot xylene). A pure microcrystalline wax such as Cosmolloid 80H would be the conservator's choice, according to the current literature. I would advise against Renaissance Wax, which is a mixture of microcrystalline and polyethylene waxes. It is widely advertised as a museum-grade product, but what was museum-grade back in the 1950s when it was developed is very different from what is museum-grade today.

ADDENDUM: In this thread at FPN, users of Renaissance Wax report it discoloring hard rubber upon application. These would appear to be instances of solvents in the wax reacting with a surface layer already degraded by exposure to light, akin to how such surfaces can instantly fade when exposed to water.

Celluloid storage and acid absorption

As celluloid ages, it emits a range of compounds. Some are benign, but as the celluloid begins to approach its expiration date, some rather nasty stuff gets released -- most notably, nitric acid. This in turn accelerates the process of deterioration, as well as attacking other nearby materials. For this reason, conservators recommend storing celluloid artifacts separated from each other and with ventilation. Keeping the temperature and especially the humidity low also slows the aging process. (see Julie A. Reilly, "Celluloid Objects: Their Chemistry and Preservation", Journal of the American Institute for Conservation, 1991, vol. 30, no. 2, article 3, pp. 145-162).

The release of acidic compounds begins well before any visible signs of material degradation, as has been demonstrated by wrapping old celluloid pens in litmus paper. Litmus paper, however, is just an indicator. It doesn't do much of anything to absorb or neutralize free acids. For that, elementary chemistry suggests the use of a mild base, such as sodium bicarbonate (baking soda) or calcium carbonate (lime). Museum conservators have indeed used calcium carbonate to trap acid gases, but as it turns out, far better options are available.

For pen storage, the best choice seems to be SPZ-grade zeolite, and in particular, MicroChamber papers. These papers are impregnated with high-absorption zeolite with an alkaline buffer, and can be used to line pen storage areas or to wrap individual pens. Standard-sized interleaving paper costs under $0.25 per 6.5 x 10.125 inch sheet, and can be bought through Amazon or directly from the manufacturer. Studies going back over twenty years show that these acid-trapping papers outperform calcium carbonate dramatically, both in thoroughness of acid capture and neutralization, and in retaining the ability to trap and hold acids and other atmospheric pollutants over time (see Siegfried Rempel, "Zeolite Molecular Traps And Their Use In Preventative Conservation" (Western Association for Art Conservation Newsletter, January 1996), Getty Conservation Institute, "Performance of Pollutant Adsorbents (2001-2003)", and the Conservation Resources website). I now have MicroChamber paper lining most of my pen and parts storage areas, supplemented by silica gel canisters to absorb excess humidity. Though standard-sized paper is what I have, 14-inch paper is required if you want to pleat it around the dividing ridges of a standard 12-pen slotter box. Finally, I have no financial stake in any of these products -- I am using them because all the sources I could find indicate that they offer the best protection option (noting that activated carbon filters are of comparable merit, though much bulkier).

NOTE: There are MicroChamber papers that also incorporate activated carbon. And as a Facebook commenter noted, one can also place strips of MicroChamber paper *inside* celluloid pens for additional protection -- though if the pen is to be used regularly, the paper will have to be cut and shaped carefully so as not to interfere with the filling mechanism. The greatest benefits would be with button-fillers and other pens with barrels that are tightly sealed.

Ammonia, stress corrosion cracking, and 14K gold nibs

Several years ago I posed the question of whether pen cleaning solutions containing ammonia could pose a risk to gold nibs via the process known as stress corrosion cracking (SCC). The original discussion at FPN can be reviewed here (the responses on Lion & Pen contained less of substance, and in any event are now lost). The most useful scientific paper cited in that discussion was David A. Scott, "The Deterioration of Gold Alloys and Some Aspects of Their Conservation", Studies in Conservation, vol. 28, no. 4 (Nov 1983), pp. 194-203. According to Scott, gold alloys in the 9-14K range are most vulnerable to stress corrosion cracking, whereas gold of 18K purity and greater is largely invulnerable. A number of reagents that could provoke stress corrosion cracking were mentioned, including aqueous solutions of ammonia. Unfortunately, details were not provided concerning the concentration of these solutions, or the length of exposure required before embrittlement took place.

That particular online discussion ended without any firm conclusions being drawn. My concerns remained, however -- enough that I drastically cut back on my own use of ammonia solutions on gold nibs. If ammonia could embrittle gold nibs, it would be a slow, cumulative process -- very possibly taking years or even decades before becoming apparent -- so I did not find any reassurance in arguments that consisted of little more than, "I've not had any problems, so it must be OK." What worried me particularly was the large number of  vintage gold nibs with cracks in locations that just didn't make sense. Cracks where a nib is subjected to repeated bending are readily explained by metal fatigue. But what about cracks in the heel of a nib, where it is firmly sandwiched between the feed and section? Fatigue would not seem to be a concern there, so why are such cracks so common? Conventional wisdom is that the gold was made too hard, but if so, why are these nibs so often cracked only where they are wedged into the section? The answer may be that while the heel is the part of a nib least subject to the cyclic loading that causes fatigue, it is the part of a nib under the greatest constant stress. And it is that very stress that makes materials vulnerable to stress corrosion cracking.

A recent reexamination of the materials science literature available online has helped fill out this picture. The studies cited by Scott, for example, are referenced in more detail in Jennifer M. M. Dugmore and Charles D. DesForges, "Stress Corrosion in Gold Alloys", Gold Bulletin, vol. 12, no. 4 (Dec 1979), p. 141:

Much of the published work on the stress corrosion cracking [of] gold alloys is that of Graf and his co-workers and results were obtained using binary gold alloys. For example, Graf (15) found that the susceptibility of such alloys actually increases as the gold contents increase from 5 to between 15 or 20 atomic per cent, in which range maximum susceptibility is observed. The precise gold content at which the maximum susceptibility occurred depended on both the corrosive media and the stress applied to the alloy. When gold-copper alloys were exposed to a mixture of ammonia, water and oxygen the maximum susceptibility occurred at a gold content of about 15 atomic per cent . . . Graf also observed that as the gold content was increased above that at which the susceptibility to stress corrosion cracking was a maximum, the susceptibility dropped rapidly and the alloy appeared to become virtually immune. In other work (3), this author reported that ultimately the susceptibility dropped to a constant low value at higher gold contents, an effect he attributed to strong general surface attack. He also observed that when the stress was increased, not only did the susceptibility to stress corrosion cracking increase but its maximum was shifted towards higher gold contents.

It is previously explained that "9 carat alloys contain about 18 atomic per cent, and 14 carat alloys often less than 30 atomic per cent gold. In each caratage, the percentages vary according to the atomic weights and the proportions of the non-gold components present in the alloys." While this might at first glance suggest that the gold content of 14K alloys puts them comfortably above the level of maximum susceptibility to ammonia-induced stress corrosion cracking, the last sentence of the longer passage quoted above indicates that under higher stress there might be no safety margin after all.

This is at least partially confirmed by another article, W. S. Rapson, "Tarnish Resistance, Corrosion and Stress Corrosion Cracking of Gold Alloys", Gold Bulletin, vol. 29, no.2 (Jun 1996), pp 61-69, which makes specific mention of fountain pen nibs, and of ink (p. 64):

SCC may be induced not only by exposure to acids during pickling but also a result of contact with reagents such as ink, traces of hydrochloric acid in the atmosphere, perspiration, etc. It has frequently been initiated at points of stress created in annealed low carat alloys by subsequent stamping. Articles such as fountain pen nibs, rings, chains, etc, provide well-known examples.

A further reference appears on p. 66:

In early production of 14 carat fountain pen nibs, for example, Loebich (27) has stated that when ternary Au-Ag-Cu alloys were used, it was found desirable to age the fabricated nibs. In the aged condition they did not undergo stress corrosion cracking in use; whereas if heated to the point where they became homogeneous, cracking by the action of the ink became likely. When certain 14 carat quaternary Au-Ag-Cu-Zn alloys are used, however, such ageing is apparently unnecessary. This could be due to the known limiting effect of the zinc on phase separation in these alloys. The susceptibility of alloys of this type to stress corrosion is apparently considerably influenced both by their zinc contents and by heat treatment. Analogous anomalies occur in the case of the white Au-Cu-Ni-Zn alloys and these have been discussed by Graf.

I have not yet been able to consult the articles by Loebich (who worked for Degussa: Otto Loebich, "Metallkundliche Probleme bei der gewerblichen Goldverarbeitung", Zeitschrift für Metallkunde 44 (1953) p. 288-92) or Graf (L. Graf and J. Budke, "Zum Problem der Spannungskorrosion homogener Mischkristalle III: Abhangigkeit der Spannungskorrosionsempfindlichkeit von Kupfer-Gold und Silber-Gold Mischkristallen von Goldgehalt und Zusammenhang mit dem "Mischkristall-Effect"", Zeitschrift für Metallkunde 46 (1955) pp. 378-385). Nonetheless, the fact that 14K nibs did have problems with stress corrosion cracking is highly significant. While that exposure was to ink -- that is, to acids and salts -- rather than to ammonia, we now have incontrovertible evidence that under the right conditions, even very weak solutions of known problem reagents can eventually leave alloy gold fatally embrittled.

What can we conclude from this? It seems clear that the observed cracking of nibs at the heel is indeed due to stress corrosion cracking -- and on balance, it is likely that this is primarily due to long-term exposure to ink, rather than to much shorter exposures to ammonia. We cannot assume that ammonia exposure is no more damaging than  ink exposure, however, given that typical ammonia cleaning solutions are far more reactive than ordinary modern inks. And given the cumulative nature of stress corrosion, minimizing ammonia exposure is only prudent.

One precaution to be considered is to remove nibs from sections before cleaning. Stress corrosion does not take place where there is no stress, and by all indications, nibs are most stressed by being wedged into their sections -- though internal stresses cannot be entirely ignored. If nibs are removed and cleaned separately, one also has the option of using other solvents not implicated in stress corrosion at all, such as denatured alcohol.

A further question is how ammonia solutions react with gold alloys in an ultrasonic cleaner. The scrubbing action of an ultrasonic will typically remove more encrusted ink in a minute than in hours of soaking, but ultrasonics are also known to be able to erode objects by driving the cleaning solution into microscopically small surface cracks and pores. Though it would seem that ultrasonic cleaning has the potential to dramatically accelerate the process of stress corrosion, I have not been able to find any relevant studies (including with other metals and other reagents).

Two final notes: While we might worry about ammonia, chlorides are a proven danger. I have personally seen a 14K gold pen overlay spontaneously crack to pieces after a few minutes of exposure to undiluted bleach. Even in very dilute form, bleach (and even heavily chlorinated water) will rapidly embrittle gold jewelry many times thicker than a pen nib. Jewelers' websites routinely caution against wearing gold rings of 14K purity or less when using cleansers, swimming in chlorinated pools, and even handling some foodstuffs (vinegar and salt, for example). And we can also learn from the jewelers regarding another aspect of stress corrosion cracking prevention, which is heat treatment. A stress relief anneal, typically 30 minutes at 250°C, is recommended after any jewelry repair. This is consistent with Loebich, cited above -- though I wonder how many of those who currently engage in nib repair have any knowledge of this.

ADDENDUM: Worth noting is the near-total absence of heel cracks on gold dip pen nibs, which were made in much the same way as fountain pen nibs, and which in use would have been subject to very similar fatigue cycling -- but very different clamping stress, and no ink exposure.

ADDENDUM: After publication of this post I was able to participate in a project involving examination of cracked gold nibs under a scanning electron microscope which left no doubt that stress corrosion cracking was the culprit. In multiple cases we could see cracks that had developed but which were not yet visible even with the aid of a standard jeweler's loupe. In subsequent discussions I found that other materials scientists with an interest in pens had done similar work, these experts unanimously confirming the stress corrosion diagnosis.

Don't snap that box!

One thing that I learned young about the handling of Old and Delicate Things was how to close a box with a spring-loaded latch. Press the release button down -- don't snap the box closed.

This beautiful and uncommon fitted Waterman box shows why. Snapping a box closed not only puts a lot of pressure on the latch plate, but also subjects it to the impact of the falling catch -- which itself can break from the repeated shocks. In this case, it was the latch plate that gave way. Subsequent snap closures did further damage, gouging away the leatherette and wood at the latch point.

Good old boxes are becoming harder to find, and the wooden parts of hundred-year-old boxes are typically dry and fragile. So please, hold that button down, and don't snap that box.