“I did my own research!,” or, Lead rot by any other name

In an online forum*, there was some discussion of the best storage methods for older metal figures. They’re usually made of a mix of an alloy of lead, tin, and various other metals such as antimony, bismuth, zinc, and so on. Many collectors have noted that some figures begin to show signs of corrosion where the metal begins to “rot” away turning into a powdery, greyish dust. Hobbyists have long called this “lead rot” and discussed the best ways to prevent it and whether it is possible to salvage miniatures with signs of lead rot. The consensus has been that lead reacts, particularly in the presence of moisture and/or extreme temperature, with acid vapors to start a self-sustaining** process of corrosion. That is, the products of the corrosion include more acid vapor that in turn corrodes more of the lead. The main culprits most often cited are wood and wood products like carboard “outgassing” acetic acid and tannins. PVA (“white”) glue, certain other materials possibly including the foam used in miniatures packaging, and so on are also implicated. The most frequent advice is to discard affected miniatures, although some will describe a process of neutralizing the acids and scrubbing away the corroded lead as a fix, with a more involved process using mineral spirits and other chemicals described by some toy soldier enthusiasts. Prevention measures are priming and/or sealing the miniatures, storage in plastic or metal containers, and good ventilation and/or desiccants in the cases.

Indeed there is a pretty well-established folklore about these topics, so I was surprised to see one commenter argue pretty strenuously that “lead rot” is not a real thing. The commenter has a history of working with various manufacturers. I can’t tell if they were ever an employee of them or more on the periphery as a consultant, freelance, etc. — they do however seem to have real contacts going way back and some insider knowledge. I am not naming them because some of their comments seem to have been deleted and their profile  seems to indicate a preference for privacy.

But the gist of their comments were that:

  1. the scientific literature does not describe “lead rot” or use the term, and even Wikipedia has no article on “lead rot
  2. zinc pest” and “tin pest” on the other hand are well-known in the literature (and on Wikipedia) and have similar presentations (metal corroding into a white powder/dust in a self-sustaining and irreversible process)
  3. in fact all cases described as lead rot are really zinc pest or tin pest, due to poorly made alloys and bad advice on cost-savings in the mixture of metals
  4. the original source for all the folklore on lead rot was a retracted article that is no longer published on the site it first appeared on

I am not a chemist and don’t pretend to be one. I am however a librarian and take an interest in rooting out disinformation and finding sources. So while I am not really competent to evaluate the chemistry involved, I am able to examine the four claims. I went with the easiest first: 4, the retracted article.

A search of the Retraction Watch Database (admittedly not comprehensive, but a good start) didn’t turn up any retracted scientific articles on lead corrosion. I was pretty sure I knew the article the commenter was talking about: a museum conservator’s report on the corrosion of lead components of ship models. It was formerly hosted here: http://www.dt.navy.mil/cnsm/lead_01.html but is now here: https://www.navsea.navy.mil/Home/Warfare-Centers/NSWC-Carderock/Resources/Curator-of-Navy-Ship-Models/Lead-Corrosion-in-Exhibition-Ship-Models/. That the old link is broken is no surprise to librarians, we’ve been discussing “link rot” since the world wide web debuted. I could find no evidence that there was any effort to “retract” this article, and suppose that the fact that the original link is most commonly given on web pages discussing lead rot, the commenter simply misunderstood the situation.

Point 1, that “lead rot” is not part of the scientific lexicon, is technically true. A federated search of many databases available through my university library*** turned up no articles using the phrase “lead rot”. However it should be noted that it’s often the case that popular usage and technical jargon is not always the same. Searching instead for articles about lead corrosion in the presence of acetic acid led me to a host of articles discussing the phenomenon, how to measure it by various methods, and most address conservation issues specifically. To be fair, the focus for these articles tend to be organ pipes and museum artifacts like lead seals. However, the articles do tend to confirm that that acetic acid (and other organic acids such as formic acid, a product of formaldehyde reacting with other chemicals in the air) catalyze lead corrosion. At least one paper does describes the reaction: Misallamova et al. (2019):

Lead and acetic acid, in the presence of oxygen, produce lead acetate and water; lead acetate reacts with carbon dioxide and water to produce another lead compound (some kind of lead carbonate — presumably known as the lead rot) and acetic acid. However this only described as “active corrosion,” and a chemist friend points out that this doesn’t qualify as autocatalytic, just catalysis, as the first equation produces one molecule of the lead compound and the second requires three molecules. I think that since the second shows three acetic acids, in the real world we could expect the reactions to be self-sustaining provided there is enough humidity and carbon dioxide present, and no ventilation, but again, I’m not a chemist.

As an aside, I’d also note that the existence or lack of a Wikipedia page is doesn’t carry a lot of weight in this controversy. I suppose this was just a simple way to show that there is not much use of the term “lead rot” outside the hobby.

Point 2 seems correct. Cornelius et al. (2017) notes that lead-tin alloys are resistant to tin pest, and indeed lead-tin solders are used in electronics for this reason (they are no longer allowed in plumbing due to toxicity). However tin pest can still occur in lead-tin alloys, so extremes of cold should be avoided. However as tin pest is supposed to occur at very low subfreezing temperatures, I find it hard to believe it’s an issue outside the arctic and Antarctic. Moreover tin pest is more of a crumbling as the tin molecules form a fragile crystalline structure, so it would not look much like lead rot.

I didn’t find much about zinc pest, but it does occur here and there in the literature.  Zinc pest hand just needs water so that could certainly form in damp places which would also be prone to lead rot.

Point 3 seems partly true. Plumbridge (2008) notes that lead-tin alloys were especially resistant to tin pest (no pest after 4 years at -18 Celsius, but another sample at -40 Celsius did show changes). Moreover the tin pest shown in his alloy tests are more of a disintegration into pieces rather than dust. But it seems likely that various alloy mixes will show different kinds of corrosion. Anecdotally, however, lead rot is not claimed to occur in lead-free alloys, but I’d expect that if folks really didn’t know the difference we’d be hearing about lead rot in them.  Interestingly, Gibson and Watt (2010) report that even a small tin content (1.2% or more) increases the resistance to lead corrosion by acetic acid, so lead/tin miniatures which may have 30% or more tin seem to be poor candidates for lead rot. Anecdotally, early miniature manufacturers were using anything they could find cheaply — a newspaper article on Ral Partha quotes the owner saying they used an alloy consisting of 80% lead before switching to the almost pure tin Ralidium.

Lastly, another commenter on the thread pointed out that the phenomenon of lead rot is remarkably similar to the manufacture of “white lead.” This is well-known process of exposing lead to vinegar fumes in order to produce a white compound for use as a pigment in paint. Vinegar after all is mostly water and acetic acid.

My hunch is that the original commenter was trying to point out that zinc pest and tin pest are problems that can present as something a lot like lead rot, and that it is the cheapest metals (high lead alloys or even pure lead) that are prone to lead rot. Moreover, tin pest is considered autocatalytic in the sense that the reaction will continue using only its own products, while lead rot is more dependent on the continuation of favorable conditions. (Good news! Ish.)

I’ll write up something a bit more thorough at some point in the future, but my references below should be a good starting point for anyone else interested in “lead rot.”

=======

*more specifically, a Facebook group; yes I know I should probably get off Facebook and social media generally

**Apparently the correct term is “autocatalytic” — the reaction products include a catalyst for the same or a related reaction

***My university is pretty strong on chemical engineering, and even has a corrosion engineering program, which the library supports as best it can by buying access to relevant resources

=========

Selected References

Cornelius, B., Treivish, S., Rosenthal, Y., and Pecht, M. (2017). The phenomenon of tin pest: A review. Microelectronics Reliability 79: 175–192.

Deflorian, F., and Fedel, M. (2013) Electrochemical analysis of the degradation of lead alloy organ-pipes due to acetic acid. Journal of Cultural Heritage 14: 254–260.

Gibson, L.T., and Watt, C.M. (2010). Acetic and formic acids emitted from wood samples and their effect on selected materials in museum environments. Corrosion Science 52:172–178.

Misallamova, S., Kouri, M., Strachotova, K.C., Stoulil, J., Popova, K., Dvorakova, P., and Lhotka, M. (2019) Protection of lead in an environment containing acetic acid vapour by using adsorbents and their characterization. Heritage Science 7(76). https://doi.org/10.1186/s40494-019-0317-3

Niklasson, A., Johannsson, L.-G., Svensson, J.-E. (2008) The influence of relative humidity and temperature on the acetic acid vapour-induced atmospheric corrosion of lead. Corrosion Science 50: 3031–3037.

Plumbridge, W. J. (2008) Recent Observations on Tin Pest Formation in Solder Alloys. Journal of Electronic Materials 37 (2): 218-223.

Plumbridge, W.J. (2010) Tin pest in lead-containing solders. Soldering & Surface Mount Technology 22 (1): 56–57.

Published in: on January 9, 2022 at 10:48 am  Comments (8)  
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8 CommentsLeave a comment

  1. Most important thing : GET your figures painted – simple as that.
    I have a LOT of minis dating back over 35 years and none that were originally painted by me have rot, the only rot i have had is in figures owned by others.
    Paint them, varnish them enjoy them.

    • Absolutely. I’d add a caution against using potentially acid materials for basing (wood, cardboard, PVA aka “white glue.” However in my experience even cheap Elmer’s Glue and and cardstock bases never affected my miniatures after decades.

    • I have had a painted miniature develop “lead rot” (after ~15 years), so painting is not 100% effective. The PbCO3 formed under the paint and made it blister out, but after treatment and a new paint job, nothing has happened with the miniature since.

      • Do you remember what was used as an undercoat/primer?

      • I suppose some paints, primers, and top coats may be more porous and allow the air, damp, and acids to react with the lead underneath. I’ve definitely seen older minis that are painted but affected by lead rot. I am guessing that a metal primer that actually bonds with the metal will do better than just an undercoat of paint (which many painters, myself included, have used as a primer). The unpainted bottom of the base would be another vector.

        I have heard anecdotally that impurities in the metal can lead to rot, with the minis corroding from the inside out, but since the reaction described above requires some air and water I’m not sure how that would work. That might be more of a thing with old hollow-cast toy soldiers.

  2. Tin pest could easily affect soldering on MRI circuit boards; the superconducting electromagnets are kept chilled with liquid hydrogen.

    • Yes, several of the articles I looked at discussed how tin solder was a problem in artic and Antarctic conditions, both for electronics and more mundane stuff (cans of fuel that were stored for an Antarctic expedition were found to be empty because the tin solder sealing them corroded away due to the cold). So super-chilled circuitry really shouldn’t be tin-based. I also read that having some lead in an alloy protects against tin pest similarly to how tin in the alloy protects protects against “lead rot”. Neither is 100% effective of course, they just reduce the chances.

  3. That was an informative read. Thank you for taking the time to collate it. I never encountered anything like it, but I am not in the hobby long enough yet, to own really old miniatures.


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