(image from The Toy Soldier Museum)
Lead Rot. The Pewter Plague. Figure Fungus. Mini Mold. Casting Canker. Whatever you call it (and I admit I made up all but the first two), it’s that dusty fluff that you find at the feet of figure, or possibly covering the whole casting, that crumbles and reveals a void, where once was figure. It is a kind of rust, basically — the oxidation of lead.
Worse yet, it seems that the oxidized lead powder itself can cause unoxidized lead to oxidize too — hence the “disease” metaphors in the names of this phenomena. It can spread!
But, wait, you think. Didn’t the Romans use lead for their water pipes? Isn’t plumbum the Latin word for lead, and the source of word plumbing, and wouldn’t a civilization as smart as the Romans use something that doesn’t rust for plumbing? Well, yeah, lead does not oxidize easily. But it will oxidize in the presence of acids.
Somehow, figure enthusiasts did not catch on to this for a while. Old military modeling books (that’s grown up for toy soldier books) even instructed collectors to clean their lead figures with vinegar before painting, in order to give the surface of the figure something for the paint to adhere to. Were they then surprised to find lead rot years later? Museum curators who noticed lead rot on parts of their models and exhibits made from lead tried to remove it with vinegar, which only causes more lead rot.
The acid catalysts of lead rot can come from direct application like the vinegar, or from outgassing from wood or composite cabinets, packaging or boxes, or from other elements present on or near the figures (bases, diorama parts, etc., including certain glues).
You can best protect your minis by priming (and painting and sealing) them and using acid-free (“archival”) glue to attach them to bases. Also consider open storage shelves, or air out your boxes now and then, and protect them from extremes of temperature which cause unstable materials (plastic, organics, etc.) to outgas (i.e., release minute amounts of gases) in the first place. Silica gel or other desiccants can help too if you leave them in your boxes or cabinet, because moisture is also a catalyst.
When miniatures companies began to shun lead in the mid-1990s (due to laws that were being introduced to restrict lead in toys), the various pewters used instead were generally tin and zinc based. Old “lead” minis are normally lead-tin alloys anyway, so the lead-free pewters are often just pure tin, or tin alloyed with small amounts of copper, bismuth, and/or antimony. Ral Patha’s “Ralidium” was, I have heard, pure zinc. I can’t say if these metals are susceptible to anything like lead rot.
I have one Minifigs figure that has some white “frost” showing on top of his paint job, and I picked up a TSR figure with some funny looking splotches that could be oxidation. I painted over it though, and no problems so far.
If you are interested in learning more about this scourge, check out the links below.
The earliest mention of lead rot I came across some time back.
One article I found about 6 or 7 years ago is no longer on the web but it is archived in the Wayback Machine.
Some discussion among minis fanatics at the Miniatures Page.
An eBay seller of military models has an article which is terribly formatted but has some advice for stopping lead rot.
Some more preservation advice at a blog.
A serious study of formaldehyde and lead oxidation. Formaldehyde is outgassed by plywood and other building materials.