Zinc pest: What it is and what to do about it

Last Updated on March 28, 2022 by Dave Farquhar

In metallurgy, zinc pest is a problem that occurs when impurities are introduced in the process of diecasting. The problem causes the metal to crack and disintegrate, in extreme cases crumbling to dust. It mostly affects products made in the 1920s-1950s.

Zinc pest or zinc rot was discovered in 1923 and well understood by the 1930s. It is relatively uncommon today, but can still occur due to sloppy manufacturing processes.

What causes zinc pest

Zinc Pest
This Dorfan train locomotive from the 1920s shows the effects of zinc pest. The cow catcher has broken off, and the wheels are chipped. The flaking paint also suggests the body may have swelled to some extent.

Diecasting uses alloys of Zinc, aluminum, magnesium, and copper. These are variously called Mazac, Zamak, or Zamac, depending on the part of the world you’re in. It’s a cheap process, because it’s mostly made of cheap metals. But it provides a really nice balance of strength, cost effectiveness, and weight.

The problem with this alloy is that it’s rather volatile. Impurities in the mix, especially lead, cause it to react. Over time, the metal expands, causing pitting, cracking, swelling, and warping. This leads to pieces falling off, distortions in the surface, and in extreme cases, crumbling.

Impurities can be introduced at any point in the supply chain, whether in the factory, or in the process of refining the zinc.

If the Zinc is less than 99.99% pure, it can be prone to zinc pest. But the manufacturing process can introduce impurities as well. Workers used to throw metal gum and candy wrappers into the molten alloy, with the company’s blessing. It was an early form of recycling, and had been a common practice with pot metal for decades. The problem was these soft metals used in wrappers often contained lead, as this was before we fully understood lead’s toxic effects. So even if the zinc started out pure enough, it might not be pure enough by the time workers were done disposing of their wrappers in the alloy.

Why zinc pest was unpredictable

Zinc pest was unpredictable because there were so many variables. If the zinc started out pure enough and few people threw wrappers into the mix that day, most of the product would be fine. But the more lead got into the mix, the more volatile the product would be.

Zinc pest literally drove the Dorfan Manufacturing Company, one of the first companies to use diecasting in making toys, out of business. Dorfan claimed its toy trains were higher quality than its competitors, who used earlier processes. Ives, notably, used cast iron. If you dropped an Ives train into a concrete floor, it would break into several pieces. A Dorfan train could survive multiple falls.

But within a few years, those Dorfan trains were breaking apart without anyone doing anything. That’s a problem when your selling point was quality. And unlike their competitors, they didn’t have an earlier design to fall back on. By 1934, Dorfan was out of business, lasting only 10 years on the market.

Dorfan was hardly the only example of this. Early Dinky diecast vehicles are also prone to this issue. But the problem grew less common over the course of the 1930s as manufacturing and refining processes improved.

Today, zinc pest can still occur, but it’s fairly rare given that we’ve had a century to figure out die casting. It tends to happen in waves as manufacturing or some other part of the supply chain moves to new countries where diecasting is an unfamiliar process. Once everyone in the supply chain understands the tricky nuances of diecasting, zinc pest disappears again, at least until pricing pressures disrupt the supply chain again.

Conditions that exacerbate zinc pest

Some people believe that high humidity, particularly humidity over 65 percent, speeds up zinc pest. High temperatures also probably contribute to zinc pest, as every 10 degree increase doubles the speed of a chemical reaction.

Fans of early diecast toys may try to slow the effects by storing items known to be affected by the problem in dry, temperature-controlled environments. But generally, once the process starts, there’s little you can do to slow it down.

Repairing affected items

Some people believe zinc pest is unrepairable, but that’s not necessarily true. There’s little you can do to reverse swelling or warping, but you can repair cracks and breaks. If you work epoxy into the cracks, you can preserve some degree of integrity. The piece may continue to crack, but future applications of epoxy will help that, and in the meantime, the existing epoxy is holding it together. If a piece breaks off entirely, you can epoxy it back into place. The piece is no longer a pristine original of course, but when few pristine originals remain, an epoxied piece is better than none.

I know of hobbyists who epoxied together Dorfan bodies in the 1980s and 1990s. The modifications they had to make, especially to get them running again, aren’t for the purist. But at least they got the trains working again, instead of just leaving them to crumble further. The repairs lasted decades.

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