Zinc pest
Zinc pest is a condition that can affect articles manufactured from zinc alloys in the 1920s to 1940s, and can cause some older diecast toys to warp, crumble or even sometimes violently explode in response to stress and temperature change. The problem was largely overcome when all manufacturers (and their employees!) finally realised the critical importance of avoiding all forms of lead contamination in the manufacturing process.
Mechanisms
The microstructure of metals and metal alloys usually takes the form of tiny ordered pockets of metal, each with their own internal crystalline atomic structure, butted up against each other. The manipulation of the sizes, and alignments of these "grains" can then give two pieces of metal with a nominally similar composition markedly different physical properties. A sword blade (for instance) may be treated with repeated hammering and holding, and heating and quenching, to produce a composite microstructure with flat aligned surface grains and a hard blade surface, but a softer and more malleable core that is less prone to brittle breaking. The addition of impurities provides a vast number of different grades of steel with different physical properties.
In the case of almost pure zinc, the metal is highly sensitive to tiny amounts of impurities that can end up between the grains and cause them to separate. When the metal is repeatedly stressed or flexed (which happens when the temperature changes), separation in the structure at the microcrystalline level causes tiny cracks, with surrounding distortions that then exert leverage to make the cracks deeper.
ZAMAK and MAZAK
A solution was devised by the New Jersey Zinc Company (who had a certain amount of self-interest in getting the problem fixed). NZC presented a zinc alloy made up with specific proportions of alloying aluminium magnesium and copper, with the zinc specially purified to 99.99% using a refluxer in the smelting process to repeatedly reprocess the metal until it reached the required purity. This was marketed under the acronym ZAMAK. In the UK, a variation using only 99.5%-pure zinc was marketed as MAZAK.
Problems with MAZAK
While ZAMAK (and MAZAK) supposedly solved the zinc pest problem, the alloy was still very sensitive to subsequent contamination before casting, and some of the early Dinky production was so plagued by zinc pest that a few batches of some models had already started falling apart by the time their boxes reached the toyshops. Investigation showed that the problem was due to people working in the factories "recycling" small pieces of soft scrap metal (such as lead tags) by throwing them into the mix - lead being fatal to the alloy's structural integrity.
More stringent quality control (and education) finally solved the problem, but there are still batches of early diecast models that have not yet shown any signs of zinc pest, but which may have hidden internal problems and may eventually disintegrate, and it is considered sensible to keep very early, very rare (and very valuable) examples of diecast models at a constant temperature, and not to send them through the post during winter months, just in case.
Manifestation
A long thin metal piece (such as a diecast model ship) can gradually curve due to stresses and strains frozen into the metal during the casting process ("banana-ing"), with the curvature promoting the propagation of more cracks through the structure until it breaks. An affected surface can crumble away into metal dust, and pieces can break off the main body of the piece and fall away until eventually the whole piece disintegrates.
Consequences
Issues with lead contamination may have changed how the industry developed: While Lesney (Matchbox) were an early adopter of lead-free paints, this may have been partly encouraged by an existing lead-phobia due to the effects on MAZAK which meant that their factory was declared a lead-free zone - any lead components were manufactured externally by subcontractors to keep the dreaded material as far as possible from the diecasting works.
It may also explain how Meccano Ltd has so little early competition in the UK for their Dinky Toys range – while Meccano Ltd. had sufficiently deep pockets to persist with MAZAK in the 1930s until they'd worked out what the problems were, other smaller companies may have found the new material more difficult to deal with, and while one might have expected Britains Ltd. to have been ideally placed to produce a range of diecast MAZAK vehicles to replace their existing lead range, it's conceivable that any early experiments may have been ruined by the omnipresence of lead in the W. Britain works.
Present-day consequences
Although the zinc pest issue is considered long solved in modern diecasting, it is still a major issue for collectors of very early examples of diecast toys, especially those produced in the 1930s. For certain "unlucky" models (such as some early diecast ships with a small production run), much of the production has already self-destructed, and it can be unusual to find a surviving example that does not show at least some banana-ing ... which means that at some unknown future date the piece will probably fail.
There are accounts of entire collections (such as "company" archive collections) consisting of examples of the very earliest production runs of a range of models, which, on investigation after years in storage, have been found to be partly destroyed by zinc pest.
Some other historic non-diecast toys and models also have smaller parts (such as cast metal bogies and wheels for wooden model railway rolling-stock) that were originally diecast using Mazac and can suffer badly from deterioration - these parts can often be replaced with modern castings taken from a "good" original example.
