CRACKING UP
By Dominic Borrelli
The distortion and break-up of die cast toys has been a well known situation and the subject of many theories over the years. Unfortunately, reading Model Collector’s letters pages, it seems that despite set criteria for Zinc alloys being set in the 1960s, the problem is very much a current one. So I’ve decided to try and find out what it’s all about. As you would expect, there are many conflicting and baffling (well, to a non-scientist, like me) reports on the causes, but I’ve tried to decipher the gist of it all. Let me say straight away, that much of the explanatory information has been gathered from Wikipedia and from the paper: Corrosion-induced Cracking of Model Train Zinc-Aluminium Die Castings by
R.J.H. Wanhill and T. Hattenberg; National Aerospace Laboratory NLR, Amsterdam, The Netherlands, to whom I am indebted.
  
The Nature of Zinc Alloy
The name ZAMAK is an acronym of the German words for Zinc, aluminium, magnesium and copper (Kupfer, in German), but the main ingredients are Zinc and aluminium. In my ignorance, I assumed ‘that was that’. However, it turns out that there’s a whole family of Zamak or Zamac alloys- with Zamaks 2,3,4,5 and 7, some with up to 3% copper content.
References to the New Jersey Zinc company as being the originator of Zamac seems confusing as both the original name and the name for the condition, ‘Zinkpest’ originate in Germany and apparently it was in Germany, around 1923, that the phenomenon of granular disintegration was first explored. Some authors even suggest that such alloys were being produced towards the end of the 19th century. Whatever the origins, the UK name, MAZAK is accredited to Morris Ashby, who obtained licences from New Jersey in the 30s. 
You may think it strange that a metal, often derisively called ‘Monkey Metal’ by engineers and associated with taps, door handles and Dinkys, should be the subject of interest to the aeronautical industry, but Zamac/Mazac castings have been used since the 1920s for engine components and ancillaries, such as carburettor bodies and granular disintegration was a problem during and post WWII, when pure metals were hard to come by.
The Zinc Pest
What it isn’t, is fatigue. Unless you have jumped up and down on it, your affected model is not suffering from fatigue. It’s also a misnomer that the addition of unintentional metals, such as lead, is solely the cause, because if the primary ingredients themselves are impure, the corrosion will occur anyway and this leads to a worry I have. But I’ll get to that later.
From what I can gather, when Mazac is produced, its crystalline structure contains boundaries between the harder zinc crystals and those crystals formed by the melting and fusion of zinc and aluminium. Impurities such as cadmium, tin and lead, are activated by moisture, which starts an electrochemical process, by which lead (insoluble in Zinc) is drawn out from between the alloy crystals and cadmium is leached from the actual crystals themselves. The impurities are then deposited at the boundaries between the two types of crystal.
The greater the moisture and the finer the crystals, the faster the corrosion.
Crystals are smaller when the cooling of the casting is rapid.
The practical upshot is that according to Professors Wanhill and Hattenberg, moisture starts a process on the surface of the metal, which induces the impurities to collect and push the crystals apart. This results in cracks and the growth or ‘blooming’ of the casting.
The Cure
Sorry, there isn’t one. However, bearing in mind that moisture is the activator, reducing it is going to help slow down, or even prevent the curse. So here are some suggestions:
. Don’t store models in cardboard boxes
. Don’t store models in the attic, garage or anywhere where rapid temperature changes are likely
. Don’t wrap models in paper or cotton wool-both trap moisture. Bubble wrap is OK as a lining to a box, or as a cover, but never wrap it tightly round a model or box
. Do invest in good quality plastic storage boxes. If you have ultra valuable or precious items, you may consider the complex process of boxing the items, then placing the box in one of those giant vacuum bags used for storing duvets and sucking the air out. Do a trial run first and if the empty plastic storage box is up to the job, repeat the process with the models in the box. I must stress, NEVER vacuum seal cardboard boxes of any sort-they WILL distort!
. Do invest in silica gel as it’s great at absorbing moisture.
There are two ways of obtaining silica gel (actually crystals): vendors on eBay will happily sell you sachets in a variety of sizes and in varying quantities. You can also DIY it, by buying large bags of crystals. The next step is to buy some Gloy paper glue, or similar and pop along to your local convenience store or supermarket. Buy the cheapest square tea bags. Cut along one edge, empty the tea out and fill with silica crystals. Seal with glue and allow seam to dry. If you don’t have access to the internet, pop along to your local pharmacy (which is what I did) and they can order tubs of silica for you. For the super-mean; get friendly with someone who works in a shoe or handbag shop.
The final thought is; always check your collection on a regular basis. Any signs of damp -dulling, pitting or bubbling of paint- means you have to act fast!
Obviously, if you have models that were made five years ago, or less, this shouldn’t happen. But unfortunately, Zinc Pest is alive and rampant. My fear is that due to the increased prices of raw materials, manufacturers (and by that, I mean the actual makers and not the name on the box) are using inferior sourced zinc or, worse still, being tempted to recycle old castings. If that’s the case, then we’re all in for a cracking time!   
Since writing this article, I have experimented with vacuum sealing models within plastic boxes and you can see the results here: