The Safety of Zinc Casting

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Since my Popular Science article about zinc casting came out in August, 2003, I've received several questions about whether casting zinc is really safe, and several people have told me it is not, including a respected chemistry professor who called me "stupid" for recommending zinc as a casting metal, on the grounds that using it would cause serious heath problems.

This is, however, not the case: Molten pure zinc does not cause health problems beyond the obvious burns if you pour it over yourself.

There are two main paths that lead people to think zinc casting constitutes a health hazard: Welding of galvanized (zinc-plated) steel, and casting of brass (a copper-zinc alloy). Both of these activities can make you sick from inhalation of zinc oxide fumes, so people's concern about zinc casting is not entirely unfounded. But these activities differ in important ways from casting pure zinc.

When assessing any risk I always like to ask first, what is the worst possible thing that can happen? In the case of melting lead, for example, the answer is pretty grim: Even minute amounts of lead in the body can cause serious, long term neurological damage or possibly cancer. Especially for children, exposure to barely detectable levels of lead has been strongly linked to brain damage. You do not want to mess around with lead.

The next question is whether there are invisible, cumulative effects that can cause you harm without your being aware of it until it's too late. For example, a hot stove either burns you or not: If you finish cooking without having burned yourself, you can be pretty sure that you won't wake up a week later with a burn. Sun tanning is different: You might tan yourself for years without getting sick, and then suddenly you find you have skin cancer. Lead is another example of something that has invisible, long-term harmful effects.

So what's the worst that can happen from exposure to zinc, and can it cause hidden long term effects?

When zinc is raised to a high temperature (at or above its boiling point around 900C), it burns and forms zinc oxide smoke. Like any kind of smoke, zinc oxide will irritate the lungs, leading to difficulty breathing: This is not a dangerous situation and it clears up immediately when you move away from the source of the fumes. People exposed to high concentrations of zinc oxide over a prolonged period of time can also develop a condition known as the "zinc chills", "metal fume fever", "brass-founders ague", or a number of other colorful terms. This involves fever, tremors, and other unpleasant symptoms.

However, metal fume fever, which is rarely seen these days, is a purely temporary condition that clears up without any known long term effects or complications. It's certainly nothing to laugh about, but the maximum downside to zinc exposure is fever and shakes that go away and then you get better. It's not like lead or sun tanning that can have life-threatening consequences down the road, or like carbon monoxide that can kill you on the spot.

And you can't even get zinc chills from melting and casting pure zinc.

Welding galvanized steel generates zinc fumes because welding happens at very, very high temperatures: The zinc plating for an inch or so in every direction from the weld is burned off in billowing white clouds of zinc oxide. Anyone who has learned to weld will have been warned many times to wear a special mask when welding galvanized steel, and that's a good idea. I use one always when welding galvanized plate.

Melting brass (a copper-zinc alloy) also generates large amounts of zinc oxide fumes, because the melting point of brass is high, close to the boiling point of zinc. In fact, the partial pressure of zinc vapor above a pot of molten brass may be almost half an atmosphere! No wonder such a pot gives off great quantities of blue-white zinc oxide smoke.

But the situation with a pot of molten pure zinc around its melting point of 420C is very different. The partial pressure of zinc vapor above just-molten zinc is about 2500 times lower than it is near the melting point of typical brass alloys. And a pot of molten zinc rapidly forms a skin of solid zinc oxide slag, which probably reduces the actual pressure of zinc vapor to zero, except when the metal is freshly stirred or poured (and then it is still only 1/2500 the amount generated by brass casting).

Zinc oxide fumes are very easy to see: They are a bright white or blue-white particulate smoke that is visible even at very low concentrations. I've seen zinc oxide being created in many different ways, including welding, burning of zinc metal with a blow torch, overheating of a pot of zinc, etc. I've even seen zinc oxide generated by melting zinc in a microwave oven. But in all my years of casting zinc, I have never seen zinc oxide smoke coming from a pot of zinc melted on an electric stove. It just doesn't happen, because the temperature is not high enough.

It's possible there is some small concentration of zinc oxide being generated below the threshold of visibility. If it were lead or lead oxide, that would be a problem, because breathing in even tiny amounts of lead is really bad. But that is manifestly not the case with zinc. Breathing small concentrations of zinc does not cause any long term or gradual effects. It does not accumulate in the blood or nervous system like lead can. If you don't feel any immediate unpleasant effect from whatever you are doing with zinc, then it's not doing you any harm.

On the basis of these facts, I am absolutely convinced, as are a number of expert chemists, metallurgists, and foundry men I've consulted, that melting pure zinc or alloys containing zinc (but not any lead) that melt at or below about 450C, represents no health hazard from inhalation of zinc fumes. Yes, you can definitely burn yourself really badly if you spill it, but that is the extent of what you have to worry about.

Here's an amusing angle on the zinc safety issue. It turns out that certain kinds of candles use wicks that have a metal wire inside them to make them stiffer. These used to be made of lead, and it seems that some countries still produce candles with lead-core wicks. This is of course a really bad idea considering that the core will be burned off into the air as the candle burns down. So what metals are used as safe alternatives to lead in candle wicks, knowing that whatever metal it is will be emitted into the air inside people's homes? The answer, not surprisingly, is zinc and tin! Here is a quote from a candle industry trade group's website:
The vast majority of wicks manufactured in the U.S. are made of 100% cotton or cotton-paper combinations. No member of the National Candle Association - which accounts for 90 percent of all candles made in the United States - uses lead wicks. (NCA members voluntarily agreed not to use lead wicks more than 25 years ago.) Some U.S.-made candles contain metal wicks; these are typically zinc-or tin-core wicks, which are known to be safe and non-toxic.

It should be noted that some imported candles, most notably from China, have been found to contain lead wicks. Fortunately, the U.S. Consumer Product Safety Commission has proposed banning lead wicks in all candles sold in the United States, a move strongly supported by the National Candle Association. This will help ensure that all candles on the market - not just those made by NCA members - use safe wicks.
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