Are microwave makers responsible to warn people about superheating explosions?

I read the letter on your website about being burned from water exploding from heating in the microwave. This recently happened to me with a cup of coffee. I thought it was because of the material of the coffee cup. I too felt ridiculous trying to explain my injuries to anyone because it seemed impossible. My question to you is: “are microwave makers responsible for advisement of this kind of hazard”? — JB

When you use a microwave oven to heat water in a glass or glazed container, the water will have difficulty boiling properly. That’s because boiling is an accelerated version of evaporation in which water vapor evaporates not only from the water’s upper surface, but also through the surface of any water vapor bubbles the water happens to contain. I use the phrase “happens to contain” because that is where all the trouble lies.

Below water’s boiling temperature, bubbles of water vapor are unstable—they are quickly crushed by atmospheric pressure and vanish into the liquid. At or above water’s boiling temperature, those water vapor bubbles are finally dense enough to withstand atmospheric pressure and they grow via evaporation, rise to the surface, and pop. At that point, I’d probably call the water vapor by its other name: steam. But where do those steam or water vapor bubbles come from in the first place?

Forming water vapor bubbles in the midst of liquid water, a process called nucleation, is surprisingly difficult and it typically happens at hot spots or non-wetted defects (places where the water doesn’t completely coat the surface and there is trapped air). When you boil water in a metal pot on the stove, there are hot spots and defects galore and nucleating the bubbles is not a problem. When you boil water in a glass or glazed container using a microwave oven, however, there are no significant hot spots and few non-wetted defects. The water boils fitfully or not at all. The “not at all” possibility can lead to disaster.

Water that’s being heated in a metal pot on the stove boils so vigorously that the stove is unable to heat it more than tiny bit above its boiling temperature. All the heat that’s flowing into the water is consumed by the process of transforming liquid water into gaseous water, so the water temperature doesn’t rise. Water that’s being heated in a glass container in a microwave oven boils so fitfully that you can heat it above its boiling temperature. It’s simply not able to use up all the thermal energy it receives via the microwaves and its temperature keeps rising. The water becomes superheated.

Most of the time, there are enough defects around to keep the water boiling a bit and it superheats only a small amount. When you remove the container of water from the microwave oven and toss in some coffee powder or a teabag, thus dragging air bubbles below the surface, the superheated water boils into those air bubbles. A stream of bubbles suddenly appears on the surface of the water. Most people would assume that those bubbles had something to do with the powder or teabag, not with the water itself. Make no mistake, however, the water was responsible and those bubbles are mostly steam, not air.

Occasionally, though, the water fails to boil at all or stops boiling after it manages to wet the last of the defects on the glass or glazed surface. I’ve made this happen deliberately many times and it’s simply not that hard to do. It can easily happen by accident. With no bubbles to assist evaporation, the water’s only way to get rid of heat is via evaporation from its top surface. If the microwave oven continues to add thermal energy to the water while it is having such difficulty getting rid of that energy, the water’s temperature will skyrocket and it will superheat severely.

Highly superheated water is explosive. If something causes nucleation in that water, a significant fraction of the water will flash to steam in the blink of an eye and blast the remaining liquid water everywhere. That boiling-hot water and steam are a major burn hazard and the blast can break the container or blow it across the room. I’ve heard from a good number of people who have been seriously hurt by exploding superheated water produced accidentally in microwave ovens. It’s a hazard people should take seriously.

After that long introduction, it’s time to answer your question. Yes, I believe that the microwave makers are responsible for advising people of this hazard. Moreover, they know that they are responsible for doing it. If you look at any modern microwave oven user manual, you will find a discussion of superheating or overheating. Look at your manual, I’ll bet it’s in there.

But that discussion will almost certainly be buried in the middle of an long list of warnings. For example, in one manual, the discussion of overheated water appears as item 17 of 22, after such entries as “4. Install or locate this appliance only in accordance with the provided installation instructions” and “12. Do not immerse cord or plug in water”. To be fair to the manufacturer, warning 17 is longest of the bunch and it suggests mostly reasonable precautions (although I’m not so happy with recommendation 17a: “Do not overheat the liquid.”). No Duh.

I think the issue is this: most product warnings are provided not out of any sincere concern for the consumer, but out of fear of litigation. A manufacturer’s goal when providing those warnings is therefore to be absolutely comprehensive so that they can point to a line in a user manual in court and claim to have fulfilled their responsibility. The number and order of the warnings makes no difference; they just have to be in there somewhere.

So all those warnings you ignore in product literature aren’t really about consumer safety, they’re about product liability. You ignore them because everything now comes with a thousand of them, ranging from the reasonable to the ridiculous. For my research, I ordered 99.999% pure sodium chloride (i.e., ultrapure table salt). It came with a 6-page Material Safety Data Sheet that identifies it as an “Xi Irritant”, noting that it is “Irritating to eyes, respiratory system and skin” and recommending first aid measures that include:

“After inhalation: supply fresh air. If required, provide artificial respiration. Keep patient warm. Seek immediate medical advice.
After skin contact: Immediately wash with water and soap and rinse thoroughly. Seek immediate medical advice.
After eye contact: rinse opened eye for several minutes under running water. Then consult a doctor.”

So much for swimming in the ocean…

By design and by accident, our society has lost the ability to distinguish real risk from imaginary risk. We treat all risks as equal and spend way too much time worrying about the wrong ones. If you want to be safer around your cell phone, for example, you should worry more about driving with it in your hand than about the microwave radiation it emits. The current evidence is that your risk of injury or death due to a cell-phone related accident far outweighs your risk from cell-phone microwave exposure. Even if further research proves that cell phone microwave exposure is injurious, we should be acting according to our best current assessments of risk, not according to fears and beliefs.

That said, I’d like to see product literature rank their warnings according to risk and put the real risks in a separate place where they can’t be overlooked or ignored. Put the real consumer safety stuff where the consumers will see it and put the product liability stuff somewhere else where the lawyers can find it. For a microwave oven, there are probably about half a dozen real risks that people should know about. Several of them are relatively obvious (e.g., don’t heat sealed containers) and some are not obvious (e.g., liquids heated in the microwave can become superheated and explode).

Maybe we’ll get a handle on risk someday. In the meantime, inform your friends and children that they should be careful about heating liquids in the microwave, particularly in glass or glazed containers. Just knowing that superheating is possible would probably halve the number of burns and other injuries that result from superheating accidents.

Why is a car’s rear window put and kept under stress, and what has this to do wi…

Why is a car’s rear window put and kept under stress, and what has this to do with polarization? — BD, Leuven, Belgium

The rear window of a car is made of tempered glass — the glass is heated approximately to its softening temperature and then cooled abruptly to put its surface under compression, leaving its inside material under tension. That tempering process makes the glass extremely strong because its compressed surface is hard to tear. But once a tear does manage to propagate through the compressed surface layer into the tense heart of the glass, the entire window shreds itself in a process called dicing fracture — it tears itself into countless little cubes.

The stresses frozen into the tempered glass affect its polarizability and give it strange characteristics when exposed to the electromagnetic fields in light. This stressed glass tends to rotate polarizations of the light passing through it. As a result, you see odd reflections of the sky (skylight is polarized to some extent). Those polarization effects become immediately apparent when you wear polarizing sunglasses.

I have read that very old panes of glass become thicker at the bottoms than the …

I have read that very old panes of glass become thicker at the bottoms than the tops. Doesn’t that show that glass flows? — MJ

While it is sometimes noted that old cathedral glass is now thicker at the bottom than at the top, such cases appear to be the result of how the glass was made, not of flow. Medieval glass was made by blowing a giant glass bubble on the end of a blowpipe or “punty” and this bubble was cut open at the end and spun into a huge disk. When the disk cooled, it was cut off the punty and diced into windowpanes. These panes naturally varied in thickness because of the stretching that occurred while spinning the bubble into a disk. Evidently, the panes were usually put in thick end down.

Modern studies of glass show that below the glass transition temperature, which is well above room temperature, molecular rearrangement effectively vanishes altogether. The glass stops behaving like a viscous liquid and becomes a solid. Its heat capacity and other characteristics are consistent with its being a solid as well.

A center punch is a device used in extricating people from cars. You put tape ov…

A center punch is a device used in extricating people from cars. You put tape over the glass window you want to pop, place the center punch in the corner of that window, and simply press inward. This does something that causes the glass to crack in a spider web pattern. The glass sticks to the tape, you push in enough glass to get your hand through, and knock the rest outward. This technique works on any window except the front windshield, which is 3 layers(glass, plastic, glass). Can you explain it? — RSG, Boston, Massachusetts

A center punch is a common tool used to dent a surface prior to drilling. The drill bit follows the pointed dent and the hole ends up passing right through it. But in the situation you describe, the center punch is being used to damage the surface of a car window. When you push the handle of the center punch inward, you are compressing a spring and storing energy. A mechanism inside the center punch eventually releases that spring and allows it to push a small metal cylinder toward the tip of the punch. This cylinder strikes the tip of the punch and pushes it violently into the glass. The glass chips.

In normal glass, this chipping would be barely noticeable. But the side and rear windows of a car are made of tempered glass—glass that has been heat processed in such a way that its surfaces are under compression and its body is under tension. Tempering strengthens the glass by making it more resistant to tearing. But once an injury gets through the compressed surface of the tempered glass and enters the tense body, the glass rips itself apart. The spider web pattern of tearing you observe is a feature of the tempered glass, not the center punch. Any deep cut or chip in the tempered glass will cause this “dicing fracture” to occur.

Can you get a suntan or sunburn through glass?

Can you get a suntan or sunburn through glass? — SD, Farmington, Utah

Yes, but not as quickly as without the glass. While glass absorbs short wavelength ultraviolet light, it does pass 350 to 400 nanometer ultraviolet. While this longer wavelength ultraviolet is less harmful than the shorter wavelength variety, you can still tan or burn if you get enough exposure. Glass is like sunscreen—it protects you pretty well but it isn’t perfect.

Is glass in a gaseous state, a liquid state or a solid state? If I remember back…

Is glass in a gaseous state, a liquid state or a solid state? If I remember back to my freshman college year, it seems my prof said it was in a highly viscous state; therefore a liquid. — GC, Garland, Texas

The answer to that question is complicated—glass is neither a normal liquid nor a normal solid. While the atoms in glass are essentially fixed in place like those in a normal solid, they are arranged in the disorderly fashion of a liquid. For that reason, glass is often described as a frozen liquid—a liquid that has cooled and thickened to the point where it has become rigid. But calling glass a liquid, even a frozen one, implies that glass can flow. Liquids always respond to stresses by flowing. Since unheated glass can’t flow in response to stress, it isn’t a liquid at all. It’s really an amorphous or “glassy” solid—a solid that lacks crystalline order.

Sometimes on television a high pitched noise breaks the windows in a house. I kn…

Sometimes on television a high pitched noise breaks the windows in a house. I know that tubular objects such as wine glasses will break when the frequency corresponds to the natural frequencies of the glass, but does flat sheet glass such as windows experience this same effect? — RF, Jackson, Michigan

In real life, only explosive sounds will break normal glass. That’s because normal glass vibrates poorly and has no strong natural frequencies. You can see this by tapping a glass window or cup—all you hear is a dull “thunk” sound.

For an object to vibrate strongly in response to a tone, that object must exhibit a strong natural resonance and the tone’s pitch must be perfectly matched to the frequency of that resonance. A crystal wineglass vibrates well and emits a clear tone when you tap it. If you listen to the pitch of that tone and then sing it loudly, you can make the wineglass vibrate. A crystal windowpane would also have natural resonances and would vibrate in response to the right tones. But it would take very loud sound at exactly the right pitch to break this windowpane. A few extraordinary voices have been able to break crystal wineglasses unassisted (i.e., without amplification) and it would take such a voice to break the crystal windowpane.

I can understand that the strings of bubbles from the side of a glass of champag…

I can understand that the strings of bubbles from the side of a glass of champagne are due to nucleating dirt or other imperfections in the glass surface, but what causes those strings of bubbles in the center of the fluid? They are quite persistent. Are they just dust? — BM, Tehachapi, CA

If there were no impurities or imperfections in a glass of champagne, bubbles would only form through statistical fluctuations—random effects would occasionally bring enough gas molecules together to form (nucleate) a bubble and that bubble would grow and rise to the surface. But such spontaneously nucleated bubbles are extremely rare and form randomly throughout the fluid, rather than in chains of steady bubbles. In fact, bubbles would be so rare in this impurity-free liquid that you would probably not even notice them—the champagne would slowly go flat by losing gas molecules from its surface alone.

In real champagne, chains of bubbles do rise upward from the center of the fluid. These bubbles are clearly forming at suspended impurities. All it takes is a tiny piece of dust to trigger bubble formation. If you swirl the champagne slightly, you should be able to see these suspended chains of bubbles move, indicating that the impurities that are triggering them are also moving with the fluid.

What is the difference between crystal and glass?

What is the difference between crystal and glass?

The “crystal” that’s used in fine glassware is actually a glass, but it is chemically different from the glass that’s used in more common glassware. Both materials are formed by melting together a mixture of silicon dioxide (also called quartz or silica) and other chemicals and both are glasses, meaning that their atoms are arranged haphazardly and not in the crystalline lattices of such materials as salt or sugar. The chemicals that are added to silicon dioxide to make normal glassware—sodium oxide and calcium oxide—make the glass easier to melt and work with at the expense of strength and increased damping. That’s why normal glassware is relatively soft and emits a dull sound when you rap it; it experiences lots of internal friction. The chemicals added to silicon dioxide to make “crystal” glassware include lead oxide, which makes the glass easier to melt and soft enough to cut and shape easily. However, lead “crystal” glassware has less internal damping than ordinary glassware and emits a ringing tone when you rap it because it experiences very little internal friction.

How is glass made?

How is glass made?

Common window glass is made by melting a mixture of quartz sand (silicon dioxide), soda (sodium oxide), and lime (calcium oxide). The quartz is the network forming material that forms the basic structure of the glass. The soda makes it much easier to melt and work with—along with making the glass weaker and more temperature sensitive. The lime prevents the soda-rich glass from dissolving in water.