Burning glass effect, shards and spontaneous combustion: Myths about the causes of fires!

I recently read a police press report in which it was noted that fields were probably catching fire due to the high outside temperatures and strong sunlight. Who can object when a police report publishes this assumption ( compare ). Everything is plausible, one would think. It's very hot outside, it hasn't rained for a long time, and a field can spontaneously catch fire. Here in Central Europe. Or not?

Another frequently mentioned cause of fire is broken glass in the wild. This is called the burning glass effect. Have you ever entered the word burning glass effect into a search engine ? The results are interesting because the term is often used in the context of fire prevention or as the cause of fire in press reports. But in reality, this effect is likely to be at most a marginal phenomenon in terms of the causes of fires.

First of all: of course the burning glass effect exists. And it is called that because this effect can cause substances or objects to catch fire. The burning glass effect is also explained quite simply. What is needed are optically curved lenses, such as magnifying glasses or other transparent bodies that can refract the light and bundle it into a focus. In this focus, also known as focal point in German, heat is created, which in turn can set other substances on fire. This is what it looks like in theory and in practice. And precisely because this effect exists and works, let's take a closer look at it.

First, an excerpt from the media reporting on the burning glass effect as the cause of the fire:

Fire chief warns: “Every piece of glass can start a forest fire”

Haltern newspaper

The fire department and police were unable to provide any information about the cause of the fire. “The forest is dry as bread, so a spark or a shard of glass is enough,” says Herrmann.

Frankfurter Rundschau

“Even a small thing can cause a forest fire, like the hot catalytic converter of a car parked on the edge of the forest over dry turf or a piece of glass that can act like a magnifying glass under the scorching sun,” emphasized the minister.

Ministry of Food and Rural Affairs

Shards as a possible cause of fire. The railway sleepers at the port of Basel could not have caught fire without an ignition source, says an expert.

BaZ

Experts are always quoted without question. But can it all be like that? Are spontaneous combustion caused by solar radiation or broken glass really being such a common cause of fire as we are told in media reports? Just because (sometimes official) bodies keep repeating it, does it mean it's true? Let's take a closer look at this focal point!

Burning glass effect and other myths!

If we look at the topic a little more closely, we will find a number of skeptical contents that deny a magnifying glass effect on shards. A simple application of search engines is enough. The magnifying glass effect, even if it works, is unlikely to occur outside of a laboratory situation or a deliberately created situation.

Spektrum.de addressed the topic in 2019 Under the title “Can glass shards cause forest fires?” we read in the article that, contrary to popular belief, it is very unlikely that “glass bottles or shards cause forest fires. A whole series of coincidences would have to come together for an ignition scenario.”

And that is completely correct! There are too many circumstances that simply do not allow a substance to ignite through a transparent body that is curved in any way. This starts with the distance to the flammable object, curvature of the material, thickness of the material, angle of solar radiation, continuity of the sun and ends with the temperature that is created in the first place.

The fact-checkers from Factfuchs (Bayerischer Rundfunk) are even clearer in a recent article with the headline “No, a piece of glass probably can’t cause a forest fire.” In this article the forestry scientist Alexander Held is quoted:

“It's almost impossible to start a fire with a piece of glass. You would have to have a magnifying glass or some kind of magnifying glass and then of course you can start a fire. But with a piece of glass that’s de facto impossible.”

Fact Fox

The likelihood that broken glass can start a fire via the burning glass effect is actually very low. This emerges not only from some fact checks, but also from a clear experiment from 2006. This experiment by the German Weather Service (DWD) asked the question of whether objects left in the forest or in fields could start a fire in the sense of the magnifying glass effect.

The experiments on the burning glass effect

The tests were carried out between the end of April and the end of July 2006 on a grassy area of ​​the DWD test site in Braunschweig ( read here ). 5 different pieces of glass were used. Originally there were 30 pieces of glass to choose from, but only five of them even made it to the shortlist. The reason: many pieces of glass do not even manage to be relevant to the burning glass effect in any way. For example, cloudy glasses. Or shards that have no curvature at all.

With this experimental setup, we also have to take into account that the glasses used were specifically arranged so that they can concentrate light in some way. This means that they were not lying on the ground at all, but were aligned in an ideal position above the fuel.

The result is sobering, because only a single shard managed to generate temperatures of over 200 degrees. And, mind you, under optimal conditions that were artificially created! However, this has not yet caused a fire, which can also be read in the test description.

Of the 120 tests in total, not a single ignition of the scattering materials could be detected. And that despite the optimal structure! In the experiment, flame formation could only be created using magnifying glasses, which were also optimally aligned. However, there was no fire.

Burning glass effect or spontaneous combustion: The ignition temperature is not reached!

All of this is logical and chemically understandable, as the ignition temperature was simply not sufficient due to the concentration of light in the shards for the fuel (in the test: spruce needle litter). And that is also an important point when it comes to “spontaneous combustion” due to dryness and strong sunlight!

To cause a fire, very high ignition temperatures are required. Wood in general is at 280 – 340 °C, the famous “straw fire” also requires 250 – 300 °C. Newsprint at 175 °C, on the other hand, is a true “cold burner” (information comes from the Chemie.de ).

So what do we do with press reports and reports of spontaneous combustion or broken glass as the culprit? Clearly, we have to be skeptical! All of this is potentially possible, but based on the current state of knowledge (since 2006), the burning glass effect is seen as very unlikely as a cause of fire under local climatic conditions. Spontaneous combustion due to pure solar radiation is even less likely, as the sun's light does not appear bundled without lenses. And we are still a long way from surface temperatures of almost 300 °C in the blazing sun.

The likely culprits

So if neither broken glass nor sunlight is responsible for fires, what then? The solution is quite simple: people and their behavior. It is more or less intentional arson or arson caused by laziness or stupidity. So human error coupled with arson.

Anyone who carelessly throws glowing cigarettes to the ground in fire-prone areas in dry and hot weather not only has not understood the issue of environmental pollution, but is also a potential arsonist. A cigarette develops temperatures of up to 900 °C in its ember zone; tobacco embers can also reach 500 °C at the end.

Certainly not every cigarette that is carelessly disposed of will start a fire, because with the amount of butts lying around everywhere, every last tree would have to have been burnt. The ash coat that forms does not transmit the heat from the embers to its full intensity. This is one reason why not every cigarette residue starts a fire, but careless throwing away should in no way be glossed over.

Another source of fire in summer: barbecuing outdoors, somewhere in nature? Hmmm …. A cold beer with that? Sounds great, right? But what about the embers? The combustion temperature of charcoal is 800 °C, which can easily start a fire.

All of this is logical, because we can remember: Whenever a fuel is exposed to temperatures that are higher than its (=fuel) ignition temperature, a flame is formed. For dry grass or straw, but also wood, we can remember around 300 °C. If these temperatures do not arise, no flames can form and no combustion can occur. Then another reason will probably be the cause of a fire.

Addendum: Myth Watering flowers and burnt skin from drops of water!

Finally, a look at children in water and flowers in relation to watering times, because these are inferior to the magnifying glass effect myth. First the flowers! There is an old rule that we shouldn't water flowers during the day. This would result in the flowers burning.

I'm noting the flowers at this point because the burning glass effect is also said to be the cause here. At this point, the optical lens is understood to be the drop of water that would concentrate the light and thus supposedly leave burn marks on the sheet. As we have now learned, it doesn't work that way. The drops also evaporate quite quickly in the blazing sun, so that spontaneous combustion or the formation of burnt spots caused by drops is not possible.

What's even more absurd (oh yes, there's a claim!) is that wet skin generally leads to more sunburn. And not because sunscreen would be washed off, but because the magnifying glass effect is said to be responsible.

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The poor children! Then they are exposed to such nonsensical statements...

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