Can broken glass really cause forest burns?

Can broken glass cause forest fires?

It is not only forest owners' associations that warn against it again and again, the police and fire brigade also list them as the cause of the fire: the broken glass in the forest. As is well known, it focuses the sunlight so strongly that when the bundles of rays hit the dry forest floor in summer, a flame is created which then sets the entire forest on fire in no time at all - a common disaster scenario. What's up

In fact, glass can focus sunlight in such a way that it can be used to ignite a fire. At least the scouts among us know that, but also all those who have dealt with the burning glass effect in physics class. But those who have paid really good attention also know that not all glass is created equal and that certain requirements must be met in order for the spark to jump over. In school, the magnifying glass effect is usually demonstrated with a magnifying glass, also known as a magnifying glass. The name burning glass can be traced back to the fact that the convex cut glass lens condenses the energy of the incident sunlight so strongly that combustible material can exceed the ignition temperature when the focused beam hits it. The targeted material catches fire. So, in principle, even carelessly discarded glass could cause an inferno. Or not?

This article is contained in Spectrum Compact, Fire - When Forests Burn

In the rarest cases, glass waste is convex cut burning glasses. As a rule, these are glass bottles or fragments of the same that have a uniform thickness. Can they, too, create a burning glass effect and start a fire under "ideal" conditions, in other words in combination with very dry and easily combustible plant material, as can be found on the forest floor in the summer months? The ignition temperature of these materials is around 300 degrees Celsius. This means that the beam of light bundled through the shard of the bottle has to reach this temperature so that the glass becomes a burning glass.

Field experiment under ideal conditions

In the summer of 2006, the German Meteorological Service in Braunschweig conducted an outdoor experiment to find out what ideal conditions would have to meet so that the forest floor could be ignited by a piece of broken glass. The bottoms of five different bottles made of colorless clear glass were used as broken glass. Stained glass was excluded from the outset due to its lower light transmission. The choice fell on intact bottle bottoms, as their shape comes closest to the convex shape of a burning glass. The researchers attached the bottle bottoms to brackets at an ideal angle to the sun at a height of 20 to 30 centimeters. In order to increase the chances of success, they had previously calculated the optimal incidence of light and the optimal focal length for each lens. The bundled rays of light hit various fuels with a moisture content of around ten percent, namely spruce needles, pine needles, beech leaves, black grass, grass and heather. A thermal imaging camera documented the experiment for ten minutes at intervals of 15 seconds. The result was clear: none of the variants made of glass and plant material caused inflammation. The required ignition temperature was far from being reached. Only the combination of the bottom shard of a ketchup bottle with spruce needles as fuel reached 300 degrees Celsius for a brief moment, but only to two millimeters and without sufficient potential for a flame.

Contrary to popular belief, it is very unlikely that glass bottles or shards will cause forest fires. A whole series of coincidences would have to come together for an ignition scenario. The mere fact that the broken glass should not be on the ground for this purpose, but should be at a distance of 20 to 30 centimeters above the forest floor, makes the probability close to zero. The broken glass is not even found in the German forest fire statistics as the cause of the fire. Although the statistics say that around a quarter of the fires can be traced back to negligent behavior by people, the unsightly disposed of drinks bottles do not pose any danger - unlike the discarded cigarette , the grill fire at the edge of the forest or even the overheated substructure of a parked vehicle.

Contrary to popular belief, it is extremely unlikely that glass bottles or shards will cause forest fires

The burning glass effect is said to have shaped the course of history early on: the Greek mathematician and inventor Archimedes is said to have worked during the Second Punic War in the years 214 to 212 BC. The ships of the Roman fleet that attacked the Sicilian port city of Syracuse were kept in check with the most varied of military equipment he had devised. Among other things, he is said to have developed burning mirrors that acted like ray cannons and set the sails of the enemy ships on fire from the shore. This story can probably also be classified in the area of ​​myths and legends, as the mirrors would have to have been immense in order to ignite the sails under the given light conditions and distances.