Glass can shatter under sudden pressure due to thermal stresses created by an impact or a rapid temperature change. This rapid increase or decrease in temperature creates internal stresses that can cause the glass to break.
Glass is essentially made of molten sand, called silica, combined with other substances like sodium carbonate (soda) or calcium carbonate to facilitate its production. Unlike metals or crystals, the molecules in glass are not arranged in a regular pattern but are disordered, somewhat like a frozen liquid. This disordered structure, called amorphous structure, explains why glass is hard yet also fragile. There are no well-aligned layers that could slide over each other, so under shock or sudden pressure, it breaks instead of deforming: this makes it very susceptible to the rapid propagation of cracks.
When sudden pressure is applied, glass does not handle it well because it has an internal structure called amorphous, meaning its atoms are not neatly arranged like in metals, but are instead loosely placed without a specific order. As a result, it lacks elasticity: instead of absorbing energy by deforming slightly, it shatters instantly under high and brutal stress.
When the tension in the material increases very rapidly, the mechanical stresses exceed the maximum resistance that the glass can endure, leading to the ultra-fast propagation of a crack. The mechanical shock creates a tension wave that travels through its fragile structure: if the wave encounters a defect or hidden weakness, that's when it breaks immediately.
In fact, glass handles slow and gradual pressures quite well, but when faced with a sudden and intense shock, it does not have time to dissipate that energy. Consequently, the bonds between atoms give way all at once. This phenomenon makes glass particularly vulnerable to any sudden impact.
Glass, even though it seems perfectly smooth, is full of tiny microscopic defects like cracks or bubbles that are invisible to the naked eye. When a sudden pressure is applied, these small defects can further concentrate stresses at certain specific points, increasing the risk of it breaking suddenly. This is compounded by mechanical fatigue, meaning that each successive pressure or shock weakens the internal structure of the glass a little more. Eventually, what seemed to be holding steady ends up giving way suddenly under what is otherwise a trivial pressure. It's exactly like bending a paperclip several times: the last twist, even if slight, ultimately causes it to snap.
Some glasses break super quickly, while others much less so: it all depends on their composition and the treatments they have received. Tempered glass, for example, undergoes a special thermal treatment to become highly resistant to shocks and sudden pressure. As a result, it is up to five times stronger than regular ordinary glass, but when it breaks, it shatters into small pieces rather than slowly cracking. On the other hand, laminated glass has a layer of plastic bonded between sheets of glass to prevent it from shattering violently in case of sudden breakage. Then there is borosilicate glass, which is resistant to thermal shock thanks to the added boron: ideal for cooking, as it withstands stressful situations like quick transitions from the freezer to the oven. In contrast, fine decorative glasses like crystal are often very sensitive and break at the slightest sudden impact due to their fragile structure.
To limit the risks of glass breaking under pressure, tempered glass is often used. This technique involves heating the glass and then cooling it rapidly, creating surface tension that makes it more resistant to impacts and pressure. Another effective solution is laminated glass: a layer of plastic sandwiched between two layers of glass. Even in the event of breakage, the pieces of glass remain attached to the plastic, thereby limiting injuries. The quality during manufacturing can also be improved by avoiding air bubbles or impurities that weaken the material. Finally, avoiding sudden temperature changes and reducing microscopic defects through rigorous controls often suffices to significantly decrease the risks of breakage.
Scratches and microscopic defects on the surface of glass can reduce its strength by up to 10 times or more. That’s why manufacturers often use special coatings to limit these cracks that are invisible to the naked eye.
A phenomenon called 'spontaneous breakage' can affect certain tempered glasses: even without direct impact, they can shatter due to internal stresses that have accumulated, sometimes long after their manufacture.
The famous case of 'Prince Rupert's Drop,' formed by dropping molten glass into cold water, features an almost unbreakable head and an extremely sensitive tail, perfectly illustrating the astonishing contrasts in the strength of glass depending on its internal structure.
Boron oxide or silicon dioxide enriched with other elements is often used to produce extremely resistant special glasses, suitable for extreme thermal or mechanical environments, such as scientific laboratories or space shuttles.
Some types of glass, such as tempered glass or Gorilla Glass, undergo thermal and chemical treatments that enhance their resistance to cracks and impacts. These processes create a controlled internal stress, allowing for better shock absorption.
The weakening of glass is not always visible to the naked eye. However, the presence of chips, superficial cracks, or deep scratches indicates an increased risk of breakage. Similarly, an unusual sound upon contact or when the glass is subjected to slight pressure can be a concerning sign.
Not necessarily. The strength of glass depends not only on its thickness but also on its chemical composition, internal structure, and the treatment process it has undergone (such as thermal or chemical tempering, for example). Thus, a thin chemically treated glass can be more robust than a thick untreated glass.
Sudden changes in temperature can cause internal tensions (also known as thermal shocks), significantly increasing the likelihood of glass breakage. Indeed, one part of the glass may attempt to expand while another part remains contracted, thereby creating stress that can lead to sudden breakage.
It is very difficult and generally not recommended to repair a cracked glass, as microcracks often continue to expand under pressure. Replacement is often the safest and most effective long-term solution.
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