Why Do Tempered Glass Lids and Dishes Suddenly Shatter?
― Inside the glass, there is always a "core" being pulled
Tempered glass is several times stronger than ordinary glass. Yet one day, with nothing touching it, it can go "bang" and turn into tiny grains all at once. The force stored inside, which is the source of its strength, is also what makes the break so violent.
After dinner, you put a glass lid back on the kitchen shelf. Or it is the middle of the night in the bathroom, and no one has touched anything. Suddenly there is a loud bang, and the glass has turned into a sheet of cracks.
Looking at the floor, you don't see sharp shards. You see small, round-cornered grains, like rock sugar.
"I didn't hit it, so why?" "Why did all of it go at once, not just one part?" Both questions have the same single answer.
There are just two reasons it suddenly shatters
Tempered glass is made by cooling only the surface quickly while it is still hot. The surface sets while squeezed tightly, so ordinary scratches rarely break it.
To balance the force squeezing the surface, the core of the glass is constantly pulled. When a flaw or a foreign particle reaches this core, the stored force is released all at once.
In other words, tempered glass is a sheet standing quietly with force locked up inside it. Let's look at it step by step.
What is going on inside tempered glass?
Tempered glass is said to be made by heating glass to a little over 600°C, then blowing air on the surface to cool it quickly. The surface sets first. Later, the core cools slowly and shrinks. The shrinking core pulls the already hardened surface inward.
So the glass sets with its surface squeezed and its core pulled. Glass is very strong when pushed and weak when pulled. A break always starts where a tiny flaw on the surface is pulled open. If the surface is squeezed, that flaw has a hard time opening.
Look at Figure 1. On the left is a cross-section of 4 mm tempered glass, and on the right is how force is spread through it. The squeezed layers on the front and back are each only about one fifth of the thickness. As you deepen the surface flaw with the slider, the tip of the flaw passes a certain depth and enters the "pulled core."
Why does "all" of it break, not just "part"?
Ordinary glass stores no force, so when it breaks, a few cracks run and then stop. The core of tempered glass is pulled all the way across the pane. Once a crack starts running in the core, the pulling force pushes it wider wherever it goes. The crack branches and spreads to the edge of the pane almost instantly.
Look at Figure 2. Ordinary glass on the left breaks into large, sharp pieces, and tempered glass on the right breaks into small grains. It breaks into grains because the stored force is spent on many finely branching cracks. So that people are less likely to be cut deeply, standards require the glass to break into at least a certain number of grains within a set area.
Why does it break when nothing was done to it?
When glass breaks "without being hit," the cause is often thought to be an older flaw. A small chip from a metal spatula or the corner of a sink can reach close to the core. The tip of a flaw in glass slowly deepens because of moisture in the air. Days or months later, it can pass the limit and the glass breaks.
Another rare cause is known: tiny particles of "nickel sulfide" mixed into the glass. After the glass is made, these particles slowly change their crystal form over a long time and swell slightly. When one swells inside the core, a crack runs from it. For building windows, panes are sometimes heated at about 290°C for several hours at the factory, so that panes containing such particles break there rather than in a building.
Heat-resistant glass is made from ingredients that hardly expand or shrink when the temperature changes. Tempered glass keeps ordinary ingredients, and is made strong by locking force inside. The label on a lid or cup tells you which one it is. Tempered glass is also less likely to break from sudden temperature changes, but it has another side: it is weak against chips and deep flaws.
So what should we do?
- Don't use chipped piecesEven a small chip on the rim may reach close to the core. If you find one, replace it with a new one.
- Don't knock it against hard thingsTake care not to bump the rim against metal spatulas, the corner of the sink, or the stove's pot rests.
- If it breaks, step back first and protect your feetThe grains fly far. Put on shoes and gloves to collect them, and pick up the fine grains with a vacuum cleaner or sticky tape.
The grains of tempered glass have rounded corners, but some sharp ones are mixed in. Do not lean in to look at it. If you think something got in your eye, don't rub it. Rinse it with water and see an eye doctor right away. For serious injuries, such as bleeding that won't stop, call emergency services (119 in Japan) without hesitation.
Summary
Tempered glass is made strong by setting it with the surface squeezed and the core pulled. The protective layer is only about one fifth of the thickness. When a flaw or foreign particle reaches the core beyond it, the locked-up force is released all at once, and the whole pane breaks into grains.
Its strength is locked-in force.
So when it breaks, it all goes at once.
How temperature differences crack stones and ice is covered in Why ice in the freezer cracks and pops and Why stones from a fire pit split and fly. A natural trick that makes materials tough by forcing cracks to take a detour is covered in Why the inside of a seashell is hard to break.
- On a sunny day, put on polarized sunglasses (the kind that cut reflections) and look at a car's rear window.
- Slowly tilt your head or change your viewing angle, and faint polka-dot or striped patterns appear. These are marks left by the air blown on the glass as it cooled, and they show how force is spread inside the glass.
- Check the labels on glass lids and dishes at home to see whether they are "tempered glass" or "heat-resistant glass." Also check the rims for chips by eye, without running a finger along them.
* Whether you can see the pattern depends on the brightness of the sky and the direction of the light. Drivers should not look while driving.
For those who want more ― terms, formulas, and links to textbooksEach part is labeled with its level, from middle-school science to university specialist courses
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school physics
- High school+Advanced high-school material, or textbook column content
- UniversityUniversity specialist courses (mechanics of materials, fracture mechanics, glass engineering), not taught in high school
- ResearchTopics researchers are still investigating, not yet taught as settled fact even at university
Middle schoolTerms: this phenomenon has names
- Air-quench tempering (physical tempering): A method in which hot glass is cooled quickly with blown air, so that it sets with its surface squeezed.
- Compressive stress and tensile stress: Inside an object, the force that tries to squeeze it together, and the force that tries to pull it apart. Both are expressed as size per unit area.
- Spontaneous breakage: Tempered glass breaking with no noticeable outside force applied. Old flaws or nickel sulfide particles are thought to be the cause.
Middle schoolHigh schoolChecking with a formula: how many millimetres is the protective layer? How hard is the core pulled?
The basic approach is to treat the force inside tempered glass as spread in a parabola shape across the depth. From this shape, you get the depth of the protective layer and the size of the force pulling on the core.
| In symbols | σ(x) ＝ σs × ( 1 − 6 × x × ( 1 − x ) )、 x ＝ z ÷ t |
| In words | Push at depth z ＝ surface push × (1 − 6 × depth fraction × remaining fraction). From this: core pull ＝ surface push ÷ 2, protective layer ＝ thickness × about 0.21 |
| Where it comes from | In a sheet with no outside force on it, the pushes and pulls inside add up to zero (force balance). The simplest shape that meets this condition is a parabola that is symmetric between front and back |
| Symbol | Meaning and unit |
| σs | Size of the force squeezing the surface (MPa ＝ newtons per square millimetre) |
| t | Thickness of the glass (mm) |
| z | Depth from the surface (mm) |
| x | Depth fraction (z ÷ t, from 0 to 1) |
| Thickness of tempered glass such as a lid | 4 mm |
| Surface push (a value taken as typical for air-quench tempering) | about 100 MPa |
| Depth fraction where the force changes direction (solution of 1 − 6 × x × (1 − x) ＝ 0) | about 0.21 |
| Area of 1 square centimetre | 100 square millimetres |
| Depth of the protective layer | 0.21 × 4 ＝ 0.84 mm |
| Force pulling on the core | 100 ÷ 2 ＝ 50 MPa |
| Converted to per square centimetre | 50 × 100 ＝ 5000 N |
| Converted to weight (1 kilogram-force ＝ 9.8 N) | 5000 ÷ 9.8 ≒ 510 kg |
The protective layer is only 0.84 mm on each side out of 4 mm. The core beyond it is constantly pulled as hard as if it were hung with about 500 kilograms for every square centimetre. This number is why a chip less than 1 mm deep can lead to "all of it" breaking.
High schoolHigh school+Force balance, and the swelling of nickel sulfide
High schoolIn terms of "force balance" in physics, the pushes and pulls inside the sheet cancel each other out. That is why tempered glass looks like a quiet sheet with nothing going on. When a crack enters the core and breaks the balance, the stored elastic energy is used as work to widen the crack.
High school+Nickel sulfide is trapped in the glass by rapid cooling in its high-temperature crystal form, which is stable only when hot. After that, over months to years at room temperature, it is said to change into the form that is stable when cold (a phase transition). This change is said to swell its volume by a few percent, and a particle sitting in the core pushes the surrounding glass apart until it cracks.
UniversityResidual stress and fracture mechanics
The force locked inside glass is called "residual stress." The conditions under which a break starts are handled by "fracture mechanics," which uses Griffith's theory and the stress intensity factor to judge, from the depth of a flaw and the size of the force, whether it will break. The slow deepening of a flaw by moisture is called "stress corrosion cracking (delayed fracture)." The distribution of force inside can be measured without breaking the glass, using "photoelasticity," which uses polarized light.
📖 Derivations and further reading: Tempered glass (Wikipedia) ／ Fracture mechanics (Wikipedia) ／ Photoelasticity (Wikipedia)
ResearchWhat is still not well understood
- Can every dangerous particle be found before shipping? Nickel sulfide particles are very small, and finding them reliably with optical inspection is still being researched.
- Can the size of the grains be predicted accurately from the stored force? It is known that the larger the force, the finer the grains tend to be, but the fine details of how cracks branch are still being studied.
- The details of how moisture deepens flaws. How water molecules break the bonds in glass is being studied, including differences by temperature and composition.
In other words, this article too is "an explanation of what is understood so far." The parabolic distribution also varies in shape and size in real products.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: "Action of forces," "Pressure" | Thinking of pushes and pulls per unit area |
| High school | Physics: "Force balance," "Elastic force" | Pushes and pulls add up to zero |
| High school+ | Chemistry: "Crystal structure" | Phase transition and swelling of nickel sulfide |
| University | Mechanics of materials, fracture mechanics, glass engineering | Residual stress, stress intensity factor, delayed fracture |
| Research | Material fracture and its prediction | Detecting foreign particles, predicting grain size |
| ― | Links to daily life | Not using chipped lids and dishes, how to clean up after a break |
- Wikipedia, "Tempered glass" (強化ガラス)
- Japanese Industrial Standard JIS R 3206, "Tempered glass" (強化ガラス)
- European Standard EN 14179, "Glass in building – Heat soaked thermally toughened soda lime silicate safety glass"
- Sakka, Sumio (作花済夫), Fundamentals and Applications of Glass Science (『ガラス科学の基礎と応用』), Uchida Rokakuho (内田老鶴圃)
- Varshneya, A. K., Fundamentals of Inorganic Glasses, Academic Press
* This article is a science explainer for general readers. The numbers given are rough guides for understanding how things work. Follow each manufacturer's instructions when handling products, and follow the directions of medical institutions, fire services and similar bodies if you are injured.