The fundamental cause of the spontaneous explosion of tempered glass lies in the impurities and defects existing inside it, which break under the long-term effect of stress. It is generally believed internationally that the self-explosion rate of tempered glass is around 0.1% to 0.3%, but this is only a statistical average. For a specific piece of glass, its risk is determined by the following factors:
Internal decisive factor (the glass itself)
This is the main cause of self-explosion, accounting for more than 98%.
Nickel sulfide (NiS) impurities
This is the most famous and primary reason. During the production process of glass sheets, extremely trace amounts of nickel and sulfur may be mixed in, forming nickel sulfide crystals in a high-temperature molten state.
The principle of "phase transition expansion" : Nickel sulfide crystals exist in two forms: the α phase at high temperatures and the β phase at low temperatures. The quenching process of tempered glass is extremely fast, and the α -phase NiS is "frozen" inside the glass. At room temperature, the α phase slowly transforms into the β phase, while its volume expands by approximately 2-4%.
Stress concentration: If this impurity happens to be located in the tensile stress zone of the glass plate (the central layer of tempered glass), the huge pressure generated by its volume expansion will disrupt the stress balance of the surrounding glass, causing the entire piece of glass to shatter instantly. The typical "butterfly spot" pattern after cracking, with nickel sulfide impurities at the center.
Other impurities and bubbles
Apart from nickel sulfide, other hard impurities in the glass raw materials, such as alumina and quartz particles, may also cause stress concentration due to their different coefficients of thermal expansion from the glass matrix.
Bubbles or stones (unmelted raw materials) in the glass can also become weak points during the tempering process, increasing the risk of self-explosion.
The tempering stress level of glass
The higher the degree of tempering, the greater the strength of the glass, but the higher the stress energy stored inside it.
If the stress is too large and uneven, it will be in an unstable state by itself. Once a minor defect is triggered, the energy will be released rapidly, leading to a self-explosion.
External Inducing Factors (Processing, Installation and Use)
Although these factors do not directly cause self-explosion, they can significantly increase the risk of self-explosion or become the "last straw that breaks the camel's back".
Processing and design factors
Edge and corner damage: During the processes of cutting, transportation and installation, if the edges and corners of the glass show chipping or cracking, these defects will become stress concentration points, greatly reducing the overall strength of the glass and inducing self-explosion.
Drilling and slotting: Any cutting or drilling of glass will alter its stress distribution, creating high-stress areas at the edges of the holes and grooves. If not handled properly or designed unreasonably, the risk of self-explosion will increase.
Size and thickness: Generally, the larger and thicker the glass, the more complex the internal stress distribution, the higher the energy it stores, and the theoretically higher the risk of self-explosion.
Installation factors
Installation stress: If the installation frame is uneven, twisted, or overly hard spacers are used, the glass will already be subjected to additional bending stress or point contact stress during installation. This kind of "forced installation" will keep the glass under high load for a long time.
Insufficient gap: There is not enough expansion gap left between the glass and the frame. When the ambient temperature changes and causes the glass to expand and contract due to heat and cold, it will be squeezed by the frame, generating additional stress.
Use environmental factors
Thermal stress: Different parts of the glass are heated unevenly (for example, one part is directly exposed to sunlight while the other is shaded), the heated part expands, and the cooled part restricts its expansion, thereby generating thermal stress within the glass. This kind of stress, when superimposed with tempering stress, may induce self-explosion.
Wind pressure and vibration: Long-term exposure to strong wind pressure or continuous vibration from nearby traffic and machinery can cause fatigue in glass, reducing its strength.
Sudden temperature changes: For instance, in the cold winter, a piece of glass that has been heated by indoor heating suddenly encounters icy rain. The intense thermal shock may also cause it to break.
How to reduce the risk of self-explosion?
Having understood the reasons, corresponding preventive measures can be taken
Select high-quality raw glass sheets: Use raw glass sheets from well-known brands with good reputation. Their production processes are strictly controlled, and the impurity content is lower.
Conduct homogenization treatment (Heat Soak Test, HST) :
This is the most effective way to prevent the self-explosion of nickel sulfide. Put the tempered glass into the homogenizing furnace, heat it to 290℃±10℃, and maintain it for a certain period of time.
This process will simulate the natural environment for several years or even decades, causing the glass containing unstable NiS impurities to shatter prematurely under controlled conditions. After homogenization treatment, the self-explosion rate can be reduced to 0.01% to 0.03%.
For high-rise building curtain walls, skylights, shower rooms and other places with safety requirements, it is strongly recommended to use homogeneous tempered glass.
Standardized processing and installation: Ensure the quality of glass cutting, edge grinding and drilling, and avoid damage to corners and edges. When installing, use compliant frames and spacers to ensure uniform force distribution and leave sufficient expansion joints.
Using ultra-clear glass: Ultra-clear glass has a low iron content, purer raw materials, and very few internal impurities (including nickel). Its self-explosion rate is much lower than that of ordinary float glass.
Summary
Self-explosion of tempered glass is a low-probability but indeed existing phenomenon. The core of it is the result of the combined effect of internal impurities (especially nickel sulfide) and external high stress. By selecting high-quality raw materials, conducting homogenization treatment, and carrying out standardized processing and installation, the risk can be controlled at a very low level.
