Standard float glass is intrinsically fragile and dangerous: when it breaks, it generates long sharp ‘bayonets’. Thermal tempering (ESG) is the engineering process that transforms this treacherous pane into an elastic structural element, capable of bearing high loads and shattering in complete safety.
The glass pane enters a furnace heated to near its softening point (about 680-700°C). It is then immediately hit by violent cold air jets. The outer surfaces cool and solidify while the core tries to contract but is held by the already-rigid skins. This generates a state of permanent pre-stress: surfaces remain in strong compression while the core is in permanent tension. To break tempered glass, a bending force must first overcome the entire surface pre-compression, explaining its extraordinary structural toughness.
The immense internal energy dictates the breakage pattern: a point impact penetrating the compression layer releases energy in a fraction of a second, shattering the entire pane into thousands of blunt, harmless fragments (cubes). This classifies it as ‘Safety Glass’. The invisible enemy is Nickel Sulfide (NiS): micro-inclusions that can expand over time causing spontaneous glass explosion. For facades, the HST (Heat Soak Test) is mandatory, a destructive furnace treatment that preemptively explodes defective panes at the factory.
Standards
European and international references applicable.
Physical properties
Usage environment
Golden rule: no material removal is permitted after tempering. A simple attempt to widen a 1 mm hole with a drill will shatter the entire pane. Drilling and milling are mandatory before the furnace. For facades, the HST (Heat Soak Test, EN 14179) is mandatory to eliminate panes with Nickel Sulfide (NiS) inclusions.