Tempering looks simple from outside — heat, then cold air — but the physics inside the chamber decides strength, flatness and optical quality. Here is the process in plain terms.
The heating half: even or nothing
Glass enters the furnace at room temperature and must reach roughly 620 °C — above its softening point — with every zone of the sheet arriving together. A sheet that is hotter on one edge leaves the furnace already bowed, and no quench can straighten it. This is why modern furnaces profile heater zones individually and why coated glass gets convection assist: the coating reflects radiant heat, so moving air must deliver what radiation cannot.
The oscillation dance
Rollers oscillate the glass back and forth through the chamber because a stationary sheet would sag between rollers and print their spacing into the surface as optical distortion. The oscillation stroke and speed are recipe parameters, tuned per thickness — the difference between glass you can read a reflection in and glass you cannot.
The quench: strength is a temperature difference
At the quench, air jets chill both surfaces rapidly while the core is still hot. The surfaces solidify first and compress as the core cools and contracts after them — locking the surfaces in compression and the core in tension. That stored stress is the strength: surface compression must be overcome before a crack can start.
Why the fragment test matters
The safety promise of tempered glass is how it fails: a dense mosaic of small, blunt fragments. The routine fragment-count test — break a sample, count particles in a defined square — verifies the stress level directly. A furnace that logs its recipes and prompts scheduled fragment tests turns compliance from a scramble into a routine.
