thermal shock
Pour boiling water into a cold, thick glass and it cracks with a sharp snap. That is thermal shock: a sudden change in temperature that shatters a brittle solid. It is the signature hazard of ceramics — a material that will happily sit in a furnace at 1500 degrees C can be destroyed by moving it too fast from hot to cold, or cold to hot. No blow, no load, no chemistry; just a temperature that changed too quickly for the material to cope.
The mechanism is temperature gradients turned into stress. When you quench a hot ceramic, the surface cools and wants to shrink first, but the still-hot interior holds it stretched, so the surface is thrown into tension. Ceramics are strong in compression but weak in tension, and they cannot yield or flow to relieve the load the way a metal does, so when the surface tensile stress reaches the material's strength, a crack starts and runs. How bad the shock is depends on the size of the temperature jump delta-T, how fast the heat is exchanged, and the material's own properties — captured by the thermal-shock parameters R and R'. Rapid cooling (a quench) is usually worse than rapid heating, because heating puts the surface in safer compression.
There are really two stories inside thermal shock. In a strong, flaw-free ceramic, no crack exists until the critical delta-T, and then it fails suddenly and catastrophically — this is crack initiation, and you fight it with high strength and low expansion. In a weaker, already-microcracked or porous material like a refractory brick, cracks are present from the start but a shock only nudges them a little; the material tolerates repeated shocks by sacrificing some strength — this is crack propagation, or damage tolerance. That is why a dense, strong alumina can be more shock-prone than a humble firebrick: strength and shock survival are not the same virtue. Thermal shock governs everything from why Pyrex survives the oven to why kiln furniture cracks and why re-entry heat shields must be designed with obsessive care.
A dense alumina crucible pulled straight from a 1200 degrees C furnace and dropped into cold water routinely cracks, because its high modulus and moderate expansion build large surface tension on quenching. A fused-silica or cordierite piece survives the same quench thanks to its tiny expansion coefficient.
Same quench, opposite outcomes: expansion coefficient and stiffness, not toughness alone, decide whether a ceramic survives thermal shock.
Strength and thermal-shock survival are not the same thing: a stronger, denser ceramic can be more shock-prone than a weak porous one, because a crack that initiates in a strong dense body runs catastrophically, while a porous body simply nibbles at its pre-existing cracks.