The process of heating a solid metal or glass to a specific high temperature and gradual cooling is called annealing. The physical property (strength, elasticity and crystalline property) of a solid can be altered by this method.
Its three main stages are – heating, holding, and cooling.
Heating
The material is heated to a specific high temperature (the "annealing temperature"), which depends on the material:
For steel/carbon steels: typically, 700–900°C (above the critical transformation temperature)
For copper: ~300–600°C
For glass: ~400–600°C (softening point)
At this temperature, the atomic structure becomes more mobile, allowing dislocations and defects in the crystal lattice to rearrange or dissolve.
Soaking (Holding)
The material is held at that temperature for a certain time (minutes to hours) so the microstructural changes can fully occur. This relieves internal stresses, recrystallizes deformed grains, and homogenizes the structure.
Cooling
The key difference from other heat treatments (like hardening), cooling is done very slowly (often in the furnace itself, sometimes just a few degrees per hour). Slow cooling prevents new stresses or phase transformations that would make the material hard/brittle again.
Some real-world examples are –
A blacksmith heats a sword red-hot and lets it cool slowly in ashes so that it becomes less brittle.
Copper wire drawn thin becomes hard and brittle and by annealing it, it stays soft and flexible for electrical use.
A hot blown glass vase when annealed in an oven overnight doesn’t crack when it cools.