Age hardening is a process in which heating is done:

Age hardening—Process of hardening certain alloys by controlled heating and cooling, which usually is associated with a phase change. Heat Treatment of Gold-Copper Alloys To soften the alloy, the casting is placed in a furnace for 10 minutes at 700°C and then quenched in water. All intermediate phases in the alloy are changed to a disordered solid solution  at  700°C,  and  the  rapid  quenching  prevents  ordering from occurring during cooling. The tensile strength, proportional limit, and hardness are reduced by such a treatment, and the ductility is increased. To harden the alloy, the temperature of the furnace is set between 200°C and 450°C and the casting is heated for 15 to 30 minutes before it is quenched in water. Ideally, before the alloy is age-hardened, it should be subjected to a softening heat treatment to relieve all residual strain hardening before the alloy is hardened again by heat treatment to produce a disordered solid solution.  Otherwise  the  amount  of  solid-state  transformation  will not be properly controlled. In metallurgical terminology, the softening heat treatment is referred to as a solution heat treatment  and  the  hardening  heat  treatment  is  termed  age hardening.  Age hardening reduces the ductility of gold alloys. Phillips 12 ed page 367, 374