Except glass and metal are completely different materials, behaving differently to applied physical forces. For one thing glass is an amorphous solid while metals have a crystalline structure. The tempering process for glass is more like that of quenching for metals. The glass surface is cooled by forced air faster than the core, which results in the different forces in the glass that toughen it. There is also a chemical tempering that replaces some of the sodium ions for potassium. Tempered glass - Wikipedia The tempering of rebar looks similar to glass tempering, but what is happening inside the metal is different. Tempering (metallurgy) - Wikipedia