For the source text click/tap here: Yoma 34
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The Mishna (31b) describes the tevilah (ritual bath immersion) that is done by the kohen gadol on Yom Kippur, and closes with the comment that if the kohen gadol was elderly or particularly sensitive, they would add heated water to the water in the mikveh so that he could immerse himself more comfortably.
It was taught in a baraita that Rabbi Yehuda said: They would heat blocks of iron on Yom Kippur eve and cast them into the cold water of the ritual bath to temper its chill.
Quenching and tempering are processes that strengthen and harden materials like steel and other iron-based alloys. The process of quenching or quench hardening involves heating the material and then rapidly cooling it to set the components into place as quickly as possible. The process is tightly controlled, with the heating temperature, cooling method, cooling substance and cooling speed all dependent upon the type of material being quenched and the desired hardness.
We explore the science of quenching but also the metaphor as applied to Isa 1:25 in our YK confession.