The quench
Harden the edge, temper the spring, don't crack it.
The leap
You heat the blade to cherry red and push it into oil. The steel goes from soft to hard in under a second, and a thousand-year-old art decides if your edge holds or shatters.
Quench, then temper
quench oil for 10-series, water for wrought iron speed tip first, no swirl — swirl warps temper 200°C for an edge, 300°C for a spring test a file should skate, not bite
Why tempering matters
A fully hardened blade is glass — hard but brittle. Tempering trades a little hardness for a lot of toughness. Two hours at 200°C and your edge will flex instead of chip.