You heat the blade to cherry red and push it into oil, and in under a second it becomes a different material. It's the closest thing smithing has to magic, and the first time you do it, you'll be sure you ruined it.

The quench

Tip first, no swirl. Swirl and the blade warps — the two sides cool at different rates. A clean, straight plunge, held until the oil stops bubbling.

The temper

Then the oven at 200°C for two hours. It trades a little hardness for a lot of toughness, and it's the difference between an edge and a shard. Skip it once and you'll never skip it again.