"We should proceed to the quenching step now."
Old George motioned for Lin Yu'an to follow him to a large iron barrel filled with black liquid beside the forge.
"This is oil," Old George explained. "For steel like 5160, oil quenching is suitable. The cooling speed of oil is slower and gentler than that of water, making it less likely to crack the blade."
He lit another small gas furnace specifically used for heat treatment, the flame making a "whooshing" sound.
"Quenching is the art of timing." He clamped the knife blank and slowly fed it into the flame. "You need to heat the blade precisely to the critical temperature of 'austenitization'. For 5160 steel, this temperature is about 845 degrees Celsius."
"How do you know when it's ready?" Lin Yu'an asked.
"With your eyes, and also with a magnet." Old George picked up a natural magnet hanging on an iron wire. "When the steel is heated to the critical temperature, it temporarily loses its magnetism. See?"
