Here’s the knife with a furnace cement coating, before getting it red hot and quenching in warm oil to harden the steel. The cement acts as insulation from the freezing action of the oil, and will result in a blade that has very hard edges, but a soft center. This makes for a strong blade, and with a little etching and polishing, will make a two-toned blade that is attractive. The Japanese call this two toned look a “hamon.”
After hardening, the blade will undergo two temperings at 425 degrees (F) for several hours. This will draw back some of that hardness (and brittleness) into a hard, but durable blade.
Here are both sides of the blade after hardening, tempering, polishing and etching with dilute ferric chloride to bring out the subtle differences between the harder edges and the softer inner areas (those that were under the clay to slow the cooling in the oil quench). Did I mention there were many rounds of polish, etch, polish, etch, and so on?
