Heart Spark Forge

Using Clay to influence the hardness of steel was an ancient Japanese innovation. Using steel (or ceramic, bronze or even bamboo--yeah, it can be done) to cut is always a battle of 2 qualities: Hardness & Toughness. In general they are inverse extreme conditions of the same material.

A knife that is too hard will break  or chip easily, a knife that is tempered too extremely for toughness will be too soft to cut without resharpening beyond convenience.

Lacking precision kilns and modern quench oils, ancient Japanese bladesmiths used different thicknesses of clay to speed hardening of the edge and slow hardening at the spine of the blade. This allowed the spine to retain toughness while the edge retained hardness. It also resulted in what many call a temper line or a hamon between harder and softer parts of the blade. Many techniques can be used to bring out a more esthetic hamon.

While there is some overlap between the hamon and the etch of a differentially hardened blade, the hamon is far more finicky and less predictable than the torch method of edge hardening. On the other hand, a hamon is quite beautiful.

Pro & Con

Hamons are lovely to look at and correlate nicely with hardness in most blades.

Unfortunately, developing a hamon takes a great deal of time and a significant percentage of blades break in the process. In my first run of blades with a hamon, I started out with 21 blades and had only 3 that I consider successes. (Some didn't harden well, some didn't take the pattern that they should have, and the majority just broke in the quench. One knife reached final polish and etching before I found more than 40 3 mm long cracks running down the length of the blade). I have been told that master smiths lose 1 in 4 blades, but I am not sure how to verify this.

The other problem with hamons is that it is impossible to create a hamon with a rust resistant steel. Delaying cleaning after cutting fruit or letting it stay wet will rust the blade rapidly. Few of us that are not professional chefs are used to wiping off and drying the knives immediately.

Immediate cleaning and drying  followed by rubbing in a little mineral oil, coconut oil or butcher's block oil will protect that patern for quite a long time.