The best way I know to explain what a hamon is the blade has been differentially heat treated. I am no master of metallurgy that can explain the real science behind this process. I just know it makes one beautifully strong knife.
Here’s an excerpt from a great article from Blade Magazine about the Hamon.
The Hamon: What, Where, Why and How
By: Steve Shackleford | October 3, 2012
To those who appreciate the tempered steel of a Japanese sword, the hamon is visual evidence of the maker’s effort to produce the finest blade work. In the West, the hamon appears in much the same fashion on Western knives, a blending of culture and craft. Where East meets West on the steel blade, there is the hamon, the graceful temper line.
“From a practical point of view, the hamon is a visible indication of a differentially heat-treated blade,” explained knifemaker Stuart Branson. “In many cases, it’s advantageous to have a harder edge supported by a softer spine in a knife. In this way, you gain the benefits of the good edge retention afforded by the harder edge with the durability of the softer spine region of the blade. In the long and graceful form of the Japanese sword this might seem obvious, but the same is true for Western blades, particularly those hard-use knives or the very popular larger chopping knives.”
According to veteran maker Gary House, the aesthetic effect of the hamon is behind its surge in demand among Western collectors, clearly defining the transition zone between hard edge and soft back.
“The popularity of the hamon on non-Japanese blades today, I believe, is the visual effect of the temper line,” he commented. “The movement and variations of the hamon are very attractive and visually appealing compared to a straight temper line.”
More and more, discriminating knife customers are looking for the hamon on the blades of some of the best-known, iconic Western-style knives.
“‘You will put a little something in the blade, won’t you?’ is a common request these days,” commented Mike Craddock, who started making knives in the 1970s, took a 40-year hiatus, and has come back strong recently. “It doesn’t necessarily make the knife better, but it does look good. I consider it the spirit of the steel.”
Certainly, the hammered and heat-treated steel is expected to be sturdy. However, the quest for the hamon is a classic case of discovering what lies beneath.
“For me, it is the mystery of the hamon,” related bladesmith Erik Fritz. “What shape is it going to take? How much work is it going to entail to bring it out so it can be seen? I think what makes it so special is that each hamon is so different and unique.”
Full article here: Blade Magazine