How important is differential hardening?
Many iaito and decorative swords have a hamon or temper line artificially added. They use a purely cosmetic surface treatment that makes the katana look like it was differential hardening. These only make the katana look more authentic (well not really if you know what you are looking at). A differentially hardened katana (as opposed case hardened or tempered) has a hardened edge and a soft spine. The edge is hardened so it will retain a razor sharp edge while the spine is left softer so it will bend and not break. This is usually done with a traditional clay coating tempering process. We believe that differential hardening is key to making a shinken katana that will be used for extensive tameshigiri (test cutting). Our expectation is that a shinken should survive at least 10,000 tatami omote cuts (10 years of heavy use) and only need minor sharpening after every 500 (six months of heavy use). We have tried case hardened katana – they did not retain an edge well and we do not recommend
