Forging a katana is extremely labor intensive, but it all comes together with a swift dunk in water to cool.

With hundreds of years of experience, the Japanese have swordsmithing down to an art, and the magnificence of the katana is proof of that. Like anything that is high quality, the process to make it is very labor intensive, with several stages of reheating and folding different metals together to create the perfect balance and strength in the blade.

Part of the process involves what is called differential hardening, or differential quenching, in order to create a blade that is well-balanced. Heating the sword until it’s red-hot and cooling it in liquid at different speeds results in a hard, strong cutting edge and a softer, resilient spine, which allows the blade to absorb shock without breaking. To accomplish this, the sword is painted with layers of clay before heating, with a thin layer or none at all on the edge of the sword, ensuring quick cooling to maximize hardening for the edge. As you can see in the video below, cooling the steel at different rates causes the blade to arch downward, before snapping back up into its final well-known curved state.

The diagram below also shows what happens during the quenching process. You can also see that quenching gives the sword a visible boundary between the hard and soft steel, as well as gives the sword its signature curve – two characteristics that Japanese swords have become known for.

Katana_diagram_of_bending_during_quenchingImage: Wikipedia/Zaereth

▼ The visible “tempering line”, showing the divide between hard and soft steel.

1024px-Katana_hardened_edge_pic_with_inset_of_nioiImage: Wikipedia/Zaereth

With so much going in to each aspect of the sword, it’s no wonder katana and other nihonto (traditional Japanese swords) have become world-renown for their excellence.

Source and top image: YouTube/supervideo via Sploid
Reference: Wikipedia