What a Friend We Have in Jizōs 14

The next advance, from shape to substance, came in the Golden Age of Samurai Sword History. The Kamakura period Robyn and I were standing on, brought forth a second legend, Masumune, who forged, in his Sōshū swordmaking tradition, some of the most beautifully crafted katana ever made. The few that survive are priceless national treasures. Masumune created blades with a unique hamon temper line of martensitic crystals embedded in a nie pearlite matrix, like ‘stars in the night sky.’
It all began with Kamakura iron sand, placed in a tatara clay tub furnace, 4 feet tall, 12 feet long, and 4 feet wide. The tub was dried and heated to a temperature of 1000 ºC, and the molten contents mixed with charcoal to provide hardness. The process continued for 36 to 72 hours, with iron sand added every 10 minutes, and the mixture turned. When the clay tub had cooled and been broken, what emerged was a raw bright silver cake of Japanese Tamahagane steel, the icing of which was where perfect blades had descended from heaven.
The next step, the forging of a kotō blade, took weeks to months, and was considered a sacred art. Several craftsmen were involved- a smith to forge the rough shape, a second apprentice to fold the metal, a specialist togi polisher, and the various artisans that would make the various koshirae fittings used to decorate the finished blade and saya sheath (including the tsuka hilt, fuchi collar, kashira pommel, and tsuba hand guard). Sharpening and polishing took as long as the forging of the blade itself. 
Two different kinds of steel were needed to form a perfect blade: a harder outer jacket wrapped around a softer inner core of steel, resulting in a unique hard, highly razor sharp cutting edge with the ability to absorb shocks in a way which reduced the likelihood of breaking or bending in combat.
The outer skin, the hadagane, was produced by hammering a heated a block of high quality raw tamahagane into a bar, cooling and breaking it up into smaller fragments, checking for impurities, and then reheating and rehammering and reassembling and reforging and repeating, resplitting and refolding the metal back upon itself many times, to weld a homogeneous complex structure of many thousands of layers. The precision of the process determined the distinctive jihada grain pattern of the blade, indicative of the period, place of manufacture and swordmaker.
The shingane inner core was hammered, folded and welded in a similar fashion, and inserted into the reheated ‘U’ shaped hadagane, like taco filling. The new composite steel billet was reheated and hammered until it approximated the final size and shape of the finished sword blade. A triangular section was cut off from the tip and shaped to create the kissaki point. 
At this point, the blank blade shape was rectangular, and referred to as a sunobe. Again reheated, section by section was carefully rehammmered to control the tendency to erratic curvature, skillfully producing the distinctive final three-dimensional characteristics of the blade. Clay mixtures, applied in various thicknesses to the rough surfaces, were allowed to dry, while the craftsman prepared the forge for the final yaki-ire heat treatment of the blade. 
The hardening of the cutting edge occurred where metallurgy turned to alchemy, in a darkened smithy, late at night, in order to judge by eye the colour and temperature of the sword, as it was passed repeatedly through the glowing charcoal. When the time was absolutely right (the colour of the moon in February and August, the two months that appear most commonly on dated inscriptions on the sword’s nakago hilt section) the blade was plunged edge down and point forward into a tank of water. The precise time taken to heat the sword, and the temperature of the blade and of the water into which it is plunged were specific to each smith, and closely guarded secrets. Legend told of a particular craftsman who cut off his apprentice’s hand for testing the temperature of the water he had used for the hardening process. 
The hard edge of martensite, was ground to razor-like sharpness. The slower cooling thickly-coated back retained the pearlite characteristics of softness and flexibility. The hamon separation line, where the shapes, colours and beauty of the steel in a Japanese sword were found, was the demarcation between durable sharpness, and resiliency. 
Once the blade was cool, and the mud scraped off, the blade was decorated, file markings cut into the later covered hilt, to show how well the blade steel ages, kanji dedications, and horimono engravings depicting gods or dragons, or other beings. Finally, the sword was tested on live condemned criminals. The best were known as ‘five-body blades,’ the number of torsos it could slide through effortlessly.