How Do Modern Japanese Blades Compare to Blades Made in Antiquity?
Specific to Japanese blades, the metallurgy today is superior as we understand the science behind the original “cookbooks” cloaked in mythology were used as guides by ancient craftspeople.
Blade quality however still relies on the smith (Toshyo) reading the molten steel and the color of the fire when forging a blade to determine how much of an impurity should be kept for best blade quality. The quality of the clay used in the hardening process can't be overlooked, nor the importance - overstated.
Modern blades are under threat. After Fukushima, the state kiln was destroyed. The best quality charcoal needed to make a fine blade was sourced here. This catastrophe was compounded by an insect blight attacking the pine trees that are used in the process for a one-two punch.
Though many smiths have their own private stashes of tamahagane, other resources are getting harder to find and more expensive to use.
Then there are marketing pressures. Many folks today that appreciate the Japanese sword, practice a form of Japanese swordsmanship called Iaido. In migrating from a practice blade made from Zn-Al to a real carbon steel blade, many practitioners prefer “lighter” hybrid blades. These blades probably wouldn’t have passed muster from the perspective of an ancient warrior but work well for kata budo.
The molecular weight of steel is constant, more steel means more weight. Conversely, when you “over” polish a blade and do a deeper hibori (groove) to lighten a blade to please a client, you are making a blade that is more efete.
It’s not wrong but it generates discussion about whether blades are better or not. They are but one has to be able to understand the difference and the trade offs available today when comparing blades side by side.