The Age of Science "Reading" Japanese Swords

For long, the authentication of Japanese swords has been conducted by the trained eye and experience of experts—visual authentication through the observation of jihada, hamon, sugata, and the rust color of the nakago. However, since the latter half of the 20th century, advances in physics and chemical analysis have led to rapid development of non-destructive techniques for analyzing the internal structure of Japanese swords. Today, scientific analysis has become an indispensable tool in museum sword collection surveys, authenticity authentication, and manufacturing method research.

This article introduces the main analytical methods used in scientific authentication of Japanese swords and their findings.

X-ray Radiography—Visualizing Hidden Internal Structures

The most fundamental scientific analysis method is X-ray radiography. While ordinary photography utilizes light reflection, X-rays have the property of penetrating matter, making it possible to visualize the cross-sectional structure of the blade, internal cracks, repair marks, and other features.

The most important information revealed by X-ray radiography of Japanese swords is the "layered structure of folding and forging." In Japanese sword manufacturing, a layered structure is formed within the steel plate by repeatedly folding tamahagane and forging it. While this layering can be somewhat inferred from hamon and jihada observation, X-ray radiography allows for more precise understanding of the internal layered distribution.

X-rays are also effective in detecting repair marks. Swords subjected to later modifications such as re-polishing (shortening the nakago) or re-hardening of the blade tip (redoing the quenching) are detected as non-uniform patterns in internal density. Such information provides important evidence for determining the authenticity of signatures.

CT Scanning—Three-Dimensional Internal Structure Analysis

As an evolution of X-ray radiography, three-dimensional analysis using CT (computed tomography) has rapidly become widespread in recent years. CT scanning is a medical technology that photographs objects from different angles using X-rays and generates cross-sectional images through computer processing; this same technology is now being applied to cultural heritage research.

The particularly notable achievement in CT analysis of Japanese swords is the "visualization of composite construction." The composite structure that combines shingan (core steel), kawagane (skin steel), and hagane (edge steel)—steels with different carbon contents—had been difficult to confirm through traditional visual inspection, but CT analysis now clearly reveals the bonding surfaces and distribution. In a joint survey conducted by RIKEN and the Tokyo National Museum, CT analysis of National Treasure swords revealed cases where complex steel arrangements differing from the composite structure previously estimated in traditional authentication were discovered. Scientific data has updated "traditional understanding" that had endured for over a century.

Fluorescent X-ray Analysis (XRF)—Identifying Origin from Elemental Composition

Fluorescent X-ray Analysis (XRF: X-ray Fluorescence) is a technique that analyzes elemental composition non-destructively by applying X-rays of a specific wavelength to a material and measuring the secondary X-rays (fluorescent X-rays) emitted from the material.

In fluorescent X-ray analysis of Japanese swords, it has been revealed that the content profiles of trace elements contained in steel—titanium (Ti), vanadium (V), silicon (Si), phosphorus (P), sulfur (S), and manganese (Mn)—differ by production region.

A particularly important discovery is the "correlation between iron sand origins and trace elements in steel." The iron sand used in tatara iron-making in Okuizumo (Shimane Prefecture) and the iron sand used in Bizen and Mino differ in their titanium-to-vanadium ratios. This difference has been confirmed to persist in minute quantities in swords even after forging, and serves as scientific evidence for determining origin.

In applications to authenticity authentication, the fact that the elemental composition of steel differs markedly between koto (swords before the Northern and Southern Courts period), shinto (Edo period), and Showa-period swords (machine-made) is utilized. Counterfeit swords with false signatures made using Showa-period electric furnace steel can be distinguished through XRF analysis, as their elemental ratio profiles do not match those of ancient and early modern tamahagane.

Electron Microscopy and EBSD Analysis—Analyzing Fine Structure of Steel Microstructure

Through the combination of scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD), the crystalline structure of steel can be observed on a fine scale. This allows visualization of the distribution of various structures such as martensite, ferrite, and pearlite.

Research that revealed the true nature of hamon—nioi and nie—at the level of fine microstructure was a groundbreaking achievement in sword research. While "nie" displays a granular brilliance visible to the naked eye, electron microscope observation has confirmed it to be an aggregate of martensite particles with a diameter of tens of micrometers.

A research group at Tokyo Institute of Technology conducted EBSD analysis of hamon samples from koto, shinto, and modern swords, quantifying the differences in crystalline structure by era and school. This research garnered attention as an attempt to provide scientific basis to hamon appreciation, which had traditionally been considered "subjective."

The Limits of Scientific Authentication and the Indispensability of Human Expertise

While scientific analysis has revolutionized Japanese sword research, its limitations are also recognized. While X-rays and CT visualize internal structure, they cannot derive aesthetic judgments such as "whether this sword is beautiful." Even though fluorescent X-rays may indicate regional tendencies, the final determination of authorship, such as "whether this sword is the work of Nagayoshi," still rests on expert experience.

Science narrows hypotheses, detects inconsistencies, and provides objective data, but the "value" of a Japanese sword is evaluated comprehensively through traditional connoisseurship and aesthetic sensibility. Scientific authentication and visual authentication are not in opposition but in a mutually complementary relationship. Cutting-edge research today continues to explore approaches that integrate both methods.