Scientific Authentication of the Nihonto (日本刀): X-ray, CT, and Fluorescent X-ray Analysis Reveal the Secrets of Forging
* Image is for illustrative purposes only.Table of Contents
The Age of Science "Reading" Japanese Swords
Japanese sword authentication has long been conducted through the eyes and experience of skilled practitioners—visual appraisal that observes the jigane, hamon, sugata, and patina color of the nakago. However, since the latter half of the twentieth century, advances in physics and chemical analysis have led to the rapid development of techniques for analyzing the internal structure of Japanese swords non-destructively. As of 2026, scientific analysis has become an indispensable tool in museum collection sword surveys, authenticity appraisal, and manufacturing method research.
This article introduces the principal analytical methods used in scientific appraisal of Japanese swords and their findings. The principal analytical methods are as follows.
| Analysis Method | What is Revealed |
|---|---|
| X-ray Radiography | Layered structure from folded forging, detection of repair marks such as polishing and re-tempering |
| CT Tomography | Three-dimensional visualization of the composite structure formed by shingane, kawagane, and hagane |
| Fluorescence X-ray Analysis (XRF) | Determination of sand iron origin and manufacturing period from the proportion of trace elements such as titanium and vanadium |
| Electron Microscopy and EBSD Analysis | Microstructural analysis of steel phases (such as martensite) that produce hamon (nioi and nie) |
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 possess the property of penetrating matter, allowing visualization of the cross-sectional structure of the blade, internal cracks, and repair marks.
The most important information revealed by X-ray imaging of Japanese swords is the "layered structure of folded forging." Repeating the process of folding and forging tamahagane creates a layered structure within the steel plate. While this layering can be inferred to some extent from observing hamon and jigane, X-ray imaging allows more precise understanding of the internal layered distribution.
X-rays are also effective for detecting repair marks. Swords that have undergone later modifications such as polishing (shortening of the nakago) or re-tempering of the blade edge are detected as patterns of uneven internal density. Such information provides important evidence for determining the authenticity of signatures.
CT Tomography—Three-Dimensional Internal Structure Analysis
As an evolution of X-ray radiography, three-dimensional analysis using CT (computerized tomography) has rapidly become widespread in recent years. CT scanning is a medical technology that photographs an object from different angles using X-rays and generates tomographic images through computer processing; this technology has also been applied to cultural heritage surveys.
The particularly noteworthy achievement of CT analysis of Japanese swords is the "visualization of composite structure." The composite structure combining shingane, kawagane, and hagane—steel materials with different carbon content—was difficult to confirm through traditional visual observation, but CT analysis has made their joining surfaces and distribution clearly visible.
In joint research conducted by RIKEN and the Tokyo National Museum, CT analysis of National Treasure swords has revealed cases where complex steel material arrangements different from those previously estimated in traditional appraisal were discovered. Scientific data has updated "traditional understanding" accumulated over a hundred years.
Fluorescence X-ray Analysis (XRF)—Identifying Origins from Elemental Composition
Fluorescence X-ray Analysis (XRF: X-ray Fluorescence) is a technique that analyzes elemental composition non-destructively by directing X-rays of specific wavelengths at a substance and measuring the secondary X-rays (fluorescence X-rays) that are emitted.
Fluorescence X-ray analysis of Japanese swords has revealed that the content profiles of the following trace elements in steel differ by origin.
- Titanium (Ti)
- Vanadium (V)
- Silicon (Si)
- Phosphorus (P)
- Sulfur (S)
- Manganese (Mn)
The particularly important discovery is the "correspondence between sand iron origins and trace elements in steel." Sand iron used in tatara iron production in Okuizumo (Shimane Prefecture) and sand iron used in Bizen and Mino have different proportions of titanium and vanadium. It has been confirmed that this difference remains in minute quantities in swords after forging, and serves as scientific evidence for origin determination.
In application to authenticity appraisal, the significant differences in elemental composition of steel among kotou (pre-Nanboku-Cho period), shintou (Edo period), and Showa-era swords (machine-made) are utilized. Swords with false signatures made from electric furnace steel of the Showa era do not match the elemental ratio profiles of ancient and early modern tamahagane, making discrimination possible through XRF analysis.
Electron Microscopy and EBSD Analysis—Analyzing the Microstructure of Steel Phases
The combination of scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD: Electron Backscatter Diffraction) allows observation of steel crystalline structure at microscopic scales. The distribution of phases such as martensite, ferrite, and pearlite are visualized.
Research that has elucidated the nature of hamon—nioi and nie—at the microstructural level has been an epochal achievement in sword research. "Nie," which appears to the naked eye as a granular luster, has been confirmed through electron microscopy observation to be an aggregate of martensite particles with a diameter of tens of micrometers.
A research group at Tokyo Institute of Technology analyzed hamon samples from kotou, shintou, and modern swords using EBSD, quantifying differences in crystalline structure among periods and schools. This research attracted attention as an attempt to provide scientific basis for hamon appreciation, which had previously been considered "subjective."
The Limitations of Scientific Appraisal and the Indispensability of Human Observation
While scientific analysis has brought a revolution to Japanese sword research, its limitations are also recognized. While X-rays and CT visualize internal structure, aesthetic judgments such as "is this sword beautiful?" cannot be derived from them. Even when fluorescence X-rays indicate the tendencies of origins, the final determination of authorship attribution, such as "is this sword made by Nagayoshi?," remains entrusted to expert experience.
Science narrows hypotheses, detects inconsistencies, and provides objective data. However, the "value" of Japanese swords is comprehensively evaluated through traditional appreciation and aesthetic sensibility. Scientific appraisal and visual appraisal are not in opposition but in a mutually complementary relationship. Contemporary cutting-edge research continues to explore approaches that integrate both.
