古墳出土刀剣の年代測定法——層位・共伴遺物・銘文から読み解く日本刀史のものさし

When discussing the history of Japanese swords, we tend to treat "when they were made" as an unquestioned premise. However, excavated swords from the Kofun and Yayoi periods have almost no inscriptions recording their production date or supporting textual evidence. What determines the dating is not the sword itself, but the archaeological method of deciphering the "context" in which the sword was buried. This article clarifies what methods are used to date excavated swords and how the results have positioned them within the broader history of weapons. Research dealing with sword dating is not merely artistic stylistic analysis, but an accumulation of empirical procedures akin to geology and statistics, and behind the commonly circulated expression "a sword from the ◯ century" lies work comparing multiple such methodologies.

Dating Cannot Be Read from the Sword Itself

Pottery shows changes in form and pattern, and wood retains a record of time in its rings. However, iron as a material shows no significant compositional change even centuries after production, making it difficult to determine the production date directly from iron itself. The reason sword dating depends so heavily on information from "outside the sword" is this material constraint. As a result, archaeology strictly treats excavation conditions as primary sources. At excavation sites, the position, orientation, and depth at the moment the sword is discovered are recorded, and detailed drawings showing the excavation location are included in reports. This is because information about "where and how the sword was positioned" itself becomes material for dating, not just the sword alone.

Stratigraphy as a Basic Principle

The most basic method of dating is stratigraphic observation. Burial facilities in ancient tombs are often sealed all at once upon construction, and grave goods excavated from the same layer or burial space can generally be considered to have been interred during the same period. When swords are discovered together with armor, horse trappings, beads, and other items, the entire combination is treated as "a grave-goods set from a single period," providing clues to the dating of swords that cannot be determined in isolation.

Chronology through Associated Artifacts

In addition to stratigraphy, relative chronology based on associated artifacts (other artifacts excavated together) is important. Since pottery changes in form relatively quickly and typological chronology research is advanced for each region, it functions as a measure for estimating "what century swords appearing with this pottery type date to." Similarly, for horse trappings and personal ornaments, the timing of their transmission from the continent and their domestic adoption processes can be traced to some degree, allowing the relative age of swords to be determined from their associational relationships. It might be said to be work of plotting not individual swords, but the entire artifact composition of a tomb on chronological coordinates.

The Limits and Auxiliary Role of Scientific Dating

Radiocarbon dating can sometimes be applied if organic materials (wooden sheaths, hilts, lacquer, etc.) adhere to or coexist with swords, but measurement error often ranges across several decades, making it insufficient to independently establish fine chronological divisions in sword history. Moreover, stratigraphy is often disturbed by preservation conditions at excavation or later disturbance (grave robbing, reburial, landslides, etc.), and the reality is that scientific measurements are used merely as supplementary material reinforcing estimates from stratigraphy and typological chronology.

Inscriptions as an Exceptional Clue

Only a small portion of excavated swords have inscriptions engraved on them indicating the circumstances or year of manufacture. Many express the year using sexagenary cycle notation (combinations of the ten stems and twelve branches), and since the calendar repeats every 60 years, the cycle alone produces multiple candidate years. Researchers then narrow down the candidates to one by cross-referencing names, titles, and writing style in the inscription, or the approximate date range obtained from stratigraphy and associated artifact chronology. Inscriptions are quasi-textual primary sources, yet the interesting point is that they cannot establish dating in isolation; they might be said to be complex sources that gain meaning only when archaeological context is combined with textual analysis.

Positioning of Swords within the Broader History of Horse Trappings and Weaponry

Research on grave goods from the Kofun period often proceeds not from individual swords, but from complete weaponry including horse trappings, bows and arrows, and armor. This is because there is a perspective of capturing the adoption of equestrian culture from the continent and the consequent transformation of weaponry systems. Changes in sword types (such as the evolution of straight-sword length and tang morphology) are organized in parallel with changes in such complete weaponry systems, being reinterpreted not as mere "sword evolution," but as indicators reflecting changes in military and political structures of the time. As the precision of dating increases, this weaponry-historical positioning becomes ever more detailed.

Conclusion

The dating of excavated swords is not determined by analysis of the sword alone. Only by combining the context of deposition as stratigraphy, associated artifacts as a measure of relative chronology, inscriptions as exceptional but powerful clues, and scientific measurements that can be applied only in limited ways, is the piece of sword dating finally positioned in the chronology of weaponry history as a whole. The very fact that dating a single sword is continuous with research on the chronology of grave goods as a whole and understanding exchange history with the continent is both the fascination and the difficulty of ancient sword research. TOUKENZA publishes explanatory columns online that take account of such historical background, and they can be viewed alongside swords currently offered.