Japanese sword technology
Japanese sword smiths used a number of different methods of combining iron and steel to produce weapons which are rightly regarded as amongst the finest examples of traditional craftsmanship. Several sections from broken blades were examined at the Royal Armouries using optical microscopy and microhardness testing to learn more about their construction.
Results of analysis
The sword shown right is of the Wari La type in which a steel envelope (shown by its dark etching) is wrapped around an iron core. This is just one way in which Japanese swordsmiths welded iron and steel together within the blade, but the steel always forms the cutting edge.
During quenching most of the blade would be protected with a clay slurry and only the edge and tip would cool at a rate sufficiently rapid to produce the extremely hard microstructure known as martensite.
The figures shown on the diagram are direct measures of the microhardness on the Vickers scale. By comparison iron gives 90-120 Hv and slowly cooled steel 200-210 Hv. An interesting effect of the exposed edge is that the more rapid cooling here results in less contraction on cooling. This gives the blade much of its final curvature.
The use of composite construction is shared by blacksmiths of many cultures worldwide, but few attained the skill and experience of the Japanese swordsmiths that led to the remarkable beauty and efficiency of Japanese blades.
The results of this study were utilised in public talks that accompanied the Royal Armouries Shogun exhibition in 2005.