It begins with sand.<\/p>\n\n
Black, heavy iron sand, washed down by rivers from the mountains of the Japanese prefecture of Shimane. Satetsu<\/em> is what the smiths call it \u2014 magnetic sand, rich in iron, poor in impurities. From this sand, in a process that lasts three days and three nights, the raw material for the Japanese sword is made.<\/p>\n\n Japanese swordsmithing is no simple story of one craftsman and one weapon. It is a system of specialists, perfected over seven centuries to such a degree that the resulting object can scarcely be called a weapon \u2014 but rather nihont\u014d<\/em>: the Japanese sword.<\/p>\n\n Display case H04V of the Samurai Museum Berlin preserves blades by Gassan Sadakazu \u2014 one of the last masters of this tradition, bearer of the imperial title Teishitsu Gigei-in.<\/em><\/p>\n\n The tatara<\/em> furnace is neither a blast furnace nor a forge hearth. It is a flat, ground-based clay furnace, about three metres long and one metre high, constructed especially for processing satetsu. It is destroyed and rebuilt after each smelting run \u2014 the building of the furnace is part of the process.<\/p>\n\n For one smelting run, about ten tonnes of satetsu and ten tonnes of charcoal are needed. The process begins in the early morning hours of the first day and ends on the third day. Throughout, charcoal is continuously added and the temperature is held at around 1,100\u00b0C by hand-operated bellows (fuigo<\/em>) \u2014 hot enough to melt the sand, but not hot enough for modern molten iron.<\/p>\n\n The result is a heterogeneous mass of steel called the kera<\/em>: a lump of about two tonnes in which various carbon contents lie side by side \u2014 from soft wrought iron with under 0.5% carbon to hard, almost brittle steel with over 1.5% carbon. The smith breaks this lump open and selects the pieces suitable for tamahagane<\/em> (jewel steel): pieces with the right carbon content, recognisable by colour, fracture and weight.[1]<\/p>\n\n The word tamahagane<\/em> means literally “jewel steel”. It is not simply “good steel” \u2014 it is a specific product of a specific process, and its legal meaning is still precise today: a nihont\u014d<\/em> in the sense of Japanese law must be forged from tamahagane. Blades of industrial steel are called gunt\u014d<\/em> (military swords) or sh\u014dwat\u014d<\/em> \u2014 they are swords, but not nihont\u014d.[1]<\/p>\n\n The selection of the right tamahagane pieces is the first and perhaps most important decision of the smith. Too little carbon: the blade becomes soft and bends. Too much: it becomes brittle and breaks. The masters of the Kamakura period had no chemical analysis \u2014 they had decades of experience, and the experience of their teachers, and the experience of their teachers’ teachers.<\/p>\n\n One of the most often cited and most misunderstood aspects of Japanese swordsmithing is the folding.<\/p>\n\n Popular accounts like to claim “thousands of layers” or even “hundreds of thousands”. The reality: a typical smelting run involves ten to fifteen folds. That yields 2^10 = 1,024 to 2^15 = 32,768 layers. More folds produce more layers, but at some point the carbon levels out and the quality drops.<\/p>\n\n The purpose of the folding is homogenisation: the heterogeneous tamahagane lump contains unevenly distributed carbon concentrations, slag inclusions and microscopic cavities. Through repeated heating to forging temperature (~800\u00b0C), hammering and folding, these irregularities are reduced. The result is a steel with a more even crystal structure \u2014 and the characteristic hada<\/em> (blade grain) that becomes visible in highly polished blades.[2]<\/p>\n\n Sesko describes various hada types: mokume<\/em> (wood grain), itame<\/em> (more irregular wood grain), masame<\/em> (straight parallel lines). These grain patterns are not decorative \u2014 they are the visible crystal structure of the steel and give the expert information about smithing school and period.[1]<\/p>\n\n The most important innovation of Japanese swordsmithing is the composite construction.<\/p>\n\n A Japanese sword does not consist of a uniform steel. It consists of at least two different kinds of steel, precisely arranged:<\/p>\n\n Kawagane<\/strong> (jacket steel): the outer steel, hard through a higher carbon content. It forms the cutting edge and the surface of the blade. It takes the sharp hamon<\/em> and keeps its edge.<\/p>\n\n Shingane<\/strong> (core steel): the inner steel, softer through a lower carbon content. It gives the blade its flexibility \u2014 when the edge is hard enough to deliver cuts, the core flexes far enough that the blade does not break.<\/p>\n\n This composite construction solves a fundamental metallurgical problem: hard steel keeps a sharp edge, but breaks. Soft steel bends, but stays blunt. The Japanese solution is the separation of both functions in a single object. Absolon aptly describes the engineering achievement: “The o-yoroi was a magnificent piece of technology… but it was a specialized tool for a specific type of warfare.” The same applies to the sword \u2014 a specialised solution for a specific problem.[3]<\/p>\n\n The hamon<\/em> \u2014 the visible hardening line, the aesthetically most striking feature of every Japanese blade \u2014 is the result of a precise thermal process.<\/p>\n\n Before quenching, the smith applies a clay mixture to the blade: thin near the edge, thicker towards the back. The exact composition of this clay mixture is a family secret of the respective school \u2014 clay, loam, wood ash, finely ground charcoal, each in secret proportions.<\/p>\n\n The blade is heated to exactly 750\u2013800\u00b0C \u2014 the critical temperature at which the steel changes from the austenitic to the martensitic phase, recognisable by the cherry-red glow. Then it is plunged into cold water, edge first.<\/p>\n\n The thinly covered edge cools quickly: martensite<\/em> forms, a crystal structure of extreme hardness and characteristic needle-like, prismatic shape. The thickly coated back cools slowly: pearlite<\/em> or bainite<\/em> forms, softer and tougher.<\/p>\n\n The boundary line between these crystal structures is the hamon<\/em>. It is microscopically visible \u2014 and macroscopically in the interplay of light and angle: depending on the lighting, the hamon casts different reflections, which in the Japanese tradition are classified as nie<\/em> (sparkling crystals) and nioi<\/em> (a milky haze).[1]<\/p>\n\n The Gassan Sadakazu sword in display case H04V is immediately recognisable as belonging to the Gassan school by its forging pattern: the ayasugi-hada<\/em>.<\/p>\n\n Ayasugi<\/em> means literally “ayasugi cypress” \u2014 and the pattern imitates the characteristically wave-shaped grain of this tree. It arises through a specific folding technique in which the smith hammers the blade in a wave-like pattern that forces the resulting layers into sweeping, parallel waves.<\/p>\n\n Sakakibara K\u014dzan \u2014 whose 19th-century handbook on armour-making also treats sword technique \u2014 describes how the ayasugi-hada can only be mastered by smiths who have fully internalised the basic technique of the masame-hada: the wave-shaped pattern presupposes precisely straight control.[4]<\/p>\n\n Gassan Sadakazu, who created the sword in display case H04V, was the last great master of this tradition. He had studied all five Gokaden and mastered the techniques of all the regional schools \u2014 which made him one of the most versatile smiths of his time. His 1907 appointment as Teishitsu Gigei-in<\/em> (imperially authorised craftsman) was the highest distinction a craftsman could attain in Meiji Japan.<\/p>\n\n The blade is finished after the forging \u2014 but not yet visible.<\/p>\n\n The togishi<\/em> (polisher) works with seven to twelve whetstones of differing grit, from the coarse arato stone to the fine hazuya finger polish. An elaborate polishing job takes forty to eighty hours. Without the togishi, the hamon<\/em> remains invisible, the hada<\/em> hidden, the nie<\/em> dull.<\/p>\n\n Sesko emphasises the independent significance of the togishi: “kaji-togi” (smith’s polishing) and “shiage-togi” (finishing polishing) are two different professions. The kaji grinds the basic form; the shiage-togishi brings out the aesthetic quality. A bad togishi can destroy a magnificent blade; a good togishi can make a mediocre blade respectable.[1]<\/p>\n\n In the Heian and Kamakura periods, the swordsmith occupied a peculiar social position: his craft was physically connected with death \u2014 he forged weapons \u2014 but his skill was honoured as a quasi-religious art.<\/p>\n\n Great smiths received imperial commissions. They were invited to court ceremonies. Their works counted as shinbutsu<\/em> (sacred property) \u2014 swords were consecrated to Buddhist temples and Shint\u014d shrines. That is a paradox: a weapon as a sacred object. But it explains why Japanese smiths never fell into the social marginalisation that affected other metalworkers elsewhere.<\/p>\n\n Japanese swordsmithing is an art of genealogy: knowledge is passed from master to student, often through adoption. Technical secrets remain in the family \u2014 the son takes over the father’s name, sometimes even his name in full.<\/p>\n\n Sesko documents in his Genealogies and Schools of Japanese Swordsmiths<\/em> over a thousand smiths and their relationships of kinship and apprenticeship \u2014 a database that maps five centuries of Japanese swordsmithing.[1]<\/p>\n\n The Gassan house is an example of this continuity: Gassan Sadayoshi (1781\u20131870) adopted Gassan Sadakazu, who in turn trained Gassan Sadakatsu \u2014 a three-generation line reaching from the Edo period through the Meiji era into the early 20th century. Every bearer of the name Gassan mastered the characteristic ayasugi-hada \u2014 but each also brought his own signature to the tradition.<\/p>\n\n The signature on the tang (nakago-mei<\/em>) is thus more than a proof of authorship: it is the document of a line of descent.<\/p>\n\n Japanese swordsmithing is alive.<\/p>\n\n Today there are in Japan about three hundred registered gendait\u014d<\/em> smiths (smiths of modern swords) who work according to traditional methods with tamahagane. The tatara<\/em> process is protected by the Japanese government as intangible cultural heritage; the Nittoho Tatara<\/em> in Shimane-ken is the only modern facility that produces tamahagane by the traditional method.<\/p>\n\n The swords of these smiths can be legally bought if they meet the registration requirements of the NBTHK \u2014 a maximum of two blades per month per smith, NBTHK certification for each piece. The best contemporary masters achieve prices of twenty to sixty thousand euros per blade.<\/p>\n\n This economic reality shows that Japanese swordsmithing is no museum survival, but a living craft tradition. Gassan Sadatoshi, today’s bearer of the name and a Ningen Kokuh\u014d<\/em> (Living National Treasure), continues a line that reaches back to the Kamakura period.<\/p>\n\n Japanese swordsmithing is more than technique. It is an argument.<\/p>\n\n The argument runs: an object that serves life \u2014 or death \u2014 deserves the highest craftsmanship. Not despite its function, but because of it. Whoever makes a sword commits to an interplay of care and material that tolerates no error. The hamon, the nie<\/em>, the hada<\/em> \u2014 all of this is the making-visible of this care.<\/p>\n\n Gassan Sadakazu made the blade in display case H04V at the age of fourteen. That is no prodigy anecdote \u2014 it is testimony to a system in which the craft began so early, was taught so deeply, had to be so consistently perfected.<\/p>\n\n What is tamahagane?<\/strong><\/p>\n\n Tamahagane is the traditional Japanese steel smelted in the tatara furnace from black iron sand (satetsu<\/em>). Only blades forged from tamahagane count legally as nihont\u014d<\/em> (Japanese sword). The term means literally “jewel steel” and denotes both the material and the production method.<\/p>\n\n How many layers does a real katana have?<\/strong><\/p>\n\n After ten to fifteen folds, 1,024 to 32,768 layers arise. The often-cited “thousands of folds” describe a real, if simplified, fact. The folding process serves the homogenisation of the heterogeneous tamahagane, not a layer record.<\/p>\n\n What is the hamon?<\/strong><\/p>\n\n The hardening line<\/em> \u2014 the boundary between the hardened edge steel (martensite) and the softer back steel. It arises through differential hardening: a different clay coating before quenching produces different cooling speeds. The hamon is at once an aesthetic feature, a technical document and a mark of the school.<\/p>\n\n Who was Gassan Sadakazu?<\/strong><\/p>\n\n Gassan Sadakazu (1836\u20131918) was a master smith of the Shinshint\u014d renaissance. He mastered all the Gokaden traditions and received the imperial title Teishitsu Gigei-in<\/em> in 1907. Two of his blades are in the collection of the Samurai Museum Berlin (display case H04V).<\/p>\n\n The sword collection of the Samurai Museum Berlin comprises over forty signed blades \u2014 living testimonies of the Japanese smithing tradition from the Kamakura period to the early 20th century. Display case H04V preserves pieces by Gassan Sadakazu, master smith and bearer of the imperial title. Open daily from 10 a.m. to 7 p.m., Auguststra\u00dfe 68, Berlin-Mitte.<\/p>\n\n \u2192 Tickets & Opening Hours<\/strong><\/a><\/p>\n\n \u2192 All exhibitions at a glance<\/strong><\/a><\/p>\n\n [1] Sesko, Markus (2014).<\/strong> Encyclopedia of Japanese Swords.<\/em> Lulu Enterprises. Used: tamahagane (p. 81), hamon (p. 19), hada types (p. 31), nie\/nioi (p. 28), nakago (p. 285).<\/p>\n\n\n [2] Turnbull, Stephen (2010).<\/strong> Katana: The Samurai Sword.<\/em> Osprey Publishing. Used: tatara process (pp. 16\u201317), folding process (pp. 18\u201319), yaki-ire (pp. 22\u201325).<\/p>\n\n\n [3] Absolon, Trevor (2017).<\/strong> Samurai Armour, Volume I.<\/em> Osprey Publishing. Used: composite construction as an engineering solution.<\/p>\n\n\n [4] Sakakibara K\u014dzan (1800\/1962).<\/strong> The Manufacture of Armour and Helmets.<\/em> Translated by H. Russell Robinson. Used: forging techniques; school discipline; choice of material.<\/p>\n\n\n [5] Samurai Museum Berlin (2025).<\/strong> SMB Catalogue 2025.<\/em> Display case H04V: Gassan Sadakazu, Tameshi-Tant\u014d, sword for the coronation of Emperor Taish\u014d.<\/p>\n\n<\/details>\n","protected":false},"excerpt":{"rendered":" Japanese swordsmithing: how a katana is made from black iron sand. Tamahagane, hamon & master smiths explained \u2013 with original blades at the Samurai Museum Berlin.<\/p>\n","protected":false},"author":10,"featured_media":52308,"template":"","wissen_category":[34],"class_list":["post-52600","wissen","type-wissen","status-publish","has-post-thumbnail","hentry","wissen_category-arsenal"],"_links":{"self":[{"href":"https:\/\/samuraimuseum.de\/en\/wp-json\/wp\/v2\/wissen\/52600","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/samuraimuseum.de\/en\/wp-json\/wp\/v2\/wissen"}],"about":[{"href":"https:\/\/samuraimuseum.de\/en\/wp-json\/wp\/v2\/types\/wissen"}],"author":[{"embeddable":true,"href":"https:\/\/samuraimuseum.de\/en\/wp-json\/wp\/v2\/users\/10"}],"version-history":[{"count":0,"href":"https:\/\/samuraimuseum.de\/en\/wp-json\/wp\/v2\/wissen\/52600\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/samuraimuseum.de\/en\/wp-json\/wp\/v2\/media\/52308"}],"wp:attachment":[{"href":"https:\/\/samuraimuseum.de\/en\/wp-json\/wp\/v2\/media?parent=52600"}],"wp:term":[{"taxonomy":"wissen_category","embeddable":true,"href":"https:\/\/samuraimuseum.de\/en\/wp-json\/wp\/v2\/wissen_category?post=52600"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}<\/span>The Tatara Furnace: The Foundation<\/span><\/h2>\n\n
<\/span>Tamahagane: Steel from Selection<\/span><\/h2>\n\n
<\/span>The Folding: Homogenisation, Not Mystification<\/span><\/h2>\n\n
<\/span>The Composite Construction: Kawagane and Shingane<\/span><\/h2>\n\n
<\/span>The Yaki-ire: Differential Hardening<\/span><\/h2>\n\n
<\/span>The School of Gassan: The Ayasugi-hada<\/span><\/h2>\n\n
<\/span>The Polisher: The Overlooked Artist<\/span><\/h2>\n\n
<\/span>The Smith in Social Context: Craft and Honour<\/span><\/h2>\n\n
<\/span>Masters and Schools: The Genealogy of the Art<\/span><\/h2>\n\n
<\/span>Today: Swordsmithing in the 21st Century<\/span><\/h2>\n\n
<\/span>Conclusion: Craft as Philosophy<\/span><\/h2>\n\n
<\/span>Frequently Asked Questions about Japanese Swordsmithing<\/span><\/h2>\n\n
<\/span>Visit the Samurai Museum Berlin<\/span><\/h2>\n\n
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