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Message: <blockquote data-quote="DouglasCole" data-source="post: 1683849" data-attributes="member: 22307">I think someone alluded to it before, but the reason that one folds steel in japanese style blades is primarily to homogenize it.  The japanese forge produces very small chunks of steel (called tamahagane, I believe) of very differing carbon content. These chunks must be homogenized to even out the carbon content and also beat out the carbon inclusions that came along for the ride.  The intent is to produce a sword with an overall carbon content not dissimilar from typical western swords.  Sometimes, the outer layer is wrapped around a lower carbon content (and thus more ductile and less hardenable) core.  This adds resilience to the blade.Both katanas and western swords can quench to very high hardness values (Rockwell C 60 to 64).  Japanese smiths coat the sides and back of the blade with clay, to retard the heat loss and cause the back and sides to not quench to this hardness, making it softer, perhaps Rc 40-45 or so.  Western swords were mostly through-hardened, but then tempered to a lower overall hardness.This produced a supremely sharpenable, but very, very brittle, edge on the japanese sword.  The smiths would put little streaks of clay into the sharp/hard zone as crack interruptors.dhc</blockquote>