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**defenderfors** · 02-02-2009, 11:40 PM

However, the method of folding metal that the Japanese used to make a sword with one sharp edge generally changes the structure of the metal such that it naturally tends to curve.

Swords with sharper edges were curved because of the process used in making the sharper edges.

Folding the steel served to homogenize it and may have aided in carburization or decarburization (the adding in or taking out of carbon from metal). The folds wouldn't have contributed to any curving action.

If I understand correctly, the steel quality available to Japanese smiths was fairly low. They'd take river sand with a high iron content, smelt it, collect the chunks, and test for the one's that were most similar to what they'd seen work well (most likely using a spark test, you can see the difference between high carbon and low carbon steels by looking at the spark pattern).

Because the steel was generally far from perfect, they'd put it through a folding process, that would basically mix the steel to make it uniform. IIRC, the pounding removes impurities as well. And, ad nauseum, depending on the carbon content, there's a temperature at which the steel begins to carburize in the presence of high carbon material, like charcoal, in which carbon begins leaching into the steel. There's also a higher temp, where where the steel will start to "burn," as carbon leaches out of the steel.

The curve of the blade comes from two things, one being the symmetry of the blade. If I understand correctly, more mass on the back of the blade "pulls" more than the sharp side (the "ha"), which introduces the curve. The differentiated tempering (the mune is cooled at a slower rate than the ha, because smiths coat it with clary) also introduces a factor wrt how much the blade curves.

The varying types of steel used could also be a contributor. Because high quality high carbon steel was hard to get, it was often used just for the edge, with lower quality, lower carbon steel used for the rest of the blade. So the mune and sides would be low carbon, the edge high carbon, and both having different amounts of "pull" when the blade is quenched.

I've seen some discussion that the Japanese smiths constructed the blades with the different types of steel in order to improve the overall qualities of the blade (strong on the mune, hard on the edge), but generally with the working smiths who've discussed it, they acknowledge that a blade heat treated by differentially quenching then tempering, and made up entirely of high quality, high carbon steel produces a better blade.

Or I could be completely wrong.

-Charles Lockhart

02-02-2009, 11:40 PM	#7
defenderfors Join Date Oct 2005 Posts 463 Senior Member	However, the method of folding metal that the Japanese used to make a sword with one sharp edge generally changes the structure of the metal such that it naturally tends to curve. Swords with sharper edges were curved because of the process used in making the sharper edges. Folding the steel served to homogenize it and may have aided in carburization or decarburization (the adding in or taking out of carbon from metal). The folds wouldn't have contributed to any curving action. If I understand correctly, the steel quality available to Japanese smiths was fairly low. They'd take river sand with a high iron content, smelt it, collect the chunks, and test for the one's that were most similar to what they'd seen work well (most likely using a spark test, you can see the difference between high carbon and low carbon steels by looking at the spark pattern). Because the steel was generally far from perfect, they'd put it through a folding process, that would basically mix the steel to make it uniform. IIRC, the pounding removes impurities as well. And, ad nauseum, depending on the carbon content, there's a temperature at which the steel begins to carburize in the presence of high carbon material, like charcoal, in which carbon begins leaching into the steel. There's also a higher temp, where where the steel will start to "burn," as carbon leaches out of the steel. The curve of the blade comes from two things, one being the symmetry of the blade. If I understand correctly, more mass on the back of the blade "pulls" more than the sharp side (the "ha"), which introduces the curve. The differentiated tempering (the mune is cooled at a slower rate than the ha, because smiths coat it with clary) also introduces a factor wrt how much the blade curves. The varying types of steel used could also be a contributor. Because high quality high carbon steel was hard to get, it was often used just for the edge, with lower quality, lower carbon steel used for the rest of the blade. So the mune and sides would be low carbon, the edge high carbon, and both having different amounts of "pull" when the blade is quenched. I've seen some discussion that the Japanese smiths constructed the blades with the different types of steel in order to improve the overall qualities of the blade (strong on the mune, hard on the edge), but generally with the working smiths who've discussed it, they acknowledge that a blade heat treated by differentially quenching then tempering, and made up entirely of high quality, high carbon steel produces a better blade. Or I could be completely wrong. -Charles Lockhart
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