[geeks] Swords (forge)

[Phil Stracchino]

On Mon, Nov 10, 2003 at 11:16:57AM -0500, Michael A. Turner wrote:
> 	It though that when hand forging steel you had to use charcoal in
> most cases to keep the iron in it's steel form. The carbon was the key to
> keeping things going the right direction, but I am not a sword smith by any
> means. IANASW? 

This really depends what you're starting with.  European smiths starting
with pig-iron, loaded with carbon, had the problem of getting excess
carbon *out* of the steel.  The way the Japanese smiths smelted iron
resulted in a very pure low-carbon iron, so they had to fold in
rice-straw ash to get the carbon content up to usable levels.


> 	Second thing, I once heard of a respected sword smith that used
> steel cable scrap for his forging. Apparently the thick twisted steel wires
> of a thick steel cable get you half way to the number of folds needed to
> make a strong katana. Anyone else heard of this? Again IANASW so the person
> who told me this may have been pulling my leg.

This is confusing two different things.

Elevator cable or other high-tensile steel cable has become popular in
the last 20 years or so for hand-forged damascus steel.  Damascus steel
was traditionally made by forging wootz iron into rods, then heating a
bundle of bars and twisting them together to form a bar which was then
hammer-welded into a single piece and forged into a sword blade.  The
parallel to steel cable is obvious, and the process leaves very visible
"watering" in the blade, especially if etched lightly with acid to bring
out the grain.  Unlike Japanese sweords, Damascus blades were a uniform
quality of steel throughout, but the grainy, bundled nature of the blade
prevented cracks from propagating far through the blade.

In Japanese swords, instead of being wrung and then hammer-welded, the
steel is repeatedly coated in rice-straw ash, folded and hammer-welded
together, incorporating carbon into the steel.  With each heating,
folding and forging cycle, more carbon is added, the layers of steel
become thinner, and the added carbon diffuses further through the steel.
The result is to progressively harden the steel.  Ten folds, and you
have 1024 layers of steel and a mild steel suitable for blade cored.
Fifteen folds and you have 32768 layers, more carbon more evenly
distributed, and a spring steel suitable for the sides and back of a
blade.  Twenty folds and you have 1024576 layers in a hard steel
suitable for the edge.  Then you assemble bars of the different grades
of steel into a sword blank, hammer-weld them together, and begin
drawing it out to the correct shape.  There is no prominent watering as
can be seen in Damascus blades -- while the grain is there, it's very
fine and almost invisible.  (Consider that by the time the blade is
fully forged, the edge has over four million laminae per inch of
thickness.) The edge may crack if abused, but a crack will not propagate
deeply into the blade because it will soon encounter a transition to a
softer steel.  The gunome and choji of the hamon are there for a reason
beyond artistry, too; they limit the size of martensitic domains in the
edge, which again limits how far a crack is likely to propagate.

	------

There is a story (possibly apocryphal) about how Masamune developed the
"modern" art of Japanese swordsmithing.  It tells how Masamune forged
swords for all of his daimyo's army, and they marched off to war past
his forge equipped with the newly-forged swords.  As the story has it,
these swords were the older, straight-bladed, double-edged Japanese ken 
(a design believed to date from around 800AD).

Several weeks later, the army marched back, victorious, but Masamune
noticed as his daimyo rode past at the head of his army that the lord
looked very displeased with him.  Then as the army marched past, with
all the troops holding their drawn swords, he realized that more than
half the blades were broken.  He directed his apprentices to go through
the passing troops, gather up all the broken swords, and pile them in
front of his forge, where he spent months studying the broken blades,
first to determine why they had broken, and then to figure out what he
could do about it.

Finally, after many months of study and experiment, he produced a new
type of sword.  It was curved and tapering, with a single edge, and
constructed much differently.  When he delivered the new swords to his
daimyo, the lord was skeptical, but agreed to try them.  Not long after,
the army marched off to war again.

When the army returned, once again they marched past the forge with
their swords drawn.  Masamune was at first concerned, worrying that the
new swords had failed.  But this time, as the daimyo passed the forge,
he beamed at Masamune with pleasure, and as the army marched past,
Masamune saw that not a single sword had broken.

	------

That, at least, is the story.


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