A sword is more than a piece of
shaped metal; as without proper hardening through differential
heat treating, quenching and tempering, steel will not
be hardened and will not reach its full potential. The
highest quality steel will be no harder or better than
normal steel without undergoing the mentioned procedures.
After the blade has been shaped, the
sword would be quenched. Traditionally there were a
variety of methods used to quench a sword; horse urine,
salty water, pure water, believed to produce better
Today we quench our swords in either
water or oil. Quenching a sword in water is more difficult
to control, but yields better results. The defect rate
is about five times higher than oil quenching, which
is much more stable and easier to control. Water quenching
produces a tougher edge which can also be hardened further
more using clay (see below). Blades quenched in oil
are still considerably hardened and do have superior
flexibility compared to a water quenched blade.
The more rapidly a blade cools down,
the harder it becomes. Thus, when a hot blade enters
the water, the water also gains heat and the blade will
cool more gradually. Therefore, the first part of the
blade that enter the water will be the hardest.
the technique of quenching was also very important.
Dao (sabres or single edged weapons) were comparatively
easier to quench than double edged blades, they would
simply enter the water edge and tip first, leaving the
spine or back and lower section of the blade softer.
This was also done for practical reasons, as the ‘softer’
sections were better for absorbing shock and impact
and employed for defensive measures.
Jian (or any other double
edged blades) were more difficult to quench effectively,
as the hardness was required evenly on both edges. To
quench a jian, the sword maker would have to
employ what was known as the ‘swallow method’,
where the sword enters flat and tip first at an angle,
dips and resurfaces, resembling the way a swallow flies.
If a blade has any flaws from forging
(air bubbles, ash), it will break immediately during
the quenching process.
quenching, the sword will be quite tough and brittle,
with little flexibility. To overcome this, the blade
would undergo a tempering process. The blade would be
reheated to a certain temperature degree then allowed
to cool naturally. The blade would be slightly less
tough afterward but have a greater degree of flexibility
– the art would be to perfectly balance the blade
for toughness, sharpness and flexibility.
Before being quenched, a special clay
mixture can be applied onto the blade to harden the
edge and obtain different hardness on the blade. The
clay mixture, was a special recipe and considered a
crucial trade secret, guarded protectively by sword
It would contain such things as feathers,
powdered bones, grass, etc. and would be applied to
the edge of the blade before being quenched. During
quenching, a chemical reaction between the clay mixture
and the hot steel occurs during the sudden temperature
drop and carbon is fed into the blade in high amounts,
creating an extremely tough edge. A clay hardened blade
can only be quenched in water, thus increasing the defect
rate even more.
Another way for clay tempering is to
apply clay along the blade but let edge exposed. Thus,
while quenching the blade into water, the uncoverd edge
will cool down suddenly, but the rest of blade
will cool down slowly. Such differential temperature
change results in the different hardness of the blade.
So the edge is tough enough to cut, where the back of
blade is soft/flexible enough to absorb the impact
during cutting. Such quenching process usually will
leave beautiful wavy tempered line on the blade, as
known as "homon" in Japanese swords term.
In traditonal Chinese sword forging, the term "Cold-forging" means forging steel blade without pre-heating . It is done by hammering blade evently into thiner piece to increase density. During the cold-forging process, the blade will naturally heat up as the metal elements are forced to restructure. Cold-forge does NOT equal simply using hammer to "reshape" or adjust the blade shape, in fact, real cold forging requires high level of skill in order to optimaize and enhance the overall strengh of blade. Cold-forging is normally conducted before final quench process.