Differential Heat Treatment and Quenching

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.

Quenching

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 results.  

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.

Therefore, 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.

Tempering

After 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. 

Clay Hardening

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 making masters.

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.

Cold-forging

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.