The katana hamon : explanations and types.
What is a hamon ?

What is called a hamon, or quench line, is this waving line that is seen on top of the edge of some sabers.
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In fact there are many shapes of hamon, but all of them have the same object : harden the blade for a better sharpening.

Quenching steel means to heat it at a very
high temperature (1200°F - 1500°F) and then to quench it in water or oil. The sudden and important change of temperature changes the molecular structure of the steel. The new structure created is called martensite.

Then, the steel becomes much harder, and that permits, when polishing, to give a better lasting sharp. A softer steel with a so good sharp, involves a cutting edge which would "crash" and becomes blunt faster.

Problem : hardness means a blade that does not absorb so easily the vibrations when having shocks and so it is uneasy to handle it (strong vibrations in hands), or it may break in extreme cases.

Therefore the Japanese have created the selective quench : this forging feat consists to apply
a refractory clay mixture to the blade, except on a small area above the cutting edge. The clay will protect the blade from the hard quenching, and the martensite will only be created on the free-clay area. The result is a blade the sharp of which is harder, but its suppleness is kept for most of its surface.
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Types of hamon :

A hamon may have many shapes, it depends on the way the clay is applied. But it is not
enough drawing a pattern to make it visible after quenching. Actually, heating modifies the clay which shrinks, and also the structure modifies the appearance of the quench line : a hamon will change according to the folds of the blade.

For this reason, the blacksmiths generally realize only a few types of hamons corresponding to their own tradition and blacksmith school.

Here are some examples of patterns (among hundreds of them) :
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Myth or reality : does the type of hamon change the cutting ability of a saber ?

Historically it has been said that some hamons optimized the cut while others were less efficient. If scientific evidences are lacking, at least it is partly true. For example, a Choji hamon requires some experience. To get the pattern as desired, the blacksmith may be tempted to remove the saber from the heat too early. Then the quench result is not so good. Apart from these difficulties inherent in some types of hamon, it is often said that, for example, a Gunome hamon would be one of the most incisive. This has been never prouved
effectively. The ability to realise a hamon at the optimal manner may nevertheless depend on the type of quench line.