The Hamon is the line of hardening of the blade, this line is often wavy and it demarcates the sharpest part of the blade from the softer part of the blade.
Realization of the Hamon
The back of the blade is coated with a special clay mixture so that when the blade is hot (about 800°) the coated part cools down much faster and is then harder and sharper.
This is one of the most important parts of making a Katana, because a hardening can be missed (you can hear it with a snapping sound) and the blade is thus disqualified. The success rate is not absolute, so this part is important for the price of the Katana.
There are many different types of dipping lines, but two are more often present, the straight lines and the wavy lines (Suguha & Midare). The style of the Hamon often depends on the tastes of the craftsman who makes the Katana, as well as on his skills.
As we said, this temper line is not just aesthetic, the martensite crystals offer a much stiffer and somewhat brittle edge (about 60 HRC) than the pearlite crystals on the back which are softer and more shock absorbing (usually around 40 HRC).
A good Hamon reveals itself in the light after polishing.
True or False Hamon
Because the Hamon is quite difficult to make and does not have a 100% success rate many sites offer Katanas with artificially made fake Hamons.
In reality it is quite difficult to recognize a real Hamon from a fake one, as there are many ways to make it look like a real temper line on the blade with acid for example and machine polishing.
You can ask the hardness of the Hamon and the rest of the blade, if they are identical then it is surely a fake Hamon. We also advise you to go for high end Katanas in order to have a fully hand forged blade with a real Hamon.