Sometimes we can find a blade what has different hamon on each side. We have a question from a customer why it's happened. Then, let's think about it.
1) At first, we can easily guess the reason that is due to the work of hardening work. Those may be caused with uneven thickness of clay, uneven heating on each side, or others.
Such blades are often found in Koto blades. On the other hand, not often found in Shinto blades. So, probably smiths in Koto period didn't care about it. Swords were weapon back then. Difference of hamon on each side never be a flaw at all as weapon.
Sample, wakizashi no signature
Especially the hamon in kissaki is quite different on each side.
2) One side is ground much more than the other side. Sometimes we can find a blade what looks like that both in Koto and Shinto.
Experiment to grind out on one side => An experiment to see the inner effect of hardening
Why one side was ground so much as its hamon had changed? We can guess some reasons. For examples, deep rust, damages through the battle, and etc. And another reason. Some collectors hate rough layers or blisters on the blade surface too much. In such case, they bring the sword to polisher to grind it off. We have seen several blades destroyed by such way. They are pitiful.
3) Distortion of steel construction of the blade. Hamon looks very weary only on one side, but the blade itself doesn't look ground off on one side. Probably it is because of poor work of steel construction. In other words, the cross section of the steel construction is not made symmetry. We often found such a blade in Koto blades. Such a poorly constructed blade is rare in Shinto blades. Probably master smiths in Shinto period would do the construction work carefully to make it perfect symmetry, while Koto smiths didn't care about it so much.
Sample, Fusamune
The hamon on the reverse side (lower sketch) is bright and neat all through the blade. On the obverse side (upper sketch), hamon of middle part is not very neat, rather blurring. But whole blade is still healthy as you can recognize it when you see the engravings. So, we can guess the outer steel (for edge) was thin around that part from its original.
Our smith said it is not very easy to construct with steel parts perfect symmetry. He suggests what happens easily in actual works.
From the left, wrapping style (kobuse), sandwich style (sam'mai), and sushi style (two block)
Sample, "Naohiro"
This blade is constructed in "sandwich style". And the red line in the sketch suggests how the border between outer steel and core steel appears. Then, we draw the images of cross section we can guess. The distortion of construction in cross section is the reason why the temper line on the reverse side (lower sketch) is neat and bright and its pattern is active, while the obverse side (upper sketch) temper line is blurring and its pattern is not so active.
4) Intentionally designed. There are some famous swords what have different hamon on each side. For example, "Konote-gashiwa Kanenaga". Kanenaga is a smith of Tegai school in Yamato province 13th century. We don't know if the hamon was intentionally designed or just an accident. But anyway, some smiths tried to design different hamon on each side just for eye attraction. Such a sword also is called "Konote-gashiwa". The word "Konote-gashiwa" suggests double-face blade.