This type is specially called "hagire".
This type of crack does not come only from the hardening failure, sometimes a hard back attack can make such a crack too. I don't know why, but hagire is most hated in all kinds of flaws at market. In fact, it is not the most terrible flaw in all kinds of them.
In the current market, collectors don't mind cracks in hamon seriously except for hagire. I have seen some number of Juyo blades that had serious cracks in hamon. "Juyo" is a title that is authorized by NBTHK. They connived the cracks.
Poor steel, Steel cracks (shinae)
It is a steel including much of slag, rough welding.
The worst is a steel that has cracks by itself. The steel is not strong. We call such cracks "shinae" in Japanese.
Cracks on the blade surface.
Cracks on the back surface.
On the other hand, there is a similar appearance called "mukade-shinae". It appears in blade surface like wrinkles, not cracks.
It is not a flaw. When a blade is heavily bent and straighten again, it appears on the blade surface. It is very thin wrinkles on the steel surface, so it can be removed by sometimes polishing.
Poor hamon, No hamon
A dark hamon means a mild cutting edge. But too hard cutting edge is not good. Such a hamon looks like a mirror.
To see the brightness of hamon area is important.
When a blade meets an fire accident, the hamon disappears by burning out.
(=> A sword in fire accident)
On some old blades, the hamon is vanishing near the tang as original. It is not a problem at all.
Some polishers forge a fake hamon on blade that has no hamon.
Some of them are so skilful that is difficult to discover the trick by a beginners eye. Some is a part of hamon, and some is all the hamon. (=> fake hamon)
Being weary (tsukare)
The shape has been broken by much grinding.
Sometimes the hamon was worn out by too much grinding.
Re-hardening (yaki-naoshi, saiha)
When a blade has been burned out by the fire, it is hardened again to recall its utility. But the originality of the blade never come again. So the value as a piece of art disappears, even if the blade has a good signature.
It is difficult to recognize if the blade was re-hardened or not, only by looking at the blade surface. To research both the tang and the blade is necessary to think about it.
Next is an example of the case of a shortened blade.
If the signature is genuine, this starting of hamon means re-hardening.
The vanishing of hamon toward the tang is correct. This blade was not re-hardened.
It is a kind of re-hardening.
Kissaki is most frequenly damaged area in a fight. So some blades are worn out there. Sometimes such blades are re-hardened only around the kissaki. But the new hamon never connect to the original hamon. Some part before the new hamon is burned out by the heat of the hardening work. So, good(?) polishing is necessary to mask the inefficient part.
The new hamon must be followed with mizukage at the start of it.
(=> Episodes by Kokaji, step hardening)
Fake signature (gimei), counterfeit
A signature put on the different blade. It was purposed to deceive collectors.
Do not be enticed by the signature. The quality of the blade is most important.
Un-original engraving (ato-bori)
If it accords with the blade, it is not so bad. We can not find any problem on it.
In old days, engravings were added on blades for prayer. On old blades, it is not easy to find if the engraving would be done originally or later. But recently it was occasionally done to mask flaws. Such a work that break blades is so bad.
An open layer is far better than a terrible dragon.
These two Sanskrit-like letters are so bad. The worth of the blade disappeared forever.
-Do not restore an original engraving-
Some engravings in old blades have been partially erased by much polishing. I wonder if you would like to restore it by some craftsman. But please never touch the original engravings. Engraving is not only a decoration putting on the blade. It is a prayer of samurai old days. You should respect original engravings even though it is erasing.
Suken by Kuniyoshi: the centre line of suken has worn out.
Kurikara by Kagemasa: the dragon has worn away in some parts.
Broken cutting edge (ha-kobore)
A small nick on the cutting edge is not a big problem, to keep a good shape is more important.
Notches by battle (kirikomi)
It should be called a pride rather than a flaw.
This cutting mark still has the enemy's edge bitten off. It came from an old day's sword fight.