Base material for this Tanto blade was carbon steel CK75 / 1075, thickness 4 mm. The blade was covered with fireclay mortar - including a coating on the back.
Quenching was performed in cold water...
Due to the hardening the blade got a nice curvature and hamon but it also got seven cracks in the edge!
My guess:
During heating the blade expands. After quenching, the back and middle part goes back to their initial volume - the hardened edge remains with a slightly higher volume.
This bending happened after the edge becomes hard and brittle and this is the reason for all cracks.
Also a thicker edge and warmer water decrease the danger of crack initiation.

Here you can see a close-up view of the cracks.

Reason for these cracks was the strong bending during quenching with already hardened edge.