This is all very useful and interesting information, thank you Lofty and Pap!

There is a very useful modern technology - ultrasonic
hardness tester UCI (Ultrasonic Contact Impedance).
It would be nice to measure the hardness of old
and new knives in this manner. The usual mechanical method
is not applicable (indentation of a diamond cone into
a metal), since it requires an even surface, that is,
it is only possible in the area of the spine of the knife.
As far as we know, Ruana used zone hardening,
cutting edge harder than the spine.
The UCI makes it possible to measure the hardness
on curvilinear surfaces.