Thread: Through hardening vs shallow hardening

I've been reading up on the different types of steel, mainly to try and determine the few steels that I want to work with. One of the things that I've read is that if you want to end up with a hamon, you need a shallow hardening steel like 1095 and not a through hardening steel like O1.

I used to think that any time you'd use differential hardening, you'd end up with a hamon, but that does not seem to be true. What is exactly the difference between the through / shallow hardening, and why does differential hardening (such as is done with e.g. 52100) not always lead to a hamon?