Trying to understand weird refelection

[heliosphere]

I recently got a Haruyuki Kuma knife from Knifewear, and when I look closely at the line between the hard cutting steel and the cladding, the reflection looks like there's an indentation between the two layers. But when I feel it with my fingernail, the transition feels totally smooth. Anyone have an explanation for this?

 

[milkbaby]

Might just be a zone of carbon migration. Typically the cladding will be something low carbon, so during forging the carbon can migrate from the high carbon core steel to the lower carbon cladding. Some bladesmiths may laminate or buy prelaminate that has nickel sheet between the core and cladding to help block carbon migration and that will typically be visible as a bright line between the core and cladding.

 

[M1k3]

As said above, carbon migration. It's more visible, usually, with a carbon steel core.

 

[DevinT]

Looks like a nickel layer to me.

Hoss

 

[holdmyphone]

My understanding is the san mai process uses three layers of steel that are fused together by another metal with a lower melting temperature, like soldering. The "groove" that you're seeing is a trick of the eye, just a contrast between the different layers of metal. Exactly WHY they are darker or lighter color has to do with the compositions of the alloys, etc...