a question for the bonding powder for laminating hard steel with soft steel

I have questions for you guys who are familiar with knife making.
When I watched some of those Japanese bladesmiths forging videos, they apply something in a powder form to laminate soft steel to hard steel.
What is the powder and its role in forging?
Is it just a bonding agent between two steel? or it prevents carbon from hard steel from diffusing into soft steel as well? It is an impurity, thus, there should be downside of using it in forging I guess. How does it affect heat treatment if any?

 

I'm no expert, but I suspect you may be talking about Borax? I think it's used as a flux.

 

yep flux. cleans out the weld aera and then keeps oxygen out of the weld

 

ah ha...thanks guys. I have another related question;
Two single bevel knives that I have show blackish lines (not continuous) here and there along the boundary between jigane and hagane, while some other knives do not show those and have very cleanly welded boundary.
Why is it? What makes such a difference between the two groups?
Coincidently the two knives showing blackish lines happen to be really good cutters - Shigefusa and Kato.