soft vice jaws

I (vaguely) remember someone in my school (as a pupil) metal working days making a mini-vice and case hardening the jaws - or more exactly the inserts.

We case-hardened other things, a standard project was an adjustable wrench (I still have mine). We used a black powder- I assume carbon based- heated the steal to red I think, buried in in the powder, left it, then clean it off when cool. I don't recall a tempering stage- but it was near on 45 years or so ago.