There are excellent videos by clickspring on case hardening, on youtube, the latest 2 from the antikythera series.
He makes his own files !!!
Using mild steel, and hand tools only.
And does controlled tests on 3 rods, of different soak times, and 20 minutes was just right for file sized objects.
The material used was leather scrap charred, and a 6:4:3 mix of stuff, with a bit of water, and a clay pack.
Leather charcoal, plain flour, table salt, 6:4:3.
He also makes probably the best videos on Youtube, as in best.
&feature=youtu.be
I wonder if std charcoal could be used instead of leather/bonemeal etc ?
Need to test this, and fire up the grill today.
I think I am happy to put a clay-packed piece in the charcoal.
(Any excuse for a barbie).
Gotta get some steaks, first.