I finally felt well enough to try a few experiments with case hardening compounds. This is not all that scientific, but represents the way I usually harden the heads of lock bolts, top jaw screws, butt plate screw heads, etc. I started off with a few 16 penny common nails.
I heated each nail head in a propane torch flame until it was bright red
I then dipped the nail head once in each hardening compound followed by a short re-heat and a second dip. Once the nail head was thickly coated with the compound, I heated the head continuously for exactly one minute (on a timer).
After a full 60 seconds of heating, I quenched the head in room temperature salt water. After a quick run against a wire wheel, all the nail heads came out looking like this....
Results: After hardening the heads on all 5 nails, I did a quick clean up on the wire wheels and then a quick file test on the edge of the head. I started with an unhardened nail and, as you would expect, the file bit easily. The hardened nails responded as follows
#1. Kasenit: As the compound is heated it melts and forms a viscus, molten ball of compound around the nail head. After the quench, the head is glass hard and a file skips off the surface leaving no mark.
#2. Brownells Hardening Compound: As the compound is heated it melts and forms a very runny thin layer of compound around the nail head. After the quench, the head is harder than it was before hardening but a file can still cut into the surface. It might get harder if you kept coating and heating, but it certainly did not harden the nail like the Kasenit did.
#3. A 50/50 mix of brownells and Sodium Ferrocyanide: As the compound is heated, it melts but forms globs that will drip off the part. After the brine quench, again, it was harder than the unhardened nail, and perhaps slightly harder than the Brownells alone, but the file bit easily and it was not as hard as the Kasenit nail.
#4. Sodium Ferrocyanide alone: As the ferrocyanide is heated it behaved differently than the others. It charred rather than melted but did form a complete, bulky coating on the nail head as it was being heated. After being quenched in the brine, it was every bit as hard as the Kasenit nail. The file test skipped over the surface without leaving a mark.
#5. Sodium Ferrocyanide + table salt, 50/50: I tried this just to see if the salt would allow the compound to melt more than the ferrocyanide alone. It did, but after the quench, the head of the nail was very hard in some spots and softer in others.
Conclusion: In the absence of being able to purchase the old Kasenit, sodium ferrocyanide seems to be a perfectly acceptable substitute. JerryWH reports that he uses potassium ferrocyanide with good success as well. I will push this a little further as I get a chance, as I would like to replicate the old Kasenit exactly, but the sodium ferrocyanide works great....and you can buy it from almost any chemical supplier.