Documentation:
From a translation of Theophilus On Divers Art
Book 3 Chapter 17-21
"Chapter 18 The hardening of files
Burn the horn of an ox in in the fire and scrape it.
Mix it with a third part of salt and grind it vigorously.
Then put the file into the fire and when it is red-hot, sprinkle the composition of it on every side.
When the coals are strongly blazing, open them up and hastily blow all arounds so that the hardening does not diminish.
Immediately take out the file and quench it evenly in water.
Take it out and dry it a little over the fire. In this way harden all files made of steel.