Hand Files Cut With a Double-Edged Hammer and Tempered in Burnt Ox Horn and Salt (Theophilus, 12th Century)

"Files are made from pure steel, large and moderately sized, triangular and round. Others also are made which are stronger in the middle, from soft iron inside, but outwardly they are covered with steel."

Beat steel (or soft iron covered with steel) to the size and shape wanted: large or moderate, triangular or round; for small files, square, round, triangular or thin. Smooth them on a planing instrument (a filing or scraping bed).

Mark out the teeth with a hammer sharp on both sides, or with the cutting iron, until the file is cut all over.

Burn an ox horn in the fire, scrape it, mix it with a third part of salt and grind it hard. Heat the file until it glows, sprinkle the mixture over it everywhere, lay hot coals on and blow quickly over the whole without letting the coating fall off; then draw it out and quench it evenly in water, and dry it slightly over the fire. He tempers all steel things this way.

For small soft-iron files, after cutting, anoint them with the grease of an old hog, bind them with small straps of buck-goat skin tied with flaxen thread, and cover each with beaten clay, leaving the handles (tangs) bare. Dry them, put them in the fire and blow strongly until the skin is burned, take them quickly out of the clay, quench evenly in water, and dry them at the fire.

From Theophilus Presbyter, **De Diversis Artibus** (12th century), Book III, chapters XVII-XIX, translated by Robert Hendrie (1847) (checked on the archive.org scan essayuponvarious00hend, pp. 223-225). He gives no sizes, temperatures or tooth spacing. The tools and the case-hardening explanation are ours.