Fascinating. I have only one comment. Files are used just as they come out of the quench, they are not tempered at all. Not even to a straw color. This of course leaves them brittle -just don't drop them. But gives the sharpest tooth.
I was the metallurgist for a company who supplied the high temperature alloys for use in furnace fixturing. First time I called on Nicholson File they were still in Philadelphia, mid-1970's. Learned a few things. Files at that time were made of 1095 steel, heated for about 5 minutes in a molten lead bath at 1440F. They were quenched straight down into a salt brine bath - not plain water - then cleaned, but never tempered.
Because a minute amount of carbon might be burnt out of that fine tooth edge, even after just 5 minutes in molten lead, a protective coating was first put on to that file, before it was ever heated. Nicholson called this stuff "cyanide loaf" Not terribly poisonous, but don't eat it neither should it get on your hands. It was a mixture of potassium ferrocyanide, flour and bone black, all boiled together in salt water. Our Gov't now prohibits the use of potassium ferrocyanide for this purpose. Of course, you will find either potassium or sodium ferrocyanide as an anti-caking agent in the best Sea Salt on your grocery store shelf. Our Gov't says it is OK to eat, just not use in heat treat.
Back in 1589 a G.B. Della Porta described a similar process for hardening files -
Take Ox hoofs, and put them into an Oven to dry, that they may be powdered fine; mingle well one part of this with as much common Salt, beaten Glass, and Chimney-soot, and beat them together, and lay them up for your use in a wooden Vessel hanging in the smoak; for the salt will melt with any moisture of the place or Air. The powder being prepared, make your iron like to a file; then cut it chequerwise, and crossways, with a sharp edged tool: having made the iron tender and soft, as I said, then make an iron chest fit to lay up your files in, and put them into it, strewing on the powder by course, that they may be covered all over: then put on the cover, and lute well the chinks with clay and straw, that the smoak of the powder may not breath out; and then lay a heap of burning coals all over it, that it may be red-hot about an hour: when you think the powder to be burnt and consumed, take the chest out from the coals with iron pinchers, and plunge the files into very cold water, and so they will become extream hard. This is the usual temper for files; for we fear not if the files should be wrested by cold waters.
Remember - do NOT temper the file. And, about a 10% mix of salt, common table salt, in water makes a faster quench.
