Nicholson Black Diamond files are higher carbon steel than used since about 1980. Used to be from 10120 to 10140 steel (1.2 to 1.4% carbon), the higher carbon used for smaller files.
The ultra high carbon of those older files makes the steel capable of taking and holding a razor edge.
Then about 1980 or so the former Republic Steel in Cleveland, Ohio stopped making this very high carbon steel. Nicholson then started using the somewhat lower carbon steel 1095 (0.90 to 1.03% carbon.
In all cases Nicholson coated their freshly-cut files with "cyanide loaf", before hardening them in lead baths. This stuff prevented any loss of carbon from the very fine tooth edges while the metal was heated about 1440F for quenching in salt water, maybe 10% table salt in water.
Then Our Gov't got into the act to Help & Protect Us All and prohibited cyanide loaf. Same reason you can no longer find Kasenit. Cyanide Loaf was a mix of potassium ferrocyanide, K4Fe(CN)6, with flour and bone black, all are boiled together in salt water. If you make it yourself it is important to use the ferrocyanide, that letter "o" is important that it be not so very poisonous.
Of course if you get the grocery store brand of table salt, even sea salt, you will find the ingredients contain a bit of potassium ferrocyanide, K4Fe(CN)6, to prevent clogging.
Gov't says you can't heat treat with the stuff but you can sprinkle it on your dinner.
I learnt this all a few decades ago when I used to call on Nicholson, first in Philadelphia, then Cullman, Alabama. My employer sold the metal used in their lead pots (yeah, they also had to change from lead to bismuth). I was the metallurgist, hence technical service, for Rolled Alloys.
Wrote this up for Muzzle Blasts a decade or so ago, under the title Old Files and New Knives. Can send a pdf of the last revision to whoever asks.
One limit to the effects of metallurgy here is how well the file is cut. I've not a clue to what the Mexicans use to make their files. But I did get a good side-by-side look at a Mexican and an American "Nicholson" file a few years ago at Woodcraft Supply. It was clear that the USA file had been cut differently. I shelled out another Andrew Jackson or so & got the American file.
Now for My Ignorance, please help - what does "pin" or "pinning" mean, with respect to files?
