Q -

One occasionally finds good old knives that aren't necessarily particularly high carbon. The "lesser" irons will take an edge, and even hold one pretty well, but they have a tendence to rust too rapidly, and the rust destroys the edge - along with the rest of the tool. An old knife that's been used, and had a lot of meat run across it, been washed for decades in lye soap with lots of fat in it, will eventually take on a "surface treatment" that's almost indestructible. Commercial and industrial surface treatments try to do it in a hurry; but the old tools that get it built up slowly can be remarkably usable and durable.

A specific instance where the "treatment" happens fairly often is with old barber equipment. Constantly bathed in "hair grease" they can form an almost indestructible skin, and if it's not taken off too agressively when sharpening they seem to "heal over" the sharpened edge so that they act like "stainless" (of a rather peculiar color) even though they're not particularly "high performance" metal.

It's a process a bit like the "cure" on granny's cast iron skillet, I suppose.

John