Interesting discussion, thought I might share an experience I have had. For the last 27 years I have hosted The Oregon Gunmakers Fair at my shop every spring. The emphasis of the Fair is a number of short seminars on traditional gun making techniques, predominantly centered around the building of flintlock and percussion firearms. Some 20 years ago we invited a professional metallurgist to demonstrate case color hardening. He used 1 1/2” square coupons of mild steel for the demo, and produced some wonderful colors, with attendant hardness. One of those example pieces has resided in a south facing window all this time, with no degradation of color, and no, it has had no coating applied. A sample of one, I know, but I suspect wear is a greater contributor to loss of color.
