This is an interesting topic, and I'm still in agreement with Stan. We simply don't know if sunlight alone can degrade case colors.
As I said earlier, I did a lot of searching and found exactly zero definitive tests done to determine the effect of sunlight. The very limited and informal test supposedly done by Dr. Gaddy would hardly constitute "good science", so it should not be accepted as Gospel by those who demand "good science" in matters of Climate Change, lead poisoning, declining quail populations, etc.
It only involved one small sample. We don't know if the samples had any protective coating such as lacquer, wax, or gun oil. The alleged sample was done with sunlight coming through a window, and we know window glass filters out a lot of the UV spectrum, especially UVB. Nobody hunts under glass or in a greenhouse. And we also know sunlight is composed of a wide spectrum of visible and invisible wavelengths, so there could be something aside from UV alone that MIGHT cause a change.
It involved only one case hardening process and one alloy of steel. So we can't say that different compositions of charcoal, different alloys, or different quenches create the same changes to the molecular structure of steel which we see as case colors. Should such a test be done in an inert gas atmosphere to exclude the effects of air or pollutants? We don't even know how much time was involved in this "test". We would also have to include the degree of surface polishing because we know that highly polished steel resists oxidation better than a rougher surface. The case hardening process is far different than dying cloth or painting a car. Stan is correct when he says that case hardening is not simply adding a pigment or dye to the steel. However, we do know that cooking steel in a crucible with different recipes of bone and wood charcoal for hours at high temperatures does in fact infuse carbon, carbon monoxide, and other compounds a few thousandths of an inch deep into the surface of the steel. This chemically changes the steel near the surface by giving it a higher carbon content than the base metal. But is it just carbon alone, when we know that different charcoal contains many other elements? The quench somehow locks things in place, and the quench itself is often done with different water at different temperatures. This is what somehow creates those "prisms" Stan spoke of, which reflects and refracts light to our eyes in differing hues and colors. So now we get into a realm of molecular and atomic bonds that are difficult to see or understand even with the use of electron microscopes. The quench may be straight rain water, city water, or it may contain additives like potassium nitrate, bat urine, or eye-of-newt. It may be agitated or infused with air bubbles. Most practitioners of the art seem to find that colors are more vibrant if they can exclude as much atmospheric oxygen as possible from the parts while dumping them into the quench. The guys who do great case hardening seem to eventually settle on a process that gives them good results, often through much trial and error. But if they know every detail of the chemistry involved, they certainly aren't sharing that knowledge.
If this thread accomplishes anything, I hope it begins to make people understand that conjecture, thinking, and even wild-assed guessing is OK... so long as it isn't stated as fact. Because as things stand right now, when you do a deep dive into this subject, you will find that books, magazine articles, and the internet is literally filled with so-called experts who state things about case hardening as fact, in their attempt to explain what causes case colors to fade over time. And most of what they are saying has absolutely no basis in proven facts.
