Oscar Gaddy was kind enough to long distance mentor me for several years before his death. He and I discussed these very issues at length. It was his opinion they both arrived at about the same point-coloring the metal via different routes. He believed that the coloration of the metal was due to the heat, much the way one sees steel pass through the complete color spectrum as it is heated to cherry red. He contended that "Nitre bluing" allowed one to hold the metal at an exact temperature while seeing the color changes. He felt "carbonia bluing" did the same thing but one could not watch/visulize the process as the part was covered and in an oven. I'm sure Ken Hurst can draw on his Colt experience and tell us what he observed. Best, Dr. Bill