Carbonia Blue is not the same as charcoal blueing, though it makes use of charcoal in the form of (bone)-charcoal in it's process. Carbonia blue (sometimes called 'machine blue') is the later post WW1 process. Prior to that period, charcoal blueing was used. Charcoal blueing was/is a labor intensive process. Each part handled manually into glowing charcoal hearth and taken out and rubbed down lightly with a slightly rough material & oil,,usually at the time whale oil, though linseed will work. EAch mfg had their own techniques and set-ups and variations in applications differed but the basics of the process were the same. Take a look at a pre-WW1 mint condition Colt to see what real charcoal blueing looks like. The first few years of 1911 production were charcoal blued. Carbonia blue (Carbonia being the brand name of the oil used in the process at the time) was a major step in advancing production and eliminating labor and costs at the time. Many parts were able to be blued at one time. Other industrys used the process as well. Remember when screws, washers and other hardware were blued? Most if not all were 'machine blued' in bulk with this process through the 20's and 30's. Most likely they used cheaper linseed oil or even pine pitch mixed with the bone char to get an acceptable result. Some gun mfg. used linseed also when blueing small parts (pins, screws, washers, internal parts, etc) with this process. Carbonia was being replaced by hot salt blueing in the late 1930's as a next step in looking for a way to produce an acceptable blued finish quicker and cheaper. Though one was a substitute for and very close match in color and looks for the other, Charcoal Blue and Carbonia Blue are different even in the basic way they are applied.
