Sometimes I think modern collectors over-think this type of question, and look for exotic answers instead of the simplest answer that is probably closest to the truth. I have never seen any old document describing how carving on horns was colored/darkened, yet many collectors recall reading about it. I've read modern articles on how horn carving was darkened, but those articles generally boil down to someone's opinion again, with authors who have handled more horns having stronger opinions... until it almost sounds like an historic fact despite still being opinion. Modern horners who antique their work down to look like old, original horns use stains, acids, etc. to yellow the surfaces, and after a while I think many begin to believe the old timers must have done the same thing to get the mellow old yellow surfaces... forgetting what time and handling oils can do to a surface.
I've seen several good Tansel horns, with mellow surfaces, where an inexperienced new owner has washed the surface with "just warm water" and suddenly the surface begins to look too light, too clean, too new. So was it originally stained to get the mellow color, or was it really a build-up of handling oils and dirt over many years, with the long period of time assisting the aging/oxidizing/darkening process?
There may be a difference between horns that were more a memento type horn, as many early F&I and Rev War horns were, versus later horns such as Tansel horns that were often working horns, carried for years and well-worn on the back side. Horns that stayed on the mantle for most of their lives would remain lighter than those carried in the field for years and probably oiled/greased down at times along with the companion rifle to help waterproof them. Oils oxidize over time, pick up more dirt, and naturally darken. while cleaner surfaces still oxidize and yellow but at a slower rate.
If many of the great F&I era horns were actually carved at the forts during idle periods by a fellow soldier [usually assumed to be John Bush], would the carver bring along "dangerous" chemicals? I doubt it. I'd guess he probably brought a small blade for carving that he was comfortable using, and the rest was all local "stuff." While Tansel horns don't always equate to the earlier horns, the tradition was undoubtedly passed on through conversations and passed-down information, and probably didn't vary too much from the carving of earlier horns. I have no documentation of what the Tansels used to darken their carving, but I would bet it was the simplest permanent black material they could find easily. Lamp black has been suggested and may have been used, rubbed into fresh carving with a damp thumb. I personally believe they used the fine dust from gun powder, also with carbon in it similar to lamp black for a permanent black filler. My reason for believing this is based on several early KY era Tansel horns I have with multiple "bug bites" in them. I have noticed the bugs, or moth larva, will eat right up to the edge of the carving, then stop, as if the coloration in the carving had something in it that the larva could not tolerate. I believe that "intolerable" material was the sulfur in the gun powder that still remained with the black color, i.e., gun powder dust, in the carving cuts, sealed into the horn by years of handling oils and perhaps waterproofing oils or even wax rubbed on the horn's surface.
If there are documented articles out there with period references to how these great old horns were colored, and perhaps surface stained, please post where they can be found and read, because I would greatly appreciate reading them, as I continue to collect and study Tansel powder horns.
Shelby Gallien