The last one I made I tried using the strong nitric acid/iron filings mix I use for stock stain. I painted on a couple of coats and let it dry then heat it over a burner on the stove until it blushes.
I did a map horn of Ohio showing the main rivers, streams, trails, and the locations of the principle indian villages prior to 1795. I used this method back in 1982 and the horn still looks great no fading.
I tried potassium permanganate but it always came out too brown for me. I couldn't duplicate the nice reddish brown to yellowish brown of the originals. I know others say they have had good results using it.
Regards, Dave
