Dr B,
Not sure what kind of hat you found but it sounds interesting. Perhaps, though brown today, at one time it may have been black(?). I have only one recipe for hatmakers' black dye and that is from 1821. This recipe was composed of mixing logwood, gum (rubber), verdigis (cupric acetate), and green vitriol (aka copperas/heptahydrate of ferrous sulfate) together. I have other black dye recipies for wool, though not specifically for hatters' use, from 1840 and 1884 - all contain logwood. I know logwood can fade quickly and turn brown with time based on repo piece-dyed gray jeans cloth and thread I have personally that have already done so. The idea of your hat being originally black that has faded is purely conjecture but perhaps might be useful info in your case at some level. At any rate, the hat sounds like it's good for your collection if nothing else.
I’m posting the following to add to the info already posted on the 1872 folding hat as it closes the chapter on these hats and where the remaining stocks ended up. From everything I have found this hat was quite an unpopular item with the troops and their officers as already posted to this thread.
The U.S. Army in the West, 1870-1880: Uniforms, Weapons and Equipment by Douglas McChristian (1995) pg 166 “…Meigs postponed their issue [the new 1876 campaign hat, initial supplier of that hat being John T. Waring & Co.] until April 1877 in order to use up some of the remaining stock of the folding campaign hat. Burdened with a large stock of the 1872 hats that few soldiers could be induced to draw, Meigs eventually found a way to salvage part of the investment. He negotiated a contract to have the brims cut down and the crowns reblocked to simulate the 1876 campaign hat. This cosmetic surgery was accomplished at a cost of fifteen cents per hat. The soldiers who drew these hats quickly discovered the ruse because the material was, of course, of the same poor quality that had led to the demise of the folding hat in the first place. In a final, desperate act to rid the army of the folding hats, Meigs generously offered to sell these counterfeits to the officers at the current price of the regulation model. Finding few officers so gullible, he capitulated and ordered the remaining hats, both altered and unaltered, to be issued to the inmates at Fort Leavenworth Military Prison.”
YMH&OS,
Monterrey J. B.