Definitely a French 25th Regiment button. One detachment was sent over with Rochambeau for the Rhode Island to Yorktown campaign. If memory serves it's the Brie Regiment. I found one in brass in Maryland in the late 80's. Good job.
Great find! I prefer my wife's hair spray over the diluted white glue method. I have pewter buttons that look fine after 5 years with the hairspray treatment.
I don't do the glue thing because of the ability of the glue to attract moisture into it. Make sure it is totally dry and I lightly spray it with flat polyurethane, and pat off the excess before it dries. Never lost a button that way in over 25 years, and the poly can be removed if you want to.
Wow I love that kind of stuff and you have the history there. Cant find that kind of stuff here where I live. That is a great relic Thanks for posting it!!! Tommy
I use water-based clear lacquer. Paint it on with small paint brush, first the sides and back - little thicker on the back. Let it dry, and then do the face and dab (not wipe) to make it a very thin coat. Gets in the cracks and seals a button very well, and if done right doesn't make it look too glossy. Happy with the results and have been using that method for 15 years.