R.H.Angier in his book FIREARM BLUEING and BROWING , Copyright Thomas G. Samworth, 1936, The Telegraph Press.Pa
says in the introduction (pp 1,2)
"The production of a dark colour on iron and steel objects, whether for ornamentation or other purposes, reaches far back in the history of metal working...
Improved methods followed the development of chemistry, and the oldest browning process for military arms, by means of "butter of antimony" (according to G. Buchner) is described in the Hanover Magazine in 1781.
In the literature on the browning of gun barrels with this substance, originated in England towards the end of the 18th century, which may be quite correct as regards military arms. The browning of barrels is in itself however much older, even in England, and according to information kindly given the author by Mr. C. E. Greener (the well-known Birmingham gunmaker), was in common use for sporting arms about 1720. A report of 1637 in the London Record Office explicitly mentions the "russetting" of barrels under the heading of "Repairs to the Arms of the Trained Bands" (London Militia).
A similar development took place at the same time on the European Continent: guns of the 18th century with blued (temper blued) and browned barrels are numerous, and of the 17th century occasionally met with, in every larger public or private collection of firearms.
Exactly the same advance took place on the American Continent, and the American backwoodsmen, using the simplest possible means, browned the barrels of their historically famous, home-made rifles, from about the middle of the 18th century onwards..."