What is Nitre Bluing?
The nitre bluing process (sometimes referred to as fire bluing, peacock bluing or niter bluing) is a gun metal finishing process involving a salt bath solution of sodium and potassium nitrates, heated to exact temperatures. Nitre bluing creates vibrant blue colors, but they are not always durable as charcoal bluing coloration. This process works best on screws and pins because they are subject to less wear and movement.
Acknowledgement: Parts of this article are adapted from “Antique Metal Finishes” by S.P. Fjestad and Doug Turnbull, found in the upcoming 2nd Edition of the Blue Book of Antique American Antiques & Values. (link takes you to 1st edition at Midway USA)