Photographs by Delbert Whitman Jr.
As we’ve noted in previous examinations of the gunmaker’s art, nearly every facet of a fine shotgun is defensible from a functional standpoint. What this means is that even those elements of a gun that possess significant aesthetic appeal (e.g., engraving, heel and toe plates, orientation of figure in the stock wood) typically bear some functional value. Nowhere is this truer than in the treatments and finishes applied to surface metal, as exemplified by the rich, inky blue-black finish synonymous with gun barrels and certain bits of furniture. Any gun worthy of its name showcases some blued steel, and any exceptional gun showcases steel blued to a depth and luster that make an impact upon even the untrained eye.
Exceptional bluing of barrels and gun furniture not only looks good but, as previously alluded to, also serves a functional purpose. It is, in a sense, a protective finish for metal surfaces that prohibits the growth of rust that can lead to pitting or other blemishes. What is unique about bluing, however, is the multifaceted chemical/molecular/physical process by which it protects the underlying metal. Bluing is a process that cultivates a pervasive layer of iron oxide that physically outcompetes the formation of “bad” rust while also creating a protective layer structured to hold oil. In tandem these physical and chemical processes create an unwelcome environment for detrimental oxidization to occur. That the blued surface also appears beautiful is a bonus.
In order to effectively describe the functional value of bluing, the process must be understood on a microscopic level. There are two commonly referred to types of bluing: rust bluing (also called blacking) and hot, or caustic, bluing. Both processes achieve similar ends, but each requires different steps. The more time-honored method as far as gunmakers are concerned is rust bluing.
Rust bluing is a process of cultivating a thin, hard, cultured surface of iron oxide on the surface of steel. This iron oxide has a unique texture that, under a microscope, exhibits multiple small fragments that fit together like interlocking three-dimensional puzzle pieces. Each fragment has an ample surface area, which in aggregate makes the iron-oxide-coated surface a welcome receptacle for protective oil. The iron-oxide surface absorbs and holds oil or other rust-inhibiting compounds like a sponge, with the capillary action of the spaces within the matrix drawing in these oils and holding them. Hence, it is not so much that the iron oxide actually inhibits corrosion, but rather it affords a surface that holds oil well.
Each gunsmith has his or her ‘secret bluing solution.’
Rust bluing is a very old method that has long served to protect gun barrels, but it has been used on trigger guards, toplevers and other furniture as well. To rust blue any piece of steel, the surface must first be extensively prepped. This requires thorough sanding at 320 grit or finer to an even polish, after which the surface must be cleaned and thoroughly de-greased. De-greasing can be done with a solvent like alcohol or acetone or with a calcium-based compound like whiting (calcium carbonate) that effectively absorbs all of the grease or oil. Once the polished metal is exceedingly clean and oil free, a gunsmith or gunmaker can apply a rusting, or bluing, solution comprised of chemicals that cause rust to take hold. Each gunsmith has his or her “secret bluing solution.” (Author R.H. Angier’s definitive Firearm Blueing & Browning has hundreds of recipes. Note that “browning” is a process applied to Damascus barrels that results in a reddish-brown finish.) The universal characteristic of these solutions is that their acidic character causes rust to take hold quickly and uniformly. This stage in the rust bluing process requires that a gunsmith wipe a coat of solution all over the surface metal and leave it to rust for a specified period in a controlled environment.
The rate of rust growth is important, to ensure even coverage, and the rusting rate is managed by controlling the humidity and temperature of the environment. Author Del Whitman uses a plywood “sweat box” to accurately control iron-oxide formation, which is only allowed to progress to a specified point. The rust that forms during this stage in the process is known colloquially as “red rust,” red oxide or ferrous oxide (Fe2O3).
Once the gunsmith witnesses the desired level of red-oxide formation on the surface of the steel, the part is removed from the sweat box and boiled in pure distilled water. The period of boiling generally takes less than 10 minutes. At this point the boiling water loosens the bonds of hydrogen and oxygen atoms, allowing free oxygen atoms in the water bath to bond with ferrous oxide (FeO), turning it into black rust, clack oxide or ferric oxide (Fe3O4). The part, removed from the water bath and allowed to dry, is now ready for carding.
Carding refers to the process by which the gunsmith uses a fine wire brush or low-velocity fine wire wheel to remove almost all of the cultured black rust down to a uniform, thin layer. Iron oxide is very hard, and carding permits only the finest, hardest bottom layer to remain intact on the surface metal. This process of adding a bluing compound, sweating, boiling and carding is then repeated numerous times, and the hard layers of ferric oxide are stacked on top of each other. For this reason, a good rust bluing should showcase a depth of color that is jet, inky black. Rust bluing is incredibly labor intensive.
Variations in the final sheen of the rust bluing depend on the beginning level of polish, how long the barrels are allowed to rust, how hard the black oxide is carded and so on. Once the desired depth of color and luster are achieved, the final surface is wiped with a light coat of quality oil, such as Clenzoil or LPS 3 Premier Rust Inhibitor, which permeates the microscopic pores of the blued metal and results in a lustrous, protective sheen.
Different hot bluing salt recipes result in very different color tones.
Though rust bluing is the time-honored method, modern gunsmiths and gunmakers have leaned increasingly on a process called hot, or caustic, bluing to achieve impressive results in a short time. The one detriment of hot bluing is that the caustic compound that initiates rust is highly corrosive, especially to lead and tin solder joints. For that reason, soldered barrels cannot be hot blued because the ribs will fall off and the barrels will fall apart. Only brazed barrels and gun furniture can be blued in this manner.
Hot bluing accomplishes a similar result as rust bluing in the surface features and color of the blued metal, but it can be accomplished in short order with heat and an instant chemical reaction. Solutions of ammonium nitrate, sodium hydroxide or potassium hydroxide and similar caustic compounds are mixed into a water bath that is in turn heated to temperatures around 350° F. Different hot bluing salt recipes result in very different color tones (e.g., the blue-purple finish of Weatherby receivers differs dramatically from the black-chrome look of Winchester Model 21 receivers). Regardless of salt composition, these baths are extremely dangerous to work around, not only because of the potential for heat burns but also due to the extreme caustic nature of the solutions and the inherent potential of chemical burns.
With the caustic bath prepared, the gunsmith must clean and de-grease the metal part and dip it in the bath. The black-oxide layer that results is extremely tight-grained, which results in a high-gloss, shiny finish. The challenge with this type of finish is that the polish level of the underlying metal is hard to disguise, so poor prep and polish will show readily. Caustic bluing affords a quick, mirror-like finish that is often used for furniture like trigger guards, screw heads and so on. As with rust bluing, the potential for rust inhibition is quite high.
Following a re-blue, there is some work required to establish crisp margins for the blued surfaces. Barrel flats, breech faces and so on must be sanded and cleaned up, if bluing ran over, and engraving must often be re-cut or chased. All of these final efforts finish the bluing job as it should be finished, allowing the blued metal to showcase great color and definition. Moreover, with a fine blued finish on barrels and furniture, a gun owner can see to it that a fine gun is afforded a due level of protection—the provision of which further enhances the gun’s beauty and aesthetic value. Once again, functional beauty is the name of the game where the gunmaker’s art is concerned.
Delbert Whitman Jr. lives near Traverse City, Michigan, and is a professional gunsmith specializing in repair, restoration, stockmaking and engraving. Reid Bryant is an Editor at Large for Shooting Sportsman.