To many, (myself included) there is no finish on a firearm quite like the deep, dark finish of a fine bluing job. Once common in firearms manufacturing from both private and military sectors, bluing is now starting to be replaced by the use of paints, anodizing, and high-technology nitride finishes. Although it is becoming less common in production guns, high polish bluing jobs are commonly found and appreciated in vintage firearms and in custom built firearms today.
Many are familiar with the look of gun bluing, but not with the technical details. A little background in the process will help answer some questions, such as: why is bluing used?; why is it being replaced by other processes?; and what are the properties of blued steel?
Properties of Bluing and Oxidation
It's common knowledge that iron and its alloys (including carbon steel commonly used in guns and tools) develop rust when exposed to humidity and open air. For as long as tools have been made from iron, rust has been a problem, and rust preventative techniques have been used to keep rust from forming.
One relatively early way of doing this was called browning or russetting. In this method, steel was encouraged to rust - often through the use of chemicals that attracted humidity to the surface. A fine layer of rust was coaxed across the surface of the metal, with the whole workpiece then carded - a technical term for using a coarse instrument to remove the loose oxides. This process was repeated until the entire piece showed a smooth red-brown color. Most notably in gunsmithing, the russetting process (sometimes referred to as "plum brown") was commonly used on 16th-18th century muskets and muzzle loading firearms.
Russetting techniques use the natural oxidizing properties of iron and steel to form a surface layer of red rust, also known as hematite (Fe2O3). Somewhere along the way, it was discovered that boiling parts during the russetting process added enough activation energy to convert hematite (red oxidation) to the deep blue-black magnetite (Fe3O4). This process became known as rust bluing. Rust bluing created a deeper, harder finish that was more scratch resistant and less susceptible to rust in common use. The deep black color produced was very attractive as well and lent itself to use in commercial and military production.
I will be rust bluing a Mauser HSc at home and sharing the results, so we will get more into that particular process at the time. For now, I'm more concerned with a newer and more commonly used process.
Hot Caustic Salt Bluing
While rust bluing provided a great way to protect metal and create an attractive finish on steel with a low investment in resources, the process had one major drawback. Chemicals used for rust bluing had to be applied in a number of coats (typically between 3 and 10), each of which had to sit for hours or even days at a time to accumulate an even layer of rust before being boiled. For this reason, the man power required to blue large quantities of guns was significant, and this bottleneck became particularly apparent with wartime manufacturing of military components.
Later, a new process was discovered that solved this problem. Known as caustic bluing or salts bluing, parts were immersed in a boiling bath of water and corrosive salts. Instead of taking days to finish a firearm, an entire action and all small parts could be done with less than an hour of contact time with chemicals, and batches could be done as large as a tank could be made to hold and heat the chemicals. This process took off rapidly among manufacturers because of its expedience.
Naturally, hot salts bluing had a number of advantages:
- The process was much faster, easier to reproduce consistently, and required less manpower
- Larger quantities could be produced at a single time
- Unlike rust bluing, where the acid involved etches the metal to a uniform surface, salts bluing will show the metal preparation very faithfully. This allows craftsmen to highly polish the parts before immersion and get a very deep, thorough finish. This was seen in Colt's famous "Royal Blue" finishes.
These advantages were largely enough for manufacturers to switch to the process despite its few disadvantages:
- Hot salts bluing resulted in a finish that was not quite as resistant to rust and scratching as the multiple layers of rust bluing
- Finish preparation became much more important commercially as the hot blue would show every scratch and imperfection in the original polish of the metal
- Hot salts bluing requires the use of very caustic and hazardous chemicals which require extra safety and disposal precautions.
Now that we've been introduced with plenty of text, let's get to an example with some pictures.
An Example - My Custom .30-06 rifle
My first experience with hot bluing was a .30-06 rifle that I built in my school for gunsmithing. The rifle was built on a Montana Rifle Co. Model 1999 with a custom chambered barrel. To save a lot of technical detail (to be shared at another time) here is an image of the rifle shaped and assembled with neither wood nor metal finished:
The metal here is still in the condition known as "in the white", meaning raw steel. The finish at this point is not quite uniform due to the varying metals and manufacturing processes. At this point, the metal was polished by hand. The higher the polish, the deeper the resulting blue finish will be (consequently, those having bluing services done can expect to be charge much higher rates for the gunsmith to perform a mirror polish vs. a simple bead blasting or low grit polish).
From this point, the parts are polished, cleaned and degreased in denatured alcohol, and handled only with gloves. Any contact with bare hands or skin will leave oil on the surface of the metal which will inhibit the oxidation process and lead to a poor bluing job.
The bare metal is introduced to a series of tanks. First, a warm water bath with cleaning solution, After this, it will be introduced to an irrigated rinse before finally making its home in the bluing salts solution. In this case, our barreled actions stayed in the solution for over an hour. After the bath, they will be introduced to pure boiling water to neutralize the salts. The boiling action is necessary to penetrate inside crevices such as the barrel threads, where salts may become trapped and lead to serious corrosion issues later down the line. Then, it is placed in water displacing oil to seal, neutralize, and protect the new finish for rust. Here it is left for approximately 30 minutes.
After the soak in heavy weight oil, they will be cleaned and degreased one last time. Patches with solvent are run through the bore to remove the heavy oils, and all nooks and crannies are carefully swabbed. Then a lighter, more proper oil (such as Rem Oil) is applied for corrosion resistance. At this point, the parts can be wiped down and finally re-assembled.