Bluing (steel)
Encyclopedia
Bluing is a passivation
process in which steel is partially protected against rust
, and is named after the blue-black appearance of the resulting protective finish. True gun bluing is an electrochemical conversion coating
resulting from an oxidizing chemical reaction with iron on the surface selectively forming magnetite
(Fe3O4), the black oxide of iron
. Black oxide provides minimal protection against corrosion, unless also treated with a water-displacing oil to reduce wetting and galvanic action. A distinction can be made between traditional bluing and some other more modern black oxide
coatings, although bluing is a subset of black oxide coatings.
In comparison, rust
, the red oxide of iron
(Fe2O3), undergoes an extremely large volume change upon hydration; as a result, the oxide easily flakes off causing the typical reddish rusting away of iron. "Cold", "Hot", "Rust Blue" and "Fume Blue" are oxidizing processes simply referred to as bluing.
"Cold" bluing is generally a selenium dioxide based compound that colors steel black, or more often a very dark gray. It is a difficult product to apply evenly, offers minimal protection and is generally best used for small fast repair jobs and touch-ups.
The "Hot" process, is an alkali salt solution referred to as "Traditional Caustic Black" that is typically done at an elevated temperature 275°F (135°C) to 310°F (154.44°C) . This method was adopted by larger firearm companies for large scale, more economical bluing. It does provide good rust resistance which is improved with the use of oil.
"Rust Bluing" and "Fume Bluing" provide the best rust and corrosion resistance as the process continually converts any metal that is capable of rusting into (Fe3O4). Treating with an oiled coating enhances the protection offered by the bluing. This process is also the only process safely used to re-blue vintage shotguns. Many double barreled shotguns are silver brazed
together and many of the parts are attached by that method also. The higher temperatures of the other processes as well as their caustic nature can weaken the brazed joints and make the gun hazardous to use.
s (0.0001 inches). For this reason, it is considered not to add any appreciable thickness to precisely-machined gun parts.
New gun
s are typically available in blued finish options offered as the least-expensive finish, and this finish is also the least effective at providing rust resistance, relative to other finishes such as Parkerizing
or hard chrome plating
or nitriding processes like Tenifer.
Bluing is also used for providing coloring for steel parts of fine clocks and other fine metalwork.
Bluing is often a hobbyist endeavor, and there are many methods of bluing, and continuing debates about the relative efficacy of each method.
Historically, razor blade
s were often blued steel. A non-linear resistance property of the blued steel of razor blades, foreshadowing the same property that would later be discovered in semi-conductor diode
junctions, along with the ready availability of blued steel razor blades, led to the use of razor blades as a detector in the crystal set AM radios which were often built by soldiers during World War II.
, sodium hydroxide, and water heated to the boiling point, 275°F to 310°F depending on the recipe. Similarly, stainless steel parts of the gun to be blued are immersed in a mixture of nitrates and chromates, similarly heated. Either of these two methods is called hot bluing. There are many other methods of hot bluing. Hot bluing is the current standard in gun bluing, both it and either method of Rust Bluing provide the most permanent degree of rust-resistance and cosmetic protection of exposed gun metal.
Rust bluing was developed between hot and cold bluing processes. It was originally used by gunsmiths in the 19th century to blue firearms prior to the development of hot bluing processes. The process was to coat the gun parts in an acid solution, let the parts rust uniformly, then immerse the parts in boiling water to stabilize the rusting process by removing any remaining residue from the applied acid solution. Then the rust was carded (scrubbed) off, using a carding brush or wheel. A carding brush is a wire brush with very soft, thin (usually about .002 thick) wires. This process is repeated until the desired depth of color is achieved or the metal simply will not color any further. This means they will not rust any further with the application of acid solution. This is one of the reasons Rust and Fume bluing tend to be more rust resistant than any other method. The parts are then oiled and allowed to stand overnight. This process leaves a deep blue/black finish.
Fume bluing is another process similar to rust bluing. Instead of applying the acid solution directly to the metal parts, the parts are placed in a sealed cabinet with a moisture source, a container of nitric acid and a container of hydrochloric acid
. The cabinet is then sealed. The mixed fumes of the acids will produce a uniform rust on the surface of the parts (inside and out) in about 12 hours. The parts are then boiled in distilled water, blown dry, then carded, (just like when rust bluing).
These processes were later abandoned by major firearm manufacturers as it often took parts days to finish completely, and was very labor intensive. It is still sometimes used by gunsmiths to obtain an authentic finish for a period gun of the time that rust bluing was in vogue, analogous to the use of browning on earlier representative firearm replicas. Rust bluing is also used on shotgun barrels that are soldered to the rib between the barrels, as hot bluing solutions would dissolve the solder during the bluing process.
There are also methods of cold bluing, which do not require heated solutions. Commercial products are widely sold in small bottles for cold bluing firearms, and these products are primarily used by individual gun owners for implementing small touch-ups to a gun's finish, to prevent a small scratch from becoming a major source of rust on a gun over time. At least one of the cold bluing solutions contains selenium dioxide
, to accomplish the bluing. Cold bluing is not particularly resistant to holster wear, nor does it provide a large degree of rust resistance. It does, however, often provide an adequate cosmetic touch-up of a gun's finish when applied and additionally oiled on a regular basis, however, rust bluing small areas will often match and blend better and wear better than any cold bluing process.
Large scale industrial hot bluing is often performed using a bluing furnace. This is an alternative method for creating the black oxide coating. In place of using a hot bath (although at a lower temperature) chemically-induced method, it is possible through controlling the temperature to heat steel precisely such as to cause the formation of black oxide selectively over the red oxide. It, too, must be oiled to provide any significant rust resistance.
Color case hardening : This is the predecessor of all metal coloring typically employed in the firearms industry. Contemporary heat-treatable steels did not exist or were in their infancy. Low-carbon steel was used, but strong materials were needed for the receivers of firearms. Initially case hardening was used but didn't offer any aesthetics. Color case hardening occurs when soft steels were packed in a reasonably airtight crucible in a mixture of charred leather, bone charcoal and wood charcoal. This crucible was heated to 1350°F for up to 6 hours (the longer the heat was applied the thicker the case hardening). At the end of this heating process the crucible is removed from the oven and positioned over a bath of water with air forced through a perforated coil in the bottom of the bath. The bottom of the crucible is opened allowing the contents to drop into the rapidly bubbling water. The differential cooling causes patterns of colors to appear as well as hardening the part.
Holster
wear will remove hot bluing, rust or fume bluing over long periods of use; it will remove cold bluing over relatively short periods of use, from any wear areas that are "touched up" with cold bluing solutions.
Browning is controlled red rust Fe2O3 and is also known as pluming or plum brown. One can generally use the same solution to brown as to blue. The difference is immersion in boiling water for bluing. The rust then turns to black-blue Fe3O4. Many older browning and bluing formulae are based on corrosive solutions (necessary to cause metal to rust), and often contain cyanide
solutions that are especially toxic to humans.
Passivation
Passivation is the process of making a material "passive", and thus less reactive with surrounding air, water, or other gases or liquids. The goal is to inhibit corrosion, whether for structural or cosmetic reasons. Passivation of metals is usually achieved by the deposition of a layer of oxide...
process in which steel is partially protected against rust
Rust
Rust is a general term for a series of iron oxides. In colloquial usage, the term is applied to red oxides, formed by the reaction of iron and oxygen in the presence of water or air moisture...
, and is named after the blue-black appearance of the resulting protective finish. True gun bluing is an electrochemical conversion coating
Conversion coating
Conversion coatings are coatings for metals where the part surface is converted into the coating with a chemical or electro-chemical process. Examples include chromate conversion coatings, phosphate conversion coatings, bluing, black oxide coatings on steel, and anodizing. They are used for...
resulting from an oxidizing chemical reaction with iron on the surface selectively forming magnetite
Magnetite
Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...
(Fe3O4), the black oxide of iron
Iron oxide
Iron oxides are chemical compounds composed of iron and oxygen. All together, there are sixteen known iron oxides and oxyhydroxides.Iron oxides and oxide-hydroxides are widespread in nature, play an important role in many geological and biological processes, and are widely utilized by humans, e.g.,...
. Black oxide provides minimal protection against corrosion, unless also treated with a water-displacing oil to reduce wetting and galvanic action. A distinction can be made between traditional bluing and some other more modern black oxide
Black oxide
Black oxide or blackening is a conversion coating for ferrous materials, copper and copper based alloys, zinc, powdered metals, and silver solder. It is used to add mild corrosion resistance and for appearance. To achieve maximum corrosion resistance the black oxide must be impregnated with oil or...
coatings, although bluing is a subset of black oxide coatings.
In comparison, rust
Rust
Rust is a general term for a series of iron oxides. In colloquial usage, the term is applied to red oxides, formed by the reaction of iron and oxygen in the presence of water or air moisture...
, the red oxide of iron
Iron(III) oxide
Iron oxide or ferric oxide is the inorganic compound with the formula Fe2O3. It is one of the three main oxides of iron, the other two being iron oxide , which is rare, and iron oxide , which also occurs naturally as the mineral magnetite. As the mineral known as hematite, Fe2O3 is the main...
(Fe2O3), undergoes an extremely large volume change upon hydration; as a result, the oxide easily flakes off causing the typical reddish rusting away of iron. "Cold", "Hot", "Rust Blue" and "Fume Blue" are oxidizing processes simply referred to as bluing.
"Cold" bluing is generally a selenium dioxide based compound that colors steel black, or more often a very dark gray. It is a difficult product to apply evenly, offers minimal protection and is generally best used for small fast repair jobs and touch-ups.
The "Hot" process, is an alkali salt solution referred to as "Traditional Caustic Black" that is typically done at an elevated temperature 275°F (135°C) to 310°F (154.44°C) . This method was adopted by larger firearm companies for large scale, more economical bluing. It does provide good rust resistance which is improved with the use of oil.
"Rust Bluing" and "Fume Bluing" provide the best rust and corrosion resistance as the process continually converts any metal that is capable of rusting into (Fe3O4). Treating with an oiled coating enhances the protection offered by the bluing. This process is also the only process safely used to re-blue vintage shotguns. Many double barreled shotguns are silver brazed
Brazing
Brazing is a metal-joining process whereby a filler metal is heated above and distributed between two or more close-fitting parts by capillary action. The filler metal is brought slightly above its melting temperature while protected by a suitable atmosphere, usually a flux...
together and many of the parts are attached by that method also. The higher temperatures of the other processes as well as their caustic nature can weaken the brazed joints and make the gun hazardous to use.
Usage
Bluing is most commonly used by gun manufacturers, gunsmiths and gun owners to improve the cosmetic appearance of, and provide a measure of corrosion resistance to, their firearms. It was also used by machinists, who protected and beautified tools made for their own use. Bluing also helps to maintain the metal finish by resisting tangential scratching, and also helps to reduce glare to the eyes of the shooter when looking down the barrel of the gun. All blued parts still need to be properly oiled to prevent rust. Bluing, being a chemical conversion coating, is not as robust against wear and corrosion resistance as plated coatings, and is typically no thicker than 2.5 micrometreMicrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
s (0.0001 inches). For this reason, it is considered not to add any appreciable thickness to precisely-machined gun parts.
New gun
Gun
A gun is a muzzle or breech-loaded projectile-firing weapon. There are various definitions depending on the nation and branch of service. A "gun" may be distinguished from other firearms in being a crew-served weapon such as a howitzer or mortar, as opposed to a small arm like a rifle or pistol,...
s are typically available in blued finish options offered as the least-expensive finish, and this finish is also the least effective at providing rust resistance, relative to other finishes such as Parkerizing
Parkerizing
Parkerizing is a method of protecting a steel surface from corrosion and increasing its resistance to wear through the application of an electrochemical phosphate conversion coating...
or hard chrome plating
Chrome plating
Chrome plating, often referred to simply as chrome, is a technique of electroplating a thin layer of chromium onto a metal object. The chromed layer can be decorative, provide corrosion resistance, ease cleaning procedures, or increase surface hardness.-Process:A component to be chrome plated will...
or nitriding processes like Tenifer.
Bluing is also used for providing coloring for steel parts of fine clocks and other fine metalwork.
Bluing is often a hobbyist endeavor, and there are many methods of bluing, and continuing debates about the relative efficacy of each method.
Historically, razor blade
Razor blade
Razor blade may refer to* A razor* The Razor Blade, a 1920s racing car* Razor blade steel, a type of steel originally designed specifically for razor blades...
s were often blued steel. A non-linear resistance property of the blued steel of razor blades, foreshadowing the same property that would later be discovered in semi-conductor diode
Diode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
junctions, along with the ready availability of blued steel razor blades, led to the use of razor blades as a detector in the crystal set AM radios which were often built by soldiers during World War II.
Hot versus cold bluing
Bluing may be applied, for example, by immersing the steel parts of the gun to be blued in a solution of potassium nitratePotassium nitrate
Potassium nitrate is a chemical compound with the formula KNO3. It is an ionic salt of potassium ions K+ and nitrate ions NO3−.It occurs as a mineral niter and is a natural solid source of nitrogen. Its common names include saltpetre , from medieval Latin sal petræ: "stone salt" or possibly "Salt...
, sodium hydroxide, and water heated to the boiling point, 275°F to 310°F depending on the recipe. Similarly, stainless steel parts of the gun to be blued are immersed in a mixture of nitrates and chromates, similarly heated. Either of these two methods is called hot bluing. There are many other methods of hot bluing. Hot bluing is the current standard in gun bluing, both it and either method of Rust Bluing provide the most permanent degree of rust-resistance and cosmetic protection of exposed gun metal.
Rust bluing was developed between hot and cold bluing processes. It was originally used by gunsmiths in the 19th century to blue firearms prior to the development of hot bluing processes. The process was to coat the gun parts in an acid solution, let the parts rust uniformly, then immerse the parts in boiling water to stabilize the rusting process by removing any remaining residue from the applied acid solution. Then the rust was carded (scrubbed) off, using a carding brush or wheel. A carding brush is a wire brush with very soft, thin (usually about .002 thick) wires. This process is repeated until the desired depth of color is achieved or the metal simply will not color any further. This means they will not rust any further with the application of acid solution. This is one of the reasons Rust and Fume bluing tend to be more rust resistant than any other method. The parts are then oiled and allowed to stand overnight. This process leaves a deep blue/black finish.
Fume bluing is another process similar to rust bluing. Instead of applying the acid solution directly to the metal parts, the parts are placed in a sealed cabinet with a moisture source, a container of nitric acid and a container of hydrochloric acid
Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....
. The cabinet is then sealed. The mixed fumes of the acids will produce a uniform rust on the surface of the parts (inside and out) in about 12 hours. The parts are then boiled in distilled water, blown dry, then carded, (just like when rust bluing).
These processes were later abandoned by major firearm manufacturers as it often took parts days to finish completely, and was very labor intensive. It is still sometimes used by gunsmiths to obtain an authentic finish for a period gun of the time that rust bluing was in vogue, analogous to the use of browning on earlier representative firearm replicas. Rust bluing is also used on shotgun barrels that are soldered to the rib between the barrels, as hot bluing solutions would dissolve the solder during the bluing process.
There are also methods of cold bluing, which do not require heated solutions. Commercial products are widely sold in small bottles for cold bluing firearms, and these products are primarily used by individual gun owners for implementing small touch-ups to a gun's finish, to prevent a small scratch from becoming a major source of rust on a gun over time. At least one of the cold bluing solutions contains selenium dioxide
Selenium dioxide
Selenium dioxide is the chemical compound with the formula SeO2. This colorless solid is one of the most frequently encountered compounds of selenium.-Properties:...
, to accomplish the bluing. Cold bluing is not particularly resistant to holster wear, nor does it provide a large degree of rust resistance. It does, however, often provide an adequate cosmetic touch-up of a gun's finish when applied and additionally oiled on a regular basis, however, rust bluing small areas will often match and blend better and wear better than any cold bluing process.
Large scale industrial hot bluing is often performed using a bluing furnace. This is an alternative method for creating the black oxide coating. In place of using a hot bath (although at a lower temperature) chemically-induced method, it is possible through controlling the temperature to heat steel precisely such as to cause the formation of black oxide selectively over the red oxide. It, too, must be oiled to provide any significant rust resistance.
Other methods of bluing and coloring
Niter bluing : This is not a chemical means of bluing. Parts to be Niter blued are steel which has been polished and cleaned, then immersed in a bath of molten salts; typically potassium nitrate and sodium nitrate (sometimes with 1/3 oz of manganese dioxide per lb of total nitrate). The mixture is heated to 590°F to 610°F and the parts are suspended in this solution with wire. The parts must be observed constantly for color change. The cross section and size of parts will affect the outcome of the finish and time it takes to achieve. This method must not be used on critically heat-treated parts such as receivers or slides. It is generally employed on smaller parts such as pins, screws, sights, etc. The colors will range through straw, gold, brown, purple, blue, teal, then black. Examples of this finish can be seen commonly on older pocket watches whose hands exhibit what is called "peacock blue", a rich iridescent blue.Color case hardening : This is the predecessor of all metal coloring typically employed in the firearms industry. Contemporary heat-treatable steels did not exist or were in their infancy. Low-carbon steel was used, but strong materials were needed for the receivers of firearms. Initially case hardening was used but didn't offer any aesthetics. Color case hardening occurs when soft steels were packed in a reasonably airtight crucible in a mixture of charred leather, bone charcoal and wood charcoal. This crucible was heated to 1350°F for up to 6 hours (the longer the heat was applied the thicker the case hardening). At the end of this heating process the crucible is removed from the oven and positioned over a bath of water with air forced through a perforated coil in the bottom of the bath. The bottom of the crucible is opened allowing the contents to drop into the rapidly bubbling water. The differential cooling causes patterns of colors to appear as well as hardening the part.
Limitations
Bluing only works on steel or stainless steel parts for protecting against corrosion. Because it changes the Fe into Fe3O4, it does not work on non-ferrous material. Aluminum and polymer parts are largely unaffected by bluing; no protection against corrosion is provided by bluing processes on them, although uneven staining of the aluminum and polymer parts can be caused by attempts at bluing.Holster
Holster
A handgun holster is a device used to hold or restrict the undesired movement of a handgun, most commonly in a location where it can be easily withdrawn for immediate use.-Function:...
wear will remove hot bluing, rust or fume bluing over long periods of use; it will remove cold bluing over relatively short periods of use, from any wear areas that are "touched up" with cold bluing solutions.
Terminology
Some prefer to call thin coatings of black oxide by the name gun bluing, and to call heavier coatings by the name black oxide, but they are both the same chemical conversion process for providing true gun bluing.Browning is controlled red rust Fe2O3 and is also known as pluming or plum brown. One can generally use the same solution to brown as to blue. The difference is immersion in boiling water for bluing. The rust then turns to black-blue Fe3O4. Many older browning and bluing formulae are based on corrosive solutions (necessary to cause metal to rust), and often contain cyanide
Cyanide
A cyanide is a chemical compound that contains the cyano group, -C≡N, which consists of a carbon atom triple-bonded to a nitrogen atom. Cyanides most commonly refer to salts of the anion CN−. Most cyanides are highly toxic....
solutions that are especially toxic to humans.
Further reading
- Angier,R.H. Firearm Bluing and Browning,London 1936.
- Fishlock,D. Metal Colouring,Teddington 1962.