Metal Treatments: Other Treatments

We will now look at some coating treatments which are usually applied on top of some of the other treatments we studied previously to give it additional resistance to corrosion.

The first treatment we will look at is Teflon coating. Teflon was originally invented by DuPont in 1938 and is one of the slipperiest substances in the world. It is used to coat non-stick pots and pans in the kitchen, but is also used in industries like aerospace, electronics, architecture and now, firearms as well. Teflon can be applied on top of stainless steel parts, blued parts and metals like titanium and aluminum. A Teflon coat has more resistance to wear than bluing. It is also completely weatherproof and prevents rust from forming by preventing any humidity from reaching the metal underneath. After a teflon coat is applied, there is no need to oil the gun further as teflon is self lubricating and only gets more slipperier as it wears down. This means that there is no chance of the oil in the gun freezing in cold temperatures. Cleanup is also simple because all it takes is an air hose and a quick wipe down. It also comes in a few different colors, so if the user desires to have a black or a camouflage coating applied to a bright stainless steel barrel, it is possible to simply apply a teflon coat to take care of this.

The above is an All Weather Pistol manufactured by C.O. Arms. It is made of SAE 4140 carbon steel and aluminium and is coated with teflon. In this example, the firearm is coated black.

The above is a Colt Defender with a stainless steel slide and teflon coating. The teflon coat is clear in this example.

Another coat is called KG Gun Kote, which was invented by KG Industries LLC. This was originally designed to the specifications of the US Navy's SEAL team 1. The requirement was for a coat that would withstand at least 500 hours of a standard salt spray test (5% salt solution sprayed continuously in a chamber) and still meet the military machine gun firing requirements. Gun Kote is a phenolic resin that is designed to be applied by spraying on or dip spin. It is then thermally cured at temperatures between 300-325 F. The resulting coat is only 0.0003-0.0005 inches thick, but is tougher than bluing or parkerizing and has a pencil hardness greater than 9H. It is abrasion resistant, chemical and corrosion resistant and has lubricity. The lubrication is produced by molybdenum disulfide. Gun Kote has also been tested against a number of chemicals ranging from sulfuric acid to hydraulic oil and successfully resisted them all. One version not only passed the 500 hour salt-spray test, it actually lasted over a 1000 hours with no problems! The South African military tested it on an equivalent of a 7 year corrosion test, which it also passed. Like the teflon coat, it also comes in a number of colors. Since the original product, they have come up with a number of different formulations. Many manufacturers such as Kimber, Buzztail etc. use KG Gun Kote and it can be applied to other vendors products by third party gunsmiths as well.

Colt 1911 pistol coated with Gun Kote

Another coating treatment is Duracoat by Lauer Custom Weaponries. It is an extremely hard polymer, which is resistant to oil and cleaning chemicals. It is also somewhat self-lubricating. Duracoat hardness is between H than 2H pencil hardness and it has good impact resistance as well. It can easily pass a 300 hour salt spray test. It can be applied to almost any surface (metal, plastic, wood) as long as proper preparation of the surface and pre-treatment is done. The substance is quick drying -- it is dry to the touch in 20 minutes and the weapon can be used in 6-8 hours. It continues to cure for 4-6 weeks afterwards though. Duracoat is available in a very wide range of colors (over 130 standard colors and 11 standard camouflage patterns). One of the very good advantages of Duracoat is that it is very temperature resistant and can easily withstand over 1800 degrees F. Therefore, it is an excellent choice for weapons with a high sustained rate of fire, such as LMGs or fully-automatic rifles.

Glock pistol coated with neon pink duracoat :D

FN rifle painted with camouflage duracoat colors.

Another coating treatment is CeraKote invented by NIC industries. This is a ceramic based coating and comes in two types: "C" series, which cures in normal temperatures and "H" series, which is a thermal cure coating. Cerakote offers very good abrasion resistance, corrosion resistance and hardness. Because of its ceramic base, it is harder and over 60% more wear-resistant than teflon. Since it is also self-lubricating, a firearm requires very little oil on top of it. The C series has a pencil hardness of 7H and can withstand over 550 hours of standard salt spray test. It can also withstand over 1200 degrees F over extended periods of time without failing, which makes it very suitable for weapons with high rates of fire. The "H" series can pass 2500 hours of salt spray test and withstand 550 F temperature for extended periods of time. It comes in 31 colors.

NIC industries offers a test page which allows the user to test various color options for Cerakote on a Glock pistol.

Metal Treatments: Stainless steel

In most of the previous methods we've studied, the method of reducing corrosion has been to infuse a thin layer of corrosion resistant material on the outer surface of the iron or steel parts of a firearm. In this post, we will study another method, where the material used to make the firearm parts is itself corrosion-resistant. We're talking about stainless steel.

Stainless steel is basically a steel alloy with at least 10.5 % chromium content by mass, or more. It does not stain as easily as ordinary steel, which is why it got its name. It also does not rust or corrode as easily as ordinary steel. Despite what the name suggests though, it is not completely stain-proof (or rust-proof, for that matter).

Some of the early research into corrosion resistant steels happened in Germany in the 1890s and 1900s and Krupp engineers even patented an autenitic stainless steel in 1912. It is fitting though, that Harry Brearley of Brown-Firth research lab of Sheffield, invented a martensitic stainless steel alloy in 1912, while they were seeking a better alloy for the manufacture of firearms! The lab was given a project to try and study how to reduce the erosion on the inside of barrels that is caused by high temperatures. Harry Brearley was in charge of researching this problem. By a stroke of luck, he noticed that one of his samples had shown no signs of rust, even after being immersed in water and exposed to air for a long time. He analyzed this piece of steel and determined that it was the high chromium content that was causing it to not rust and thus "rustless steel" was born in 1912. The name was later changed to "stainless steel" due to a suggestion from a local Sheffield firm that specialized in making cutlery items.

There are many different grades of stainless steel with differing mechanical and chemical properties. For firearms, the grade usually chosen is SAE 416 grade. This is a martensitic steel with sulfur content, that has good machining properties and can be hardened later with heat treatment. Due to its good machining properties, barrels made with stainless steel exhibit better accuracy than made with chrome-molybdenum steel. This is why target shooters prefer stainless steel barrels for their weapons. Stainless steel is more expensive though and also has slightly less life than chrome-molybdenum steel barrels due to less hardness, which is why military weapons use chrome-molybdenum steel.

The finish looks very much like a nickel plated steel weapon. Like nickel-plate, the finish can be bright, brushed or matte. However, since nickel plated steel only has a thin outer finish, it is subject to flaking and peeling off of the plated layer. This is not the case with a stainless steel weapon.

While there were revolvers with some stainless steel parts before, the Smith & Wesson Model 60 revolver pictured above has the distinction of being the first regular production model revolver to be completely made of stainless steel. It was been in continuous production since 1965. When it was first introduced, the model was so popular that most gun shops had a waiting list of 6 months to purchase one.

The above Smith and Wesson M1911 model is also made of stainless steel. Feel free to compare and contrast this with the 1911s that have been nickel plated and chrome plated. Smith & Wesson did not produce the first all stainless steel 1911 model though. That honor goes to the Arcadia Machine and Tool company which produced their all stainless-steel model of the M1911 in 1977.

Stainless steel does look better on some weapons and it is easier to machine, so that makes it easier for manufacturers. However, it also has some disadvantages. While it is rust resistant, it is not completely rust-proof and so, care of the weapon is recommended. Also, it is not quite as hard as a carbon-steel weapon, so stainless steel with bend and scratch easier than an equivalent blued carbon-steel weapon. Light scratches are easier to buff out on stainless steel though and will not show as easily as a blued weapon. Stainless steel is only hardened to about 25-32 HRC on the Rockwell Hardness scale, so it is not so hard as some of the other metal treatments. Galling of the moving parts was an issue with early stainless steel weapons, but the problem has largely been solved now with better lubricants.

Metal Treatments: Ferritic Nitrocarburizing/Melonite/Tenifer

In the last few posts, we looked into metal treatments like plating, parkerizing and bluing. In this post, we will study a newer form of metal treatment that has rapidly gained popularity in the firearms industry since the 80s. This metal treatment process is called Ferritic Nitrocarburizing, but there are several variants of this. The most popular variants are known by their trademarked names of Tenifer and Melonite and the main difference between these two are the chemicals used during the process. Both these trademarks, along with Tufftride, are owned by the same company, Degussa of Germany. The Degussa website explains that Tenifer and Tufftride are actually the same process and the only reason for the different names is because they couldn't get the trademark Tenifer registered in all countries and therefore use Tufftride in the countries where they couldn't get the name Tenifer registered.

To understand this method, we must first understand a few basic properties of iron and steel. Some materials, such as iron and steel, can exist with different crystalline structures. These different crystalline structures cause the same material to have different physical properties (e.g. different hardness, elasticity etc.). These different crystal structures are called "phases". Examples of such phases are: ferritic phase, austenitic phase, martensitic phase, ledeburite phase, pearlite phase etc. Both iron and steel can be switched from one phase to another by heating to different temperatures and adding other elements and cooling at different rates to change the crystalline structure of the product. The diagram below illustrates the temperatures and carbon content % that cause steel to change from one phase to another.

Steel Phase Diagram

Licensed under the Creative Commons Attribution-Share Alike 3.0 Unported License by Christopher Dang Ngoc Chan.

The basic concept behind the Ferritic Nitrocarburizing method is to introduce nitrogen and carbon to the steel when its crystal structure is still in the Ferritic phase. The temperature when this is done ranges from between 525-650 degrees centigrade. The first Ferritic Nitrocarburizing treatment process was invented by UK chemical giant, Imperial Chemical Laboratories (ICL), who came up with a process of dropping the workpiece in a sulfur salt bath at 550 degrees centigrade. This process was called Sulfinuz treatment because of the sulfur salt content. It was mainly used for cutting tools and high speed spindle parts, but it had problems with cleaning the solution off.

Degussa of Germany came up with a more environment friendly salt-bath process, which they called Tenifer in most of Europe and Tufftride in England and Asia. They later improved on this by inventing an ion nitriding process in the early 1980s. The ion nitriding process was faster and more precise to control. As far as the firearms industry is concerned though, the processes used for metal treatment use the salt-bath. Tenifer and Melonite use the same process, but slightly different chemicals and temperatures. Melonite treatment is known to be the simpler of the processes. The process starts by creating a salt bath of alkali cyanate in a steel vessel. The steel vessel has a pipe that creates bubbles and aerates the salt bath. The workpiece is introduced into the bath and the cyanate reacts with the workpiece to form an alkali carbonate. The bath is then treated with a regenerator chemical to convert the carbonate back to a cyanate. This causes two layers to form on the surface: the compound layer and the diffusion layer. The compound layer has iron, nitrogen and oxygen and is resistant to abrasion and wear. The diffusion layer has nitrides and carbides and provides extra hardness. The end result is a corrosion resistant finish that is extremely hard and corrosion resistant.

The Tenifer process was traditionally used in the German automotive industry for years, by manufacturers such as BMW and Mercedes. Glock GmBH, which was then an unknown Austrian manufacturer, was the first to use it in the firearms industry in the 1980s. All Glock pistols come with Tenifer treatment and it became well known to the firearms industry because of their success. It is now used by other manufacturers as well, such as Steyr, Walther, Heckler & Koch etc. It is renowned for its hardness and toughness. Tenifer has a dull-gray color and has a hardness of 64 HRC on the Rockwell scale, which is very hard, considering that diamond has a hardness of 70 HRC. Tenifer is also extremely corrosion resistant and is at least 85% more corrosion resistant that hard chrome plating and almost completely salt-water resistant as well. It also has excellent anti-friction properties. Glock generally applies a tenifer coat of 0.5 mm thickness to the slides and barrels of their pistols. The slides are further subject to parkerizing treatment on top of that. So, even if the parkerized finish were to wear off, the slide is still protected by the Tenifer layer.

Glock 17 pistol. The gray slide on the top of the pistol is treated with Tenifer, as is the barrel.

Tenifer's properties have reached semi-legendary status. There are lots of videos and articles available on the Internet showing how hard and tough it is. People have subject their Glocks to ocean immersion for months and sharpened their knives with glocks, all without any effect on the finish!

While tenifer treatment is a very excellent process of metal treatment, it has one legal disadvantage -- it cannot be done in the United States, due to environmental laws regulating the use of certain cyanide salt chemicals and the amount of byproduct cyanide generated by the process allegedly exceed EPA limits. The original process as done in Europe uses 60% Sodium Cyanide and Cyanate and 40% Potassium Cyanide and Cyanate.

Hence, American companies such as Smith and Wesson or Springfield Arsenal use the Melonite treatment process instead, which is also a ferritic nitrocarburizing process, but uses different salts and a slightly modified process to produce the same results. Melonite can be used on such steel grades as 416 stainless and 4140. However, it has the disadvantage of actually removing some of the properties of 416 stainless steel. The melonite process also produces a black finish instead of the gray color of Tenifer.

Ferritic Nitrocarburizing treatments produce some of the most corrosion resistant and hardest metal treatments in existence. After Glock pioneered their use in the industry, traditional firearms manufacturers are slowly beginning to adopt this technology to their products.

Metal Treatments: Plating

The next method of metal treatment we will study is called plating. There are two forms of plating: electroplating and electroless plating.

The general technique behind electroplating is to take the part to be plated and connect it to the negative terminal (cathode) of a DC current power source. The metal that forms the coating is connected to the positive terminal (anode) of the power source. Both are placed in a electrolyte solution that contains one or more dissolved metal salts that permit the flow of electricity. Metal from the positive terminal end (the anode) dissolves in the electrolyte and then deposits on the metal connected to the negative terminal (the cathode). In some cases, the anode is not consumed by the process, but instead, the metal salts dissolved in the electrolyte are deposited on the cathode. When electroplating, the cathode is generally placed in the middle between two anodes, so that the deposits are made evenly on both sides of the part.

The oldest type of electroplating used for gun parts is nickel plating, which is at least 125-150 years old. It was originally used as a plating over steel to protect it from corrosion. Nickel plating works very well on surfaces that are highly pitted, by sealing off the pores and preventing further rusting. Parts that have been nickel plated have a slight yellowish tinge to them. The nickel coat does about 45 HRC on the Rockwell Hardness scale, which is about the same hardness as the tip of an axe or a chisel. When nickel plating, the anode used is a non-consumable one and the plating occurs from the dissolved nickel salts in the electrolyte. Nickel plated guns were once very popular in the Wild West because they don't rust as easily as guns treated with bluing. Also, wiping them down at the end of the day is a lot easier with a nickel plated gun because it provides a smoother finish than a gun with bluing.

Another form of nickel plating is called Electroless Nickel Plating which came into vogue in the 1950s. Instead of using electricity to deposit the metal, the reaction is auto-catalytic and deposits a layer of nickel-phosphorus on the surface of the metal. The part is first prepared by adding a pre-plating catalyst to the surface, before dipping it in the solution, which causes the dissolved metal in the solution to deposit on the part. It has the advantage of being able to be used on oddly shaped surfaces, as well as non-conducting and non-metallic materials, such as certain plastics. It provides a very uniform thickness coat. The hardness of this is about 48-50 HRC. One particular type of electroless nickel treatment is called NP3, by Robar Corporation. This type combines nickel with teflon to provide less friction between the coated parts.

Some examples of nickel plated firearms are shown below. Note that the finish of the nickel plating can be adjusted to be bright and shiny or matte finish to reduce glare.

Another form of plating, which is very commonly used these days, is Hard Chrome Plating. This method was invented in the 1920s and perfected in the 1940-1950s. In ordinary chrome plating which is used for decorative purposes, the coating is very thin and beautiful, whereas hard chrome plating is not as beautiful, but is a much thicker surface. Hard Chrome Plating is also called Engineering Chrome Plating, as it is very much used in surfaces that need a good corrosion resistance as well as lubrication. This type of plating is much harder than nickel plating. It rates between 65-70 HRC on the Rockwell Hardness scale, which puts it harder than high-quality steel alloys such as custom knife edges. They can also show superior resistance to rusting in punishing environments such as salt spray. Chrome plated firearms generally come in bright or matte finishes.


There is also another type of finish called Black Chrome Plating, where the steel firearm is initially nickel plated, and then chrome plated on top of the nickel plating. It is a softer finish than hard chrome plating, but looks more beautiful in some people's eyes.

Other forms of plating include gold plating and silver plating, but these are generally done in the interests of looks rather than protection from wear and tear.

The above two gold-plated weapons were originally owned by former Iraqi dictator, Saddam Hussein.

Generally, the following processes are used when plating a weapon:

1. The surfaces are first polished to as smooth as possible, because small bumps will show up even more after the surfaces are plated.
2. The parts are cleaned in detergent to remove all traces of grease and oil
3. The parts are then rinsed in water to remove all traces of detergent.
4. In case of electroless process, the parts are coated with the catalyst
5. The parts are placed in the electrolyte solution or electroless solution and the solution is agitated gently with air bubbles to prevent formation of gas bubbles sticking to the part. Gas bubbles sticking to the surface prevents metal from depositing there, so it is important to get rid of them.
6. The parts are taken out and dried and oiled.
7. The parts are then reassembled.

Care must be taken that the plating does not add to much to the thickness of the weapon, especially a very close fitting one.

All types of plating are generally superior to bluing or parkerizing. Nickel plating is cheapest can be applied to various surfaces (even non metallic ones), but does not provide as durable a finish as Hard Chrome plating. Electroless nickel plating is a bit more expensive, but it provides the best corrosion protection because it is less porous than nickel plating or hard chrome plating. However, it is less durable than hard chrome plating. Overall, hard chrome plating provides the best combination of hardness, protection against corrosion and durability of all the different plating types.

Metal Treatments: Parkerizing

In our previous post, we discussed metal treatments such as browning and bluing. In this post, we will discuss another metal treatment that came into vogue shortly after the bluing process. This process is called parkerizing. The process is also known as phosphating or phospatizing. As the name indicates, parkerizing is a generic term for applying a coating of zinc phosphate, iron phosphate or manganese phosphate on a steel surface. The phosphate coating provides resistance to rusting, corrosion and wear. In addition, it reduces a highly reflective surface into a non-reflective surface, which makes it easier to aim a gun in bright sunlight.

The process of applying a phosphate coating was originally invented in the 1850s, but really became prominent in the 1900s, thanks to improvements done by the Parker Rust-proof Phosphating Company in the 1915-1945 period, who were responsible for making it possible to mass produce parts using this metal treatment process. In fact, the name "parkerizing" is derived from the name of the Parker Company.

The first appearance of a phosphate coating process came from England, patented by one William Alexander Ross in 1869. Thomas Coslett, also of England, developed another phosphating process, which he patented in Britain in 1906 and the United States in 1907. The Coslett process was an iron phosphating process, and in 1912, an American named Frank Rupert Granville Richards, invented the manganese phosphating process, for which he was granted a separate patent. Another American, Clark W. Parker, bought the patent rights to the Coslett and Richards processes, and after making several experiments in his home kitchen, went on to found the Parker Rust-proof Phosphating Company of America in Detroit in 1915, along with his son, Wyman C. Parker.

By 1919, the Parker company had managed to secure a patent for an improved manganese phosphate coating process. In 1929, they filed another patent which reduced the time of coating to 1/3rd of the previous time, by heating the solution and controlling the temperature to a very precise range of 260-285 degrees centigrade. During this time, the Great Depression set in, and along with it, Clark W. Parker and his son, Wyman C. Parker, were both convicted of investor fraud, for a scam involving shares in an automobile company and two other companies they'd founded. Despite the imprisonment of its founding owners, the Parker Rust-proof Phosphating Company continued its business. Manganese phosphate was a somewhat expensive process because of limited availability of manganese compounds in this period. In 1938, the company was granted a patent for inventing a zinc phosphate coating process, which came just in time, since World War 2 started soon after, and the United States lost access to some of its main sources of manganese. On the other hand, the United States had plenty of access to zinc ores and thus this process became much more cheap than using manganese. By 1941, the company had discovered a way to carry out the zinc phosphating process even more faster and cheaper than before. By adding a bit of copper, they could reduce the alkalinity of the solution, and by adding a chlorate to the nitrates in the solution, they could reduce the temperature of the solution to 46-54 degrees centigrade, which made it possible to make it a very efficient process.

The Parkerizing process was used to coat many of the small arms made by the United States during World War 2. It was also used to protect vehicle parts and aircraft parts during this period. This is when the process really became famous.

The modern parkerizing process consists of the following steps.

1. The steel part is initially cleaned of all traces of grease, oil and dirt by washing it a slightly alkaline detergent solution. This is very key to the process, as most of the problems of parkerizing are usually traceable to incorrect washing.
2. The steel part is then washed in a flowing water tank to remove any traces of detergent. The water must be clear and free of certain chemicals so that they don't interfere by reacting with the solution in the next step. The water may be passed through a charcoal filter to remove most tiny particles.
3. The steel part is dropped into another tank containing a phosphoric acid solution, along with some zinc or manganese. Amounts of copper, nitrates and chlorates are also added to the solution to speed up the coating process and also reduce the temperature that the solution has to be heated to. In one process, the solution is heated to about 88-99 degrees centigrade and the part is immersed for 5-45 minutes. As the chemical reaction takes place, bubbles are emitted from the steel part. When the bubbling stops, the coating process is complete. The process may be carried out at a lower temperature (which saves energy costs) or a higher temperature (which reduces the time needed to coat the part) depending on need.
4. The steel part is then taken out and washed again in running water to rinse off the parkerizing solution.
5. The parts are dried and oiled lightly to increase resistance to corrosion.

The steel part develops a light gray to dark gray coating depending n the solution used. After a few years, the parkerized surface turns into a greenish-gray color.

The above picture shows a beautifully parkerized M1911 after a few years. Note the greenish-gray shade of the finish.

Now for some pictures showing how the process is done, from start to finish from http://www.theboxotruth.com/. Note that, unlike the hot-bluing process we studied in the previous post, this process does not need a factory environment and may also be carried out at home. The pictures show a person carrying out the process in his kitchen, after purchasing some parkerizing solution at his local sporting goods supplier.

First, we start with a "before picture", i.e. before the parts were parkerized. Note the finish of the weapon and the wooden handgrips showing lots of wear and tear. The old finish that was on the gun had worn off almost completely. The author bead-blasted the remaining off before starting the parkerizing process.

The first step is to disassemble the weapon and wash the parts to be parkerized in a detergent to remove dirt, oil and grease.

The next step is to rinse in clear hot running water a few times, to remove all traces of the detergent.

Then, a parkerizing solution is prepared in a separate stainless steel pot. The person used 1 part of solution to 5 parts of water in this case. The pot is heated on a stove to about 85-95 degrees centigrade and then the steel parts are suspended from a wooden plank into the parkerizing solution. Bubbles rise from the solution as the coating starts to take place.

After 15 minutes, the parts are taken out, rinsed in running water to remove any traces of parkerizing solution and dried on a towel. They are then oiled.

The person then bought two fake-ivory plastic handgrips from his local sporting goods supplier, to replace the original worn out wood handgrips. The final reassembled pistol that looks like this:

Note that the fresh coating is dark-gray in color now, but it will change in a few years time into a greenish gray finish, without losing any of its corrosion and wear resistant properties.

The advantages of this process are many. It produces a corrosion and rust resistant finish, but is also much more durable than bluing. It is also a very cheap process. Unlike hot-bluing, this doesn't require a factory environment to perform the process and it may also be done by hobbyists. The chemicals required for parkerizing are all sold by most major gun parts suppliers in the United States. It also reduces the glare of the treated parts.

The downsides are that this can only be performed on certain steel parts. It cannot be carried out on stainless steel or steel containing large amounts of nickel. It also cannot be done on non-ferrous metals. Traditional parkerizing process has recently also been criticized for adding large amounts of phosphorus impurities to the used water, which encourages rapid algae growth.

Metal Treatments: Browning and Bluing

Since the early days of gun-making, people were looking for different ways to better protect the iron and steel parts of their weapons. There are actually two forms of "bluing", one is called "cold bluing" and the other is called "hot bluing". We will study both processes here. We will also study the process of "browning".

So what is the purpose of these processes. The purpose is to provide a thin coating on the outside of the iron and steel parts, so that they are protected from rusting and corrosion. The idea is to intentionally rust the outside surface of the part (such as the barrel) and then stop the rusting processes. The outside layer then protects the inside parts from further rusting and corrosion. Additionally, these processes reduce the glare of what would otherwise be a very shiny part. It is much harder to aim an untreated barrel in broad daylight, since the glare of the sun's rays reflect off the shiny barrel into the user's eyes.

Browning and Cold Bluing are actually very similar processes, with just one minor difference, so we will do well to study both these processes together. The process of browning is the older process and has been known for centuries, even before firearms were invented. In Europe, this process was originally called "russetting" and the term "browning" came to be used later. Later on, a modified browning solution and process resulted in a dark blue/black finish, which began to be known as "bluing". It has been theorized that the result of browning its barrel is what gave the Brown Bess musket its name.

The compositions of browning solutions was a very closely guarded secret among old-time gunsmiths. However, as the centuries passed, different people came up with various formulae to achieve the same effect. The end result is that today there are literally hundreds of browning solutions available. One of the more common ones was using salt-water, which has been known as a rusting agent since the Iron Age. Salt water was definitely used at least until the middle 1800s or so. There are other solutions involving strong acids, metallic salts, chlorides, antimony etc. In general, concentrated browning solutions act more rapidly than diluted ones, however the diluted solutions provide coatings of better durability, appearance and closer grain finish. Also, old time gun barrels were non-uniform in composition, so a concentrated solution could quickly produce a non-uniform finish, which is why weaker solutions were preferred. Also, since each manufacturer used a different steel alloy in their guns, what would work with excellent results for one manufacturer's guns would not work the same way for another manufacturer's guns.

The formula for the solution used for the Brown Bess musket (taken from General Regulations and Orders for the Army, 1811) is as follows:

• Nitric acid - 1/2 ounce
• Sweet spirits of wine - 1/2 ounce
• Spirits of wine - 1 ounce
• Blue vitriol (a.k.a. copper sulphate) - 2 ounces
• Tincture of Steel - 1 ounce

Water is used to dissolve the copper sulphate first and the rest of the ingredients are added to the solution and more water added to make up 1 quart of solution. Other solutions may also be used depending on the type of iron or steel alloys used in the weapon.

The browning process starts as follows: The barrel is first removed of all greasy impurities by washing with soap or detergent. Then a plug of wood is placed on both ends of the barrel to make sure that the insides are sealed and only the outsides are rusted. The barrel is hung in the air and then the browning solution is thoroughly applied to the outside with a clean cloth or sponge. The barrel is left exposed in the air for about 24 hours, after which a thin layer of reddish brown rust is formed on the surface. After this, the barrel is "carded" by rubbing it with a hard brush or steel wool, which removes the acid from the surface. The whole process is repeated for two or three times to get the desired shade of reddish-brown finish on the barrel. The barrel is then cleaned and oiled. The same process may also be carried out on the other iron or steel parts of the gun as well.

A browned damascus barrel

A Brown Bess musket with a characteristic browned barrel

It was later discovered that before carding, if the part is immersed in boiling hot water first, this stops the rusting process. Additionally, the part turns a deep blue or black color instead of reddish brown. This is due to the black oxide of iron (Fe3O4) being formed instead of the red oxide of iron (Fe2O3). This process is called "cold bluing" instead of "browning" and is the only real difference between the two processes.

The above picture shows a civil war era percussion-lock weapon with a cold-blued barrel.

The following two videos demonstrate cold-bluing processes in some detail, using different chemicals:

Browning and cold bluing add a very thin layer of protection, which does not provide any appreciable extra thickness to precisely machined parts. However, they do not provide any different hardness to the surface and hence offer no extra protection against wear. The process is also an extremely slow, labor intensive process and therefore it is currently used by small one man shops and hobby experimenters only and not large manufacturers.

Instead, what some modern manufacturers use is the hot bluing process. In this method, the parts are initially degreased, as in the browning and cold-bluing processes. Next, the steel parts to be blued are dropped in a hot solution of potassium nitrate, sodium hydroxide and water. Stainless steel parts are dropped in a hot solution of nitrates and chromates. The process causes a layer of black oxide of iron (Fe3O4) to form. Care must be observed during this process as these solutions will eat through leather, wool, skin etc. and the fumes can cause deterioration of any metals in the area. Hence, hot bluing is best left to factory environments and not suitable for home experimenters. After a while, the parts are removed and rinsed in hot water to stop the bluing process. The parts are then oiled and reassembled.

The above example is a Taurus pistol based on the famous Colt M1911 pistol. Note the beautiful dark blue parts of the weapon.

Hot bluing is the best of all three processes when it comes to corrosion resistance and rust protection. It is also much faster to manufacture and provides the best looking finish of the three methods. Hence, this is the method often used by modern manufacturers. The only downside to hot bluing is that the chemicals used are corrosive and hence it is best done in a controlled factory environment.

While bluing is the least expensive method of finish, it is not the best protection against rust compared to some other modern finishing methods. It also only works on ferrous materials, steel or stainless steel parts and doesn't work on other materials such as aluminium alloys. We will study some of the other metal treatment methods in the following days.

Metal Treatments: Case Hardening

In the earlier days of gun-making, many gun parts (such as flintlock actions, percussion lock hammers etc.) were made of low carbon steel, because it was easier to machine, cheap and reasonably strong. Unfortunately, such parts would not often withstand the battering stresses they received on the field. To counter this, the process of case-hardening was invented.

Case hardening consists of adding carbon to a low carbon steel. The extra carbon is added to the surface of the part only, thereby making the surface of the part harder, but leaving the center of the part tough and malleable. This allows for a part that has a hard surface, but is not brittle and can withstand shock well.

The process of case hardening (also called pack hardening) starts by taking the low carbon steel parts after they've been machined for proper fit and packing them in a sealed container with compounds having high carbon content. In the olden days, this high carbon compound was simply animal hides or animal bones. These days, bone charcoal is often the chemical of choice. The container is made of ceramic, steel or graphite and is sealed with fire clay after being filled. The container is then heated in a furnace till it turns red-hot (approximately 950 degrees Centigrade or 1750 degrees Fahrenheit) and held at that temperature for hours depending on how many parts are being treated at a time. The animal bones and hide slowly get converted into carbon and form carbon-rich gas (carbon monoxide mainly), which migrates upon the surfaces of the steel parts and penetrates to a few thousands of an inch deep into the parts. The container is held at this temperature depending on how much depth the user wants the carbon to penetrate through the surface. Then the container is removed from the furnace and the contents are quenched in a liquid bath, typically a lot of water with a thin layer of oil added on top to lessen the shock of quenching. The surface skin of the parts become hard as glass, but the inner areas remain softer and more malleable.

Color case hardening uses a similar process to the above, but the parts are initially cleaned of all oils and greases before being put into the container with the high-carbon compounds. The process is the same as above, except during the quenching process. If the steel in quenched unevenly, different colors (blues, yellows and oranges) are produced on the steel's surface. Some people would move the steel in a jerking motion into the quenching bath to produce color bars. Many London gun makers of the 1800s used a slightly different technique: the quenching bath would be agitated with lots of air bubbles, which produces a mottled color effect on the part. Adding a little potassium nitrate to the water in the quenching bath also adds a bit of extra brightness to the colors formed.

After case hardening, the parts are taken out, cleaned, dried and then oiled or lacquered to prevent rusting.

This technique was heavily used in the 1700s and 1800s. Most parts of a gun's action or lockwork, except for the springs, were often hardened with this technique.

In the above picture, we see a lever action rifle whose lock work is case hardened in brilliant blue, yellow and dark orange shades. With color case hardening, such as the example above, the results are extremely obvious to be seen by everyone, since the bright colors are easily visible. In ordinary case hardening, there is no such indication that the case hardening operation was done. However, it is equally effective as color case hardening.

With modern steel manufacturing processes, it is possible to control the amount of carbon in the steel uniformly, which has removed much of the original reason for case hardening. However, it is still desirable in some cases to harden the surface, while leaving the inside of the part resilient and tough. In this situation, case-hardening works better, because the modern steel manufacturing processes distribute carbon uniformly through the sample, not just at the surface. Hence, in the present day, firing pins and bolt action faces are still case-hardened, since these parts are subject to shock impact and high pressures. Replica weapons are also case-hardened, so as to highlight the metal working techniques of the period.

Metal Treatments: Basics

From the earliest guns, iron and steel were the materials of choice for making certain parts of the gun, such as the barrel, action, trigger mechanism etc. We've already looked at some techniques that lend aesthetics to the finished product, such as pattern welded or damascus barrels. Now we will look at some metal treatment processes used in gun manufacturing.

The aim of metal treatment process is to preserve the life of the weapon. Both iron and steel parts are vulnerable to corrosion and rusting and it is imperative to prevent these as much as possible. It is also a good idea to harden the metal parts in order to give them longer life and prevent deformation.

A second reason for metal treatment is to reduce the glare of the gun barrel, so that the user doesn't get blinded by the glare when aiming the weapon.

A third reason is merely to improve the cosmetics of the weapon. For instance, a browned barrel has a pleasing reddish-brown finish compared to an untreated barrel.

There have been several treatments developed throughout history: Case Hardening, Browning, Bluing, Parkerizing, Chrome plating, Tenifer etc. The metal treatment techniques that we will study in subsequent posts will deal with how these techniques were developed.