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Metals Used in Firearms - IV

October 1, 2014, 8:26 pm
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In our last three posts, we looked at usage of steel and stainless steel in firearms. In today's post, we will look at the usage of alloys of another metal: aluminum.

Aluminum is one of the most abundant elements on the earth and is widely found in many minerals. In fact, it is a very commonly used metal in today's world and we find it in soda cans, aluminum foil, vehicles, aircraft, windows, doors etc. However, throughout most of mankind's history, people did not know how to extract the aluminum metal out of ores. The first successes were discovered in the mid 1850s or so, but the yield was small and slow. In fact, before the mid 1880s, pure aluminum was more expensive than even gold! It is no coincidence that the top of the Washington Monument was topped off by an aluminum cap stone when it was first built in 1884. Napoleon III of France held banquets where honored guests were supplied with aluminum utensils, whereas less honored guests were given gold utensils! Think of that the next time you throw a soda can into the trash.

Once the process of extracting aluminum from ores via electrolysis was discovered in 1886 and factories using this method started to open shortly after, the price of aluminum began to drop. The invention of airplanes made the demand for the metal even more and after World War II, aluminum prices dropped even more. In modern times, there are many factories around the world producing aluminum.

The main advantage of aluminum is that it is pretty strong compared to its weight. It can be easily shaped and offers pretty good corrosion resistance. Aluminum is usually never used in its pure form, but usually in the form of an alloy, with other elements such as copper, zinc, magnesium, silicon etc. added. These other elements improve the mechanical properties of the aluminum alloy. When aluminum is exposed to air, it forms a thin layer of aluminum oxide, which prevents further oxidation of the inner layers and gives it corrosion resistance.

The two common aluminum alloys used in firearms are 6061 and 7075 aluminum. 6061 aluminum is about 95.8-98.5% aluminum and contains magnesium and silicon as its major alloying elements. It exhibits ease of machining and welding. It also exhibits good corrosion resistance and is used for cans, boats, scuba tanks etc. 7075 aluminum is an alloy that contains about 5.6-6.1% of zinc as its major alloying element. It is much stronger than 6061 aluminum, but is harder to machine and weld than 6061 aluminum. It is also more expensive.

Aluminum is used to construct the frames and receivers of some pistols and rifles, most notably the M16 family. It is also used for magazines, sight rings, scope bodies etc.

In Vietnam, the original M16 used aluminum receivers made of 6061 aluminum originally, but later switched to 7075 aluminum. The reason given was that when the receiver was forged from 6061 aluminum, the forging process made them prone to intergranular exfoliation in environments of high temperature and humidity, such as that found in the jungles of Vietnam, especially when combined with human sweat. Upon a suggestion by Eugene Stoner, the receivers were changed to use 7075 aluminum instead.


Of course, after the machining process, the aluminum has to be hardened to withstand stress forces. This is done by a process called anodizing. The parts to be anodized is connected to a positive electrode (the anode) and dropped into a tank containing an acid solution. Direct current is applied to the anode and cathode and a layer of aluminum oxide forms on the piece. The coating forms a thick layer pretty quickly, much quicker and thicker than if the aluminum was to be exposed to air directly. The layer is very hard, but it contains pores, which could let air or water go through into the inner layers of the piece. Therefore, a sealant is applied after anodizing to seal off the pores.

US military specifications for aluminum alloys used in firearms talk about 7075-T6. The T6 at the end specifies the treatments applied to the aluminum at the end (heat treated and artifically aged). US military spec MIL-A-8625 specifies how the anodization of aluminum should be done.

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Metals Used in Firearms - V

October 2, 2014, 8:36 pm
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In our last post, we looked at the use of aluminum in firearms. In today's post, we will look at a more exotic material: titanium.

Titanium is a metal, which like aluminum, has a very high strength-to-weight ratio. It also has a very high melting point. It is also a fairly commonly occurring element on earth (it is the seventh most common metal) and is found combined with other elements in nature. Several minerals and most igneous rocks contain a bit of titanium, but only a few minerals are commercially viable to extract titanium from. The processes to extract titanium from its ores are laborious and costly.

Titanium is more dense than aluminum, but is twice as strong. It is also fairly hard. Therefore, some manufacturers, such as Smith & Wesson and Barrett, make some firearm parts out of titanium.

A Smith & Wesson 342PD with a titanium cylinder

While titanium is hard, it is not as hard as some grades of steel, therefore it isn't normally used for parts like the barrel and bolt, which experience large forces. Machining it is also a tricky business, as it needs good tools and adequate cooling when machining, otherwise the metal turns soft. Welding it is also expensive as it easily combines with gases like oxygen, hydrogen and nitrogen when heated, therefore it must be welded in an inert gas atmosphere. According to Ben Rich, one of the engineers behind the SR-71 Blackbird airplane, which was made of titanium, chlorine also reacts with titanium when heated and they found that they had to use distilled water when washing spot welds, otherwise the welds would fail in a few weeks. Also, they had to throw out entire sets of wrenches away because they were plated with cadmium and found that these were contaminating the titanium and causing structural failures later.

All this means that titanium parts are pretty expensive to make. For instance, NEMO arms made an AR-10 model with several parts made of titanium: upper and lower receivers, muzzle brake, handguard, buffer tube and gas block. Total cost of the firearm: $100,000. And your humble editor wonders if the person that plans to buy this will ever fire it.

Most manufacturers generally only use titanium for a few components, such as revolver cylinders, frame components, pins, bipods, muzzle brakes etc.



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Metals Used in Firearms - VI

October 4, 2014, 10:23 pm
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In our last few posts in this series, we've studied different metal alloys used in modern firearms: steel, stainless steel, aluminum and titanium. In the next couple of posts, we will study metals that were used in centuries past. In today's post, the object of study will be a metal alloy that was used with some of the earliest firearms: bronze. We will also study a class of bronze alloys called gunmetal (sometimes called red brass in the US) in this post.

Bronze is an alloy of copper mixed with some other elements. Early types of bronze consisted of copper mixed with a small quantity of arsenic ("arsenical bronze"). It seems to have occurred some time after man learned to melt copper. It is theorized that some unknown potter in Eastern Europe or Asia Minor discovered that heating certain rocks in a potter's kiln produced a metal (copper) that could be used for tools and jewelry. About 1000 years after the discovery of copper smelting, we find evidence of people in the Balkans melting copper and arsenic together to form bronze around 4200 BC.  Since some copper bearing minerals also contain some arsenic as impurities, the discovery of bronze might have been an accident, but there is evidence that people later deliberately began to add arsenic to copper to make bronze. Another 1000 years or so later, in 3200 BC, we begin to see the use of tin and copper to make bronze as well. Again, this may have been a lucky accident as well, as some copper ores to contain a little bit of tin, but we also see that by 2000 BC, people were mining tin bearing minerals deliberately in England, France, Spain and Portugal. It is interesting to note that tin from England has shown up in bronzes found in the Mediterranean and Near East. In fact, given that tin is not a common metal and deposits are only found in a few places in the world, the trade in tin to be used in bronzes established some of the earliest international trade routes.

Now we skip past the bronze age and past the iron age as well, into China around 1290 AD, where both gunpowder and cannon were invented and in use. The earliest Chinese cannons were made of bamboo, but they later started to make them out of metal and the metal of choice was bronze. Cast iron and wrought iron were also known to the Chinese at this point, but the early metal cannons made around 1300 AD were all found to be made of bronze. Even though bronze was more expensive than iron, the reasons for using it were probably because bronze is stronger than wrought iron and not as brittle as cast iron. It is also far more resistant to corrosion than iron is and is easier to cast into barrels.

Bronze Chinese hand cannon from the Yuan Dynasty (1271-1368 AD). Click on the image to enlarge
Image licensed under Creative Commons Attribution-Share Alike 3.0 Unported license  by Yannick Trottier.

When firearms technologies reached Europe, many of the early European firearms were also made of bronze. Bronze cannon continued to be used in America as well. In fact, during the Civil War, a majority of the field artillery used by both sides were made of bronze. While cannons made of iron were also invented, people continued to use bronze cannons for a while after for several reasons. Even though cast iron guns can withstand greater amounts of gunpowder and thereby have greater range, bronze cannons are lighter for the same caliber. Therefore, they were preferred for campaigns, as they could be moved around more easily. On ships, bronze firearms (both small arms and cannon) were preferred over iron mainly because bronze doesn't corrode as easily in the presence of sea water.


A pair of bronze barreled travelling pistols from England circa 1803-1820

Gunmetal is one of the strongest types of bronze used to make guns. It dates back to the middle of the 19th century, when it was discovered that adding a little bit of zinc to bronze improves its casting characteristics. The British Admiralty gunmetal formula was 88% copper, 10% tin and 2% zinc, whereas the US Ordnance formula was 88% copper, 8% tin and 4% zinc.

Interestingly, even though literature from the 18th and 19th centuries refer to brass firearms, many of these were actually made of bronze (i.e. alloy of copper and tin) rather than brass (which is an alloy of copper and zinc). Examples include the Henry rifle and several confederate weapons, which are often mentioned to have brass frames, when they are really more bronze than brass.

Winchester Model 1866 rifle

The above image shows a Winchester Model 1866 rifle (a.k.a. the improved Henry rifle), sometimes called the "Yellow Boy", because of the color of its receiver. Although some sources call it a brass receiver, chemical analysis shows that it is actually a gunmetal of 83% copper, 14.5% tin, 2% zinc and 0.5% lead.

While bronze was used for firearms for quite a while, it was always more expensive than iron and less strong. With the rise of modern steel making techniques, it became possible for steel to overcome the advantages that bronze had over iron (i.e.) ease of casting, elasticity and corrosion resistance. This is why bronze stopped being used for firearms, as improvements in steel manufacturing and improved casting and machining techniques were discovered.


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Metals Used in Firearms - VII

October 5, 2014, 11:04 pm
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In our last post, we looked at how bronze was used for early gun barrels. In today's post, we will look at early barrels made of a form of iron called wrought iron.

As was noted in our previous post, some of the problems with bronze were the price and availability of materials. Iron ore is much more common than copper and tin. However, it is harder to extract iron from its ore, which is why the iron age started after the bronze age. Early iron implements were made from iron from meteorites, which is relatively pure iron and date back to 3000 BC or so. Therefore, it was rare and expensive during this period. Later on, people discovered how to extract iron from minerals on earth. It is not entirely certain how the knowledge of processing iron was spread, because various cultures around the world seem to have discovered it. For instance, around 1800 BC, there is evidence of iron smelting in both India and Turkey, by 1500 BC, smiths in West Africa were processing iron ore as well. Once the art of processing iron from minerals found on earth was mastered, the price of iron became much cheaper than bronze, since iron ore is commonly found on the earth's crust.

Wrought iron is a form of iron, with very little carbon content (less than 0.1%). If more carbon is added to it, the result is steel and if even more carbon is added, the result is called "cast iron", which we will study in our next post, as it has some importance in the history of firearms as well. Wrought iron can be shaped by heating and hammering it and it is easily welded as well. It cannot be easily hardened though. Early blacksmiths produced wrought iron in small furnaces called bloomeries and this technique persisted for many centuries. The method consists of heating iron oxide ore to burn off the oxygen and remove the other impurities by using a flux to form slag, which can be separated from the metal.

These days, TV programs show steel mills pouring molten metal into molds, but for many centuries, people could not produce fires hot enough to melt iron. Instead, iron was purified by heating the ore enough to burn off impurities and making the iron soft enough to be extracted. The fuel of choice was charcoal, made from the wood of trees. The use of coke and coal did not start until people had burned up most of their forests and had to look for alternate sources of fuel.

Charcoal was traditionally produced by heating wood without access to air, so that it doesn't burn, but removes water and volatile chemicals from the wood. The standard technique was to form a pile of wood in a conical shape, with a central opening for a chimney, then cover it with mud, clay etc. to make it air tight. Then, some burning fuel is introduced into the chimney and the air is slowly cut off while the wood burns, which makes it burn slowly and form charcoal. The process takes a number of days to finish and is a delicate operation. People who specialized in making charcoal were called "colliers" (your humble author has an English friend with that last name, perhaps one of his ancestors specialized in this business). The resulting charcoal burns hotter than wood.


The above method is somewhat inefficent and modern methods use sealed metal containers, which produce a much higher yield of charcoal.

The next bit was to pre-process the iron ore before extracting the iron from it. Iron ore usually consists of iron oxide, with other impurities in it. First, the metallic ore was separated from the non-metallic earth and rocks by washing it, and then it was roasted. The purpose of roasting the ore is really for a few reasons: First, it removes the moisture from the ore, as well as reduces the content of any sulfur impurities. Second, it makes the ore easier to crush, before smelting it.

A bloomery made of heat resistant materials, such as earth and clay, was constructed next. A bloomery is basically a conical furnace with one or more openings in the bottom to allow air in. An opening is also made in the bottom, to extract the metallic iron.


The bloomery is first preheated by burning charcoal and after it becomes hot, the iron ore, limestone and more charcoal are added on top. Air is blown in from the bottom using bellows, either operated by hand for smaller operations, or operated by water wheel in the medieval era.


The hot carbon in the charcoal combines with the oxygen in the iron oxide ore, to form carbon monoxide and carbon dioxide, leaving behind the iron metal. The small pieces of iron left behind fall to the bottom of the furnace and form a spongy mass called "sponge iron". Silicate impurities in the ore combine with the limestone flux to form a slag, which also falls to the bottom and mixes with the sponge iron. The hot sponge iron mass is pulled out from the bottom of the bloomery and hammered while it is still red hot. This helps shape the iron bar and also forces out the slag from the iron. The result is wrought iron.

Incidentally, the name "wrought" is an old English word that means "worked" and this process of hammering the sponge iron is what gave "wrought iron" its name. This method was used from the beginning of the Iron age until the 18th century or so. It isn't a very efficient method though, as it only extracted about 50% of iron from the total iron found it the ore, even in the best case scenario. In most situations, only about 20% of the iron was extracted and the rest was lost in the slag. However, the slag could be recycled in the next batch to extract more iron.

Later on, the process was improved in the 18th century to use a finery forge and later, a puddling furnace, to produce wrought iron more efficiently. We'll study these processes in the next post or two.

Since wrought iron is very malleable (i.e. it can be beaten into a flat shape easily) and weldable, gunsmiths could easily make parts out of it. A flat sheet of metal would be forged into a long tube and then the two edges would be welded together to form the barrel. We discussed this process in detail a few years ago and the reader is advised to read that article if needed. Other parts of the firearm were also likewise forged out of wrought iron.

Wrought iron isn't as hard as cast iron, which we will study in a future post, but it was used for quite a while because it was cheaper than brass and widely available. For now, enjoy a couple of videos showing how wrought iron is made in a bloomery.



As you can see from the videos, when using a bloomery, there's a lot of work that goes into producing just one small bar of iron. We will study more improved methods in the next couple of posts.
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Metals Used in Firearms - VIII

October 7, 2014, 12:02 am
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In our last post, we studied how a bloomery was historically used to produce wrought iron from the raw iron ore. The problem with bloomeries is that they weren't very efficient when it came to extracting the iron metal from the ore (typically, a run would yield only 20% of the iron from the ore). A more efficient process of extracting ore was developed using a blast furnace and used to produce an alloy of iron called pig iron. We will study that in this post.

The use of blast furnaces first started in China around 400 BC or so. The technology spread to the West and the Roman empire was definitely aware of it. However, with the collapse of the Roman empire, the technologies were forgotten in Europe during the Dark ages and were only rediscovered around the late middle ages (1325-1500 AD).

In a blast furnace, the fuel (initially charcoal and later coke), iron ore and limestone flux are poured on top and air is continuously supplied from the bottom, usually with mechanically powered bellows. Unlike a bloomery, the heat generated in a blast furnace is hot enough to melt the iron. The burning charcoal produces carbon monoxide, which rises up in the furnace, heats up the iron ore and removes the oxide from the ore to form carbon dioxide. The heat produced melts the iron, which drips down to the bottom of the furnace. Other impurities in the ore combine with the hot limestone, which forms a slag that also drips to the bottom of the blast furnace.


Since the slag is lighter than the molten iron, it tends to float on top and can be separated out. As the molten iron drops down through the charcoal (or coke), it absorbs some of the carbon in it. The resulting alloy is iron combined with a high percentage of carbon (greater than 4% or so). As we noted earlier, when we studied steel, adding carbon to pure iron makes it harder, but adding too much carbon makes it brittle.

The molten iron alloy is tapped off from the bottom of the furnace and poured into sand molds. Traditionally, the mold was shaped as a long central channel line, to which were branched several smaller depressions at right angles to the central channel. The molten metal would flow into down the central channel into the smaller branches and cool down to form ingots. To the more poetic minded, the configuration looked like a bunch of piglets suckling milk from their mother. This is why this type of iron alloy is called "pig iron".

Once the mold has cooled down, the ingots can be easily broken off the central channel, since the alloy contains so much carbon that it becomes brittle enough to be broken by a few blows from a hammer.

Pig iron ingots. Image licensed under Creative Commons Attribution 3.0 Unported by Mfields1 at wikipedia.

Blast furnaces are much more efficient than bloomeries and can extract much more iron from raw ore. The only problem is that the pig iron produced by this method contains so much carbon (and some sand impurities) that it is not of much use by itself. The pig iron is so hard and brittle that it cannot be easily shaped and will easily break if struck with a hammer. It also melts at a lower temperature than iron or steel. So what use is it for gun-making, the reader wonders. Well, this pig iron can be put into a finery forge or a puddling furnace and further processed in one of three options.

  1. Remelt the pig iron, add more flux and remove the excess carbon in the slag formed. This removes most of the carbon to below 0.1% and forms wrought iron, which can be easily shaped and used for gun making (as we saw in our previous post).
  2. Add scrap iron to the pig iron, remelt the whole thing and add other alloys to form cast iron. Cast iron has about 2-4% of carbon and contains other elements like silicon. We will study how cast iron is used in gunmaking in a future post.
  3. Melt the pig iron, add more oxygen to remove the excess carbon, then add other elements like manganese, molybdenum and chromium to form steel.
Blast furnaces got so large that the supply of charcoal could not keep up with the demand. Entire forests were cut down to convert wood to charcoal and then people had to pay extra to haul wood from locations far away from the factories. Some factories even failed because they could not get enough hardwood from close by any more. In England, Abraham Darby started to switch his furnace to use coked bituminous coal instead of charcoal in 1709. Since coal does not involve labor for cutting trees down and converting the wood to charcoal, the price of pig iron produced using coal became cheaper than that produced using charcoal. Additionally, it became possible to build larger furnaces using coking coal. The availability of cheap iron in England was one of the key factors responsible for the start of the modern Industrial revolution!

In the next couple of posts, we will study how pig iron is converted to cast iron or wrought iron.


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Metals Used in Firearms - IX

October 11, 2014, 9:00 pm
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In our last post, we looked at how blast furnaces were used to make an iron alloy called pig iron. While blast furnaces are good at extracting iron from the ore more efficiently than bloomeries, the iron produced by this method contains a high amount of carbon (greater than 4%), some silicon and other impurities. The high carbon content makes pig iron very brittle and it also has a lower melting point than pure iron. Therefore, this form of iron alloy is very cheap to produce, but useless for firearms. Instead, pig iron is reworked to form cast iron, steel or wrought iron, which are much more useful for firearms. We will study how that works in today's post and perhaps the next one as well.

Cast iron is a form of iron alloy that contains about 2-4% of carbon and 1-3% of silicon, along with some other alloying materials. It is somewhat brittle and cannot be shaped by heating and hammering like wrought iron. However, it can be cast into shapes using sand molds. It also has good wear resistance and has some resistance to rusting as well. 

Cast iron was known to the Chinese around 500 BC and was used by them to make pots, pans, farm tools etc. Almost 2000 years passed before it started becoming available to Western Europe in the 15th century.

Some blast furnaces can produce cast iron directly, simply by adjusting the amount of carbon absorbed by the iron ore. If not, cast iron is produced by heating pig iron back into a molten state, often along with a good quantity of scrap iron and steel (both of which have far less carbon content than pig iron). Limestone is also added as a flux to remove some of the other impurities, such as sulfur and phosphorus. The resulting alloy's carbon content is reduced to be about 2-4% and silicon to 1-3% and other alloying elements (such as manganese, nickel, chromium, copper, vanadium etc.) are added to change the properties of the cast iron as required. The molten cast iron is then poured into molds to solidify into the final shapes desired.

Since it is much cheaper to manufacture than bronze, many of England's navy cannons started to switch to use cast iron rather than bronze. This allowed ships to be fitted with more guns at a cheaper price. For instance, in 1570, the price of just the raw tin and copper needed for a single bronze gun cost about £60, whereas the cost of a complete cast iron gun (including raw materials and manufacturing cost) was about £20. The price of cast iron guns dropped even more, as the technologies for producing cast iron improved. By 1670, a ton of bronze cost about £150 in England, whereas a ton of cast iron cost £18. Another benefit of using cast iron was that the guns could generally be loaded with more gunpowder and therefore had more range.

However, early cannon were generally made of bronze or wrought iron rather than cast iron initially. In the first part of the 16th century, cast iron was not thought of as a suitable material to cast large guns from and bronze was preferred instead. The problem was that molding technology was somewhat simple in those days and the cast iron would often harden in the mold before all of it had been poured in. Also, bronze is more resistant to corrosion. Cast iron guns also had the problem that they would occasionally burst with no warning, whereas a bronze gun would slowly wear down. Therefore, most of the European navies used bronze guns initially.

Around 1625, during the reign of King Charles I of England, it was realized that if England's navy had to expand, there needed to be a cheaper way to make guns. At that time, the cost of bronze guns alone was about 35% of the cost of the entire ship. The Commissioners of the Royal Navy were directed to see if iron guns could be used instead. John Browne, who was then a royal gun maker, was one of the few people who showed interest in solving the problem and he delivered a set of six cast iron guns in 1626, which successfully passed the Royal Navy's specifications. Incidentally, John Browne came from a family of people who dealt with cast iron guns -- there are records of his father, Thomas Browne, obtaining a license to make cast iron cannon in 1589 and in 1609, he testified that he had delivered 469 tons of cast iron ordnance since 1591, which showed that he was a big armaments maker. In 1613, John Browne stated to a commission that he had four factories making cast iron cannon and employed 200 people and exported mostly to the Dutch. Later, he was granted a royal monopoly to provide iron guns to the Navy. His grandson George, was also a royal gun-founder for King Charles II.

As cast iron was a lot cheaper to make than bronze, therefore England's Royal Navy decided to go with cast iron cannon so that they could build more ships with their limited budget. Of course, there was the risk of cast iron guns exploding without warning, but the cost savings was determined to worth making the switch. Other countries followed them and soon, cast iron cannon became common around the world. For smaller firearms though, wrought iron or steel were still the materials of choice. In our next post, we will study how raw pig iron was converted to wrought iron or steel.


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Metals Used in Firearms - X

October 13, 2014, 12:18 am
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A couple of posts ago, we studied about the blast forge and how it is used to produce pig iron. While blast forges are much more efficient at extracting iron from ore than bloomeries, they have the side effect of adding excess carbon to the iron, along with other impurities. The result is an iron alloy called "pig iron", which is rather brittle and has a lower melting point than pure iron, which makes it useless for firearms. However, this pig iron alloy can be converted into a much more purer iron alloy called "wrought iron", which we studied earlier when we studied bloomeries. Wrought iron contains a lot less carbon than pig iron and is therefore much more malleable, can be shaped and also welded easily. It is more efficient to use a blast forge to produce pig iron from the ore and then refine the pig iron into wrought iron than it is to produce wrought iron directly from the iron ore in a bloomery. We will study how that was done in today's post.

The first technique to refine pig iron or cast iron was invented in China around 500 BC and involved using a finery forge. Like cast iron, the technique of refining it didn't reach Western Europe until the 15th century or so. In the area of Wallonia (now part of Belgium), the process was improved and spread to some other parts of Europe. Most of Sweden used a type of finery forge called the German forge for the process, but the area in Uppland, north of Stockholm, used the Walloon process, as did most of England. Another type of forge that was used in England and South Wales, also was popularized in Sweden as the Lancashire forge. We will study them in this post.

The German process only uses a single finery forge for all operations, whereas the Walloon process uses two forges, a finery forge to refine the pig iron into wrought iron and a second chafery forge to shape the wrought iron into bars. We will study the German finery forge first:

A German Forge. Click on the image to enlarge. Public domain image.

In the above figure, H is the hearth in which the operation is carried out. It is line with thick cast iron plates and is about 12 inches deep and width about 24 to 26 inches. Air is blown in through a nozzle called a "tuyer", which is labelled 't' and projects about four inches into the hearth. There are usually two tuyers or more in the hearth. The tuyers are made of sheet copper and they are fed by bellows B, which are driven by a wheel powered by water A. The wheel has cams 'c' attached at the axle, that raise the lids of the bellows and the levers 'e' regulate the bellows from falling too rapidly by adding or subtracting weights in the boxes 'w'. A hole to drain slag is present at the bottom of the hearth. Above the furnace is placed a brick hood 'v' which serves to carry off the smoke.

The process starts by filling the hearth with charcoal and heating it. The pig iron is either introduced into the middle of the fuel pile or piled on top of the charcoal and air is fed in through the tuyers. After a short while, the pig iron melts and passes through the current of air from the tuyers and falls to the bottom of the hearth. This takes about 3.5 hours. As the molten metal falls, it combines with the oxygen being pumped in via the tuyers and the carbon present in the pig iron becomes carbon dioxide and escapes, leaving behind an alloy that contains much less carbon than before. Any silicon impurities also oxidize and become slag. The molten iron forms as pasty mass (called a bloom) beneath the fuel that it has passed through. Any slag formed during this process is run off through the slag hole, leaving behind just enough to continue the process of decarburization of the iron. When the partially refined iron bloom has become large enough, a workman rolls it up into a ball using a strong bar of iron and then pushes is back to the top of the fuel and adds more charcoal as needed. As the iron melts and falls down to the bottom of the hearth for a second time, even more carbon is removed as carbon dioxide and the remaining relatively pure iron forms a spongy mass. This mass is rolled into a large ball again and then removed and hammered by a large tilt hammer powered by water. The hammer head is about 800-1200 lbs in weight and made of cast iron or wrought iron. The hammering process compresses the iron mass together and pushes out any slag through the pores. The result is wrought iron which contains less than 0.1% carbon. The slag that contains a  relatively higher portion of iron is not thrown away, but is recycled for the next round of melting, along with any bits of iron that fly off during the hammering process. The process is pretty efficient in that about 100 lbs. of pig iron will produce about 85 lbs. of wrought iron. For every 100 lbs. of wrought iron produced, the process uses up around 150 lbs. of charcoal. It must be mentioned that the fuel used in this process must be charcoal, because impurities in other fuel types can affect the iron alloy and add other undesirable elements to it, changing its properties.

In the Walloon process, the finery forge is used to melt the iron as described above and then it is hammered to remove the slag. Then the iron is heated again in a separate chafery forge, not to melting temperature, but just enough to make the iron soft, so that it can be shaped into bars of standard sizes. The bars of iron can now be sold to customers. The finery forge must use charcoal as its fuel, for the reasons explained above in the previous paragraph, but the chafery forge can use other fuels such as coal or gas as well, because it does not heat the iron enough to melt it and therefore cannot add impurities to the alloy. Typically, the Walloon process would use one chafery for every two or three finery forges.

The wrought iron produced by these processes is relatively pure iron and is easily shaped and weldable, therefore, it can be used to produce barrels using methods we studied a while back.

In 18th century England, the best quality grade of iron available was called "Oregrounds iron". The name is actually because the iron was exported from a small Swedish city called Öregrund. Most of Sweden used the German forging method, but the area around Uppland (where Öregrund is located) used the Walloon method. The Walloon process was taken from Belgium to Sweden by a Walloon/Dutch merchant named Louis De Geer, who also brought a group of Walloon workers with him to work in his factories. Other Walloon and Dutch people followed his footsteps into Sweden and established more finery forges. Their products became very famous in England for their high purity. Interestingly, one of the reasons for the iron's purity was because oregrounds iron was chiefly made of ore from a Swedish mine called Dannemora, and this ore had some manganese in it. The manganese in the ore caused some impurities that would have normally stayed in the iron, to instead combine with the manganese and run off as slag. This pure iron was particularly suitable to be converted to steel and was therefore imported by England for the cutlery industry and also for the Royal Navy. At one point, there was a cartel of merchants in London and Bristol that was controlling the supply of oregrounds iron to the extent that they'd bought up the entire output of the Swedish forges several years in advance!

While finery forges can produce high quality wrought iron, there is one rather huge disadvantage that they have. Finery forges have to use charcoal as the fuel, because other fuel types such as coal, peat or gas can add other impurities to the iron alloy, thereby affecting its properties. The charcoal also has to be of high quality for best results. As we mentioned before, by the 18th century, the supply of charcoal was becoming a problem in Europe and entire forests were cut down to meet the demand and there was still a shortage of charcoal. Therefore, other techniques were invented to replace finery forges, the most successful of which was the puddling furnace, which could use other fuels such as coal, coke or gas. This allowed the iron industry to not depend on the growth of trees and ushered in the industrial revolution. The invention of puddling furnaces meant that finery forges began to become obsolete by the latter part of the 18th century. In our next post, we will study the puddling furnace.


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Metals Used in Firearms - XI

October 22, 2014, 11:09 pm
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In our last post, we saw how people converted pig iron (or cast iron), an alloy of iron useless for making firearms, to a more useful wrought iron, which is much more suitable for making firearms, using finery forges. The one problem with a finery forge is that it needs charcoal for its fuel, as using any other type of fuel will add impurities to the wrought iron and change its properties. However, as the demand for wrought iron rose, the supply of charcoal could not keep up with the demand and entire forests disappeared. Experiments were made using other fuels and the puddling furnace was developed to replace the finery forge. Puddling furnaces could not only produce wrought iron, but were later used to produce steel from pig iron as well. We will study how this worked in today's post.

The invention of the puddling furnace is credited to Henry Cort of Hampshire, England in 1784. Another invention of his was the modern rolling mill, which also was key to starting the industrial revolution.

In our earlier article on the production of pig iron from iron ore, recall that while the iron is melted to separate it from the ore, it comes in contact with the fuel (coal) and combines with the carbon and silicon in it to form the pig-iron alloy. Therefore, one way to remove these elements from the pig iron alloy is to melt it without making it touch the fuel and then blowing air over the molten metal. The oxygen in the air combines with the impurities such as carbon, silicon, phosphorus, sulfur etc. and forms gases (such as carbon dioxide, sulfur dioxide etc.) which escape through the exhaust and leave a purer form of iron (wrought iron) behind. This is the operating principle of the puddling furnace.

An early puddling furnace. Click on the image to enlarge. Public domain image.

In the above image, we see an early type of puddling furnace. The fuel is placed on an grate 'b' at the right of the furnace and can be refilled through door 'c'. The puddling chamber 'e' is in the middle of the image. It consists of a bed of sand, upon which the pig iron is placed. 'i' is the chimney flue through which the gases escape. The door 'j' is used to access the puddling chamber and it is opened and closed by lever 'k'. As you can see, the fuel in 'b' does not come into direct contact with the pig iron in 'e', therefore it cannot contaminate it. The heat is transferred from 'b' to 'e' via convection and radiation only. As the pig iron melts in 'e', it forms a pool of molten metal, which is then stirred with an iron rod via the door 'j'. At this intense temperature, the carbon in the pig iron burns off and forms carbon dioxide, which escapes via the chimney 'i', leaving behind a pasty mass of relatively pure iron behind. A worker, known as a 'puddler', then uses a pair of tongs to pull the ball of puddled iron out of the furnace and takes it to a power hammer to work it into shape. This process is called shingling. It compacts the iron by welding all the internal cracks, expelling all the slag out and breaking off the chunks of impurities. The iron can later be reheated and passed through heavy rollers (in a rolling mill) to roll it into bars or cylinders, or it can be shaped by using a pair of heavy mechanically operated jaws.


The original process, as patented by Henry Cort, could only be used by a particular type of pig iron called white cast iron, not grey cast iron, which was much more common. One way to handle this was to melt the pig iron beforehand and add flux to remove the silicon (as slag) from the iron alloy, leaving behind white cast iron, which can then be used in the puddling furnace. This process is called 'dry puddling'. A better technique was discovered by a puddler named Joseph Hall in England. He discovered that if a bit of rust (a.k.a iron scale) is added to the grey cast iron before melting in the furnace, the oxygen in the rust combines violently with the carbon in the grey cast iron and forms carbon dioxide. Other elements such as silicon, sulfur and phosphorus also combine with the oxygen from the rust and are removed, leaving the iron behind. This process is called 'wet puddling' and is much more efficient than dry puddling.

The process of the carbon combining with oxygen is exothermic (i.e.) it gives off heat. Therefore, when the carbon first starts burning off, the temperature is around 1150 degrees centigrade (2100 degrees fahrenheit), but since the reaction gives off heat, the temperature of the molten metal rises to about 1540 degrees centigrade (2800 degrees fahrenheit). The formation of carbon dioxide causes the molten metal to puff up. When most of the carbon is burned off as carbon dioxide and escapes out, the iron becomes a pasty/spongy mass (i.e. it was called "coming to nature") and can be removed by the workers and then shingled. Judging when the iron has "come to nature" was an acquired skill that had to be learned by the workers. This is one of the reasons why puddling could never be fully automated.

The use of sand in the bed of the puddling furnace caused a lot of the iron to be removed with the slag, but the above mentioned Joseph Hall found a way around this by using roasted tap cinder for the bed instead, which reduced the waste massively (from around 50% to less than 5%). Further refinements in the process meant that by the mid 19th century, the yield of wrought iron from the pig iron alloy by the wet puddling process was close to 100%.

In 1850, the process of making mild steel in a puddling furnace was invented in Westphalia, Germany and quickly spread to England and France. It only worked with pig irons made of certain types of ore though.

After the pig iron is puddled, shingled and rolled, the resulting wrought iron or steel produced can be used to make gun barrels. We discussed this process in detail many months ago, when we studied how pattern welded barrels were produced. It will serve the reader well to reread the process again.

There were some massive advantages of the puddling furnace over the older finery forge process to produce wrought iron. For one, a finery forge was restricted to using charcoal as its fuel, as any other fuel could cause contamination of the iron. The supply of charcoal was becoming a problem as demand increased and forests were chopped down, therefore finery forges were severely restricted. Since puddling furnaces do not allow the fuel to come into contact with the pig iron, other cheaper types of fuel can be used instead -- coke, coal and even dry pine wood were all used in puddling furnaces. A puddling furnace produces more efficiently than a finery forge: two workers (a puddler and a helper) could produce about 1500 kg. of iron in a 12 hour shift.

There are some disadvantages of the puddling process as well, chiefly due to human factors. The point when the iron can be removed from the puddling furnace (i.e. when the iron has "come to nature") to be shingled, has to be judged expertly by the puddler, therefore this process could never be fully automated. This also means that the process depends on how much the puddler and his assistants can handle at one time, so larger furnaces to handle over 500 kg. of pig iron could not be built and if you wanted more capacity, the solution was to build more puddling furnaces and employ more workers. The heat, smoke, ashes, fumes and strenuous labor involved in a puddling furnace caused many puddlers to have short lives. It was unusual to find a worker in a puddling furnace that lived to be 40 years old, as most of them died by their 30s.

In the next couple of posts, we will study how steel for firearms was produced.


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Metals Used in Firearms - XII

October 26, 2014, 9:20 pm
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In our last post, we saw how a puddling furnace could be used to convert pig iron/cast iron into wrought iron, which is much more suitable for manufacturing firearm parts. Today, we will study some early attempts to convert this wrought iron to steel. Earlier, we alluded to how carbon is one of the important elements that alloys with iron to alter its properties. It might be well to go over how the percentage of carbon content creates different grades of iron alloy, before we go any further.

Wrought iron is an iron alloy that contains very low carbon content (0.02% - 0.08%). Steel is an alloy of iron that has around 0.2% - 2.1% of carbon in it. Other elements such as nickel, chromium, molybdenum etc. may also be added to steel to alter its properties further, but carbon is the main alloying element. If the iron alloy has more carbon content (about 2.1% - 4.0%), then it is called cast iron and if it is above 4%, it is called pig iron. As the percentage of carbon content increases, the hardness of the iron alloy increases as well, but it also becomes brittle (i.e. it will break if subjected to a sudden hard blow). The increase in carbon content also makes the alloys less weldable and shapeable. Therefore, cast iron and pig iron have to be cast in molds, rather than hammer forged over an anvil.

Since wrought iron has very low carbon content, it is very malleable and weldable and therefore, it was used for forging gun parts since the early days of firearms. However, it is also relatively soft, therefore it wears down easier than other iron alloys.

Pig iron, which has very high carbon content, is very hard, so it cannot be welded or shaped other than by casting, and it shatters easily, which means it is useless for use in firearms. On the other hand, it is very cheap to mass-produce pig iron from iron ore and this pig iron can be converted to other more useful iron alloys, as we saw previously.

Cast iron also cannot be welded, but it can be cast into shapes like pig iron. Like pig iron, it has a tendency to shatter, but this can be compensated by making the parts thicker (which is how several cannon and naval guns were made). This increases the weight of the gun, but since cast iron is cheap to produce, the cost savings made it worth using for larger guns. Since cast iron has a tendency to shatter, explosive artillery shells and grenades were made of cast iron for this reason as well.

Steel has carbon content in between wrought iron and cast iron, therefore it has some useful properties common to the other two alloys. Like wrought iron, steel can be welded and forged into shapes. However, the higher carbon content of steel means it can be hardened much more than wrought iron and therefore lasts longer. On the other hand, the carbon content of steel is not high enough to make it brittle like cast iron. Therefore, the flexibility and tensile strength of steel, combined with its hardness, make it much more useful for firearms than either wrought iron or cast iron. Steel was known for centuries, but the process of making steel was more difficult and expensive to manufacture, until the mid 1850s or so. Ancient India was famous for the superb quality of Wootz steel (otherwise known in the west as Damascus steel), but the techniques of production were not well known elsewhere and also not mass produced enough to be adopted in large scale. The tricky part to manufacturing steel was how to manage the carbon content properly. Too little carbon and the steel would be too soft and too much carbon and the steel would become cast iron and therefore brittle.

Remember that in the previous posts, we saw how carbon was removed from pig iron and cast iron, to form wrought iron (which has very low carbon content). This was done using finery forges and later, puddling forges. It was noticed however, that this process could also be used to produce steel, by not removing all the carbon content in the pig iron. According to The Ordnance Manual for the Use of the Officers of the United States Armyfrom 1862 (page 418), we have the following observation:

"If, in the operation of puddling, the process be stopped at a particular time, determined by indications given by the metal to an experienced eye, an iron is obtained of greater hardness and strength than ordinary iron, to which the name semi-steel, or puddled steel, has been applied. The principal difficulty in its manufacture is that of obtaining uniformity in the product, homogeneity and solidity throughout the entire mass. It is much improved by reheating and hammering under a heavy hammer.

A tenacity of 118,000 lbs to the square inch has been obtained from semi-steel made in this country in this way. Field-pieces have been made of this material, and it is believed that it will answer well for this purpose."

This means that the metal is pulled out of the puddling furnace, before the process of removing all the carbon is complete. Of course, this means the process requires a highly skilled and experienced worker to decide exactly when to pull out the metal from the furnace. Also, different workers could have different ideas about when the metal should be removed and therefore, the quality of steel produced by this method would vary a lot.

Another better method that was used before the industrial revolution was the "cementation process". The idea is that after wrought iron is produced (by removing all the carbon content from pig iron), a little bit of carbon is added back to the wrought iron in a controlled manner, to make steel.

The process was originally described in a treatise published in Prague in 1574, but was reinvented by Johann Nussbaum of Magdeburg in 1601. It made its way to England in 1614.

A cementation furnace. Click on the image to enlarge. Public domain image.

Wrought iron bars and charcoal are packed in several alternating layers in a closed furnace and exposed to heat of about 1500 degrees fahrenheit (815 degrees centigrade) for 7 to 8 days and then the bars are examined to make sure that the correct conditions are reached and then the heat is removed and the furnace is allowed to cool for two weeks or more. The carbon in the charcoal gets absorbed into the iron bars, making steel. The gases produced during this process leave bluish gray bubble marks (blisters) on the steel's surface, therefore the product was called "blister steel".

The problem with producing blister steel with this method is that the carbon tends to be absorbed in a non-uniform manner and there is usually more carbon on the outside of each bar. Therefore, if someone makes multiple pieces from the same bar of steel, some of the pieces could be harder than the others, even if all the pieces are forged and heat treated identically! To work around this problem, the blister steel would be sheared into smaller strips of steel and then the strips would be stacked together in a pile, heated and forge welded back to each other, to even out the carbon content throughout the steel. The result was called "shear steel". For even better product, the process would be repeated (i.e.) shear steel bars would be sheared once again, stacked together, heated and welded together again to produce "double shear steel", "triple shear steel" and so on.

The quality of blister steel also depends on the quality of the wrought iron bars used in the process. It was discovered that the best wrought iron bars for making steel came from Russia and Sweden (the famous "Oregrounds Iron" that we talked about earlier). This is one of the reasons why British and Dutch merchants bought up the entire output of some Swedish iron factories many years in advance.

For your viewing pleasure, here are a couple of videos showing blister and shear steel being produced.



Of course, the problem of distributing the carbon evenly in the steel was not entirely solved by shear steel. In our next post, we will study more major advancements in steel making technologies and how the town of Sheffield became a major center of steel manufacturing.

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Metals Used in Firearms - XIII

November 2, 2014, 11:07 pm
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In our last post, we saw how wrought iron could be converted into steel, by adding carbon to wrought iron in a closed furnace, in a controlled manner. Recall that, in our previous post, we mentioned that the problem with this method was that the distribution of carbon throughout the steel bar was non-uniform, resulting in some parts of the bar being harder than other parts. As we saw in the previous post, one way to handle this was to shear the blister steel bars into smaller pieces, stack the pieces on to a pile, re-heat the pile and then weld them together, so that the carbon content would be more evenly distributed. For better product, the process would be repeated multiple times. However, all this increased the cost of the steel and it did not necessarily result in even distribution of carbon in the steel either.

By the early part of the 1700s, steel was being used to make some parts of firearms (e.g.) lock springs, frizzens etc., as well as the tools to make firearms. In Europe, England and Germany were two major sources of steel during this period. The next development in steel making was due to an English clock maker and the technology he developed was crucible steel. We will study the process in this post.

The process of making crucible steel is actually much older -- as early as 300 BC, there were several places in southern India making a type of crucible steel called "wootz steel". This steel was exported to the middle-east, where it was encountered by Europeans during the crusades and was labelled by them as "damascus steel". The source of iron ore for the Indian steel was an area in South India, where the iron ore came with small amounts of vanadium and other rare earths. As a result of these trace elements, wootz steel has carbon nanotubes in it, contributing to its superior ability to hold an edge. Unfortunately, by the 1700s, with the rise of British power in India, the secrets of its production died with the blacksmiths. However, we have several earlier descriptions of many travelers to India (Arabs, Persians, French, English, Scottish etc.) from which we know that they were definitely using a crucible process.

Over in England, Benjamin Huntsman was in the business of making clocks, tools and locks in Doncaster, in the early 1720s. Later on, he also practiced as a surgeon and an oculist. Like most people in the clock making trade, he bought most of his steel from German sources. However, he found that this steel was not always good enough for springs and pendulums for his clocks, where consistency in the steel is the key to accuracy. Therefore, he performed several experiments to try and find a more uniform steel production process. Since he needed a large amount of suitable fuel for his steel furnace, he moved his business from Doncaster to Sheffield in 1740, because of the better availability of coke and coal in Sheffield. He continued his experiments in secret in Sheffield for many years and gradually re-discovered the crucible steel process. Essentially, his process consists of melting the iron in a clay crucible, adding a precise amount of carbon. The carbon distributes evenly throughout the molten steel, resulting in a more consistent product. The molten steel is then poured out into a mold to harden. Since the steel is poured out into a mold, it is sometimes called "cast steel" as well. However, unlike cast iron, this steel is flexible enough that it can be heated and forged by a hammer as well, or even welded.

The process starts off by using a crucible made of clay, to which is added wrought iron bars and powdered charcoal. The amount of charcoal added to the crucible is calculated based on the amount of wrought iron. A flux consisting of ordinary glass pieces is also added to the crucible. The crucible lid is then sealed and it is heated in a furnace. Since the glass has a lower melting point than the iron, it melts first and forms a liquid in the bottom of the crucible. After a few hours, the iron starts to melt and absorbs some of the carbon from the powdered charcoal as it becomes a liquid. Since iron is denser than glass, the liquid iron sinks past the liquid glass to the bottom of the crucible. Any oxygen is released in the form of carbon monoxide gas, which bubbles out through the layer of liquid glass. In a few hours, the iron is fully melted into a liquid and absorbs enough carbon to transform to steel. The liquid steel is at the bottom of the crucible, with a layer of liquid glass above it. The liquid glass seals the steel and prevents any oxygen or excess charcoal carbon from being absorbed by the molten steel. At this point, a worker, called a "puller-out", (sometimes, it was two people) reaches down into the furnace and pulls out the crucible pot. The crucible pot can be left to cool until the metal turns solid, at which point, the glass layer is broken with a hammer and the steel ingot underneath is retrieved. Alternatively, immediately after pulling the crucible from the furnace, another worker, called a "teemer", can open the crucible lid and pour the liquid steel into a mold, with another worker using a tool to dam the glass slag floating in the crucible on top of the steel. The steel has to be poured into the mold quickly (in under two minutes or so) and then a lid is placed on the top of the mold, to limit the amount of oxygen combining with the cooling steel. In about five minutes, the steel becomes solid enough inside the mold. If the steel ingot is to be sold to someone else, then the mold is allowed to cool for several hours before being opened. However, if the foundry has its own forging shop, then the mold is broken after 5 minutes and the still hot ingot is carried off to a hammer to be forged into the final shape, as it is still soft enough to be easily shaped (incidentally, this is the origin of the English saying, "strike while the iron is hot"). The crucible can be re-used a few times before it has to be disposed off, because it weakens due to the intense heat and erosion, every time it is used.

The "puller-out" and "teemer" had to be strong men, to lift and handle the crucible, since the weight of the steel alone in a single crucible was usually around 20 to 45 kg. (45 to 100 lbs.). The mold was typically about 50-100 cm. (about 20 to 40 inches) in length and square in cross section. It was made of two halves, held together by rings. The hole on top of the mold typically had a width of only 7.5 cm. (about 3 inches). The mold was deliberately kept narrow so that the molten steel cannot be exposed to much oxygen as it is poured into the mold. A good teemer could pour molten metal from the crucible through this narrow hole of the mold in under 2 minutes, without any splashing or spilling. Teemers were trained to do this by making them pour cold lead pellets into molds, until they could do it perfectly, before they were allowed to handle hot steel.

A teemer at work. Public domain image.

In the beginning, Huntsman remelted blister steel instead of wrought iron, in his crucibles, but he refined his process over several years. He realized very early on, that his steel could be used for other purposes besides clock springs and tried to interest other local manufacturers of cutlery and tools to use his steels, but they were not interested, since his steel was harder than everyone else's steel. Therefore, he exported his steel to France, where it was very well received. Pretty soon, the Sheffield cutlery manufacturers began to lose market share to superior products from French manufacturers and as a result, they actually tried to obtain a government order to force Huntsman to stop exporting his steel! Due to their efforts, Huntsman even contemplated moving his factory elsewhere. Luckily, cooler heads prevailed and the Sheffield manufacturers abandoned their attempts to sabotage his business and started buying from him instead and the demand for his steel went up tremendously. He established a larger steel factory in 1770 and the city of Sheffield started becoming famous for its steel. Within 100 years of his discovery, the city of Sheffield was producing about 40% of the steel produced in the entire world!

Click on the image to enlarge.

Huntsman worked in secret and never patented his process, so other companies elsewhere also tried manufacturing crucible steel. However, they could not duplicate the Huntsman process immediately for a few reasons.

The first reason was the crucible -- it had to be able to withstand high temperatures and therefore, it needed to be made of a special type of fire-clay. As luck would have it, the place where Huntsman went to dig his clay from, in the north western part of Sheffield town, happened to be one of the few places in England where this special type of clay existed. We now call this type of clay as "Stannington clay". When people in other parts of England, Europe and the United States tried to duplicate the process, their attempts failed because their clay pots could not withstand the intense heat of molten steel. It took other people a few decades to figure out that the type of clay used was crucial to the process.

The second reason was the flux that he used -- his secret was broken glass. The glass melts before the steel does and coats the surface of the molten steel ingot. As legend has it, this secret was finally discovered by one of his competitors, using industrial espionage tactics. The story goes that a person by the name of Samuel Walker had a rival foundry at Grenoside, on the northern part of Sheffield. One cold winter night, Walker disguised himself as a poor beggar and showed up outside Huntsman's factory, pretending to be ill and begged to be let inside for shelter and warmth. The workers took pity on him and led him to a corner of the factory floor to sleep in. Walker pretended to sleep, but what he was actually doing was carefully watching the whole process of making the steel. He observed the workers breaking green glass bottles and putting them in the crucibles. About three months later, Walker's factory in Grenoside was also making crucible steel. Whether the story about the disguised beggar is true or not, it is definitely true that Samuel Walker did exist and he did learn details of Hunstman's secret process somehow. Samuel Walker is recorded to have built his rival factory for making steel in 1750, although he did not expand his factory until 1771, indicating that his original furnace had only limited success. Perhaps he didn't figure out the other secrets, such as the clay, until many years later. Other people in Sheffield also started making cast steel, once they had figured out Huntsman's secrets and Sheffield became the first "Steel city" in the world.

The following three videos show some experiments made by a couple of geeks (one is Niels Provos, who is well known in computer security circles and now works in Google):




In the United States, steel was mainly imported from England during this period. The Remington company was one of the first to start offering crucible steel barrels for firearms in the late 1820s. In 1845, Samuel Remington appeared before the Ordnance Trial Board, to persuade them to use Remington steel barrels for military firearms.

By the time of the Civil War, both Remington and Colt were supplying crucible steel barrels, while most of the other manufacturers were still making wrought iron barrels only. Both companies stamped "cast steel" on their barrels, to show that they were of a superior quality. It must be noted though that wrought iron barrels were still cheaper than cast steel at this stage, so both companies also offered wrought iron barrels for sale as well. From a catalog dating from 1871, Remington is listed as offering both cast steel barrels and iron barrels of different grades. During this time, Remington's "cast steel barrels weighing 6 lbs. or less" are listed at a price of $5.00 each, whereas the price of their "iron barrels weighing 7 lbs. or less" are listed at $3.00 each.

The invention of the Bessemer steel process dropped the price of steel even more and was really responsible for many other firearm manufacturers to switch from wrought iron to steel. We will study that process in the next post.


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Metals Used in Firearms - XIV

November 8, 2014, 2:55 am
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In our last post, we saw how crucible steel was manufactured after around 1740 or so, using the process invented by Benjamin Huntsman. While crucible steel was a significant improvement over blister steel in terms of quality, it was still somewhat expensive to produce. Therefore, many firearm manufacturers used steel for smaller parts, such as sear springs, frizzens etc. and many barrels were still made of wrought iron, instead of steel. As we saw in our previous post, some larger manufacturers like Remington and Colt did offer superior steel barrels after 1820 or so, but they cost over double the price of wrought iron barrels and therefore, both companies sold wrought iron barrels as well, as a cheaper alternative to their steel barrels. High end firearm manufacturers combined steel and iron to make damascus barrels. These were beautiful to look at, but they were expensive to produce and generally designed for rich clients.

So what was the reason for the higher cost of steel. Well, let's look at the processes involved to convert iron ore to steel using the crucible steel method, as done before the 1850s:

  1. Convert the iron ore to pig iron or cast iron, using a blast furnace.
  2. Convert the cast iron into wrought iron, using a finery forge, or later on, a puddling furnace.
  3. Convert the wrought iron into blister steel, using the cementation process.
  4. Convert the blister steel into crucible steel. using the Huntsman process.

All four steps needed to be done to produce crucible steel, whereas producing wrought iron only required the first two steps. Steps 2, 3 and 4 also required skilled workers with specialized training (we studied about specialized workers called puddlers, puller-outs and teemers in the last few posts). Step 2 was also not geared towards mass production. Using finery forges was a slow process and work-intensive in nature. While the puddling forge replaced the finery forge, it also required specialist workers and puddler workers generally had short life spans as well, due to the unhealthy and stressful nature of their work. Step 3 took wasn't a continuous process either and took the longest time to finish (typically, a batch would take 2 weeks to convert from wrought iron to blister steel). Step 4 was also done in batches, since it was limited by how much puller-outs and teemers could lift at a given time. Step 4 also typically took around 4 hours to finish. No wonder, crucible steel/cast steel cost so much more than wrought iron.

Improvements in the crucible steel manufacturing process, done in the United States in the middle of the 19th century, rendered step 3 unnecessary, as it was now possible to convert wrought iron to crucible steel directly in the crucible. However, the improved process still took a few hours to accomplish, was still a batch process and required skilled workers. Therefore, wrought iron was still the material of choice for many gun makers. Incidentally, large construction projects like bridges and towers of this era also generally used wrought iron, because of the non-availability of large volumes of steel to meet the demand.

The price of steel did not drop until an English engineer named Henry Bessemer invented the Bessemer process in 1856. With his invention, the cast iron produced in step 1 above could be directly converted to quality steel, without going through steps 2, 3 and 4. It could also be produced in larger volumes than using the crucible process and could be done in 30 minutes, further reducing costs. In fact, the steel produced by his method cost the same price or cheaper than wrought iron. Since steel is generally harder and tougher than wrought iron, after this low-cost production method was invented, most industries stopped using wrought iron altogether and switched to steel completely. In fact, in today's modern world, the only people producing wrought iron are traditional blacksmiths in tiny shops employing only one or two people. We will study how the Bessemer process worked in today's post.

The process consists of melting cast iron in a large vessel (called a Bessemer converter) and blowing air through the molten iron from the bottom of the vessel, through nozzles called "tuyers". The oxygen in the air oxidizes impurities such as silicon, manganese and excess carbon and forms oxides, which either escape as gases or form lighter slag which floats on top of the molten iron and can be separated. The oxidation of impurities also raises the temperature and keeps the iron in a molten state. The materials used to line the insides of the Bessemer converter vessel also play an important part in removing some impurities, as we will see below. The production of oxides causes a large flame to appear in the mouth of the vessel and monitoring this flame gives an indication of how the oxidation process is proceeding. After the oxidation is complete, the slag is removed and a precise quantity of carbon and other elements are mixed into the molten metal to form steel. This molten steel is then poured into molds to solidify.

A Bessemer converter. Click on the image to enlarge. Public domain image.


The process of converting cast iron to steel only takes about 20 to 30 minutes and doesn't use as much coke as some of the other processes we've studied in the past. Also, large vessels can be built to handle about 30 tons of metal at a time, making it more efficient for producing large volumes of steel. Typically, a factory has at least two converter vessels for efficiency, so that while one vessel is being filled or emptied, the other one is busy melting the iron ore.

The process of oxidizing iron (decarburizing) with forced air was actually known to people outside Europe, many centuries before the Bessemer process was invented. We know that the Chinese had a decarburizing process in the 11th century AD and there are European traveler accounts of Japanese using a similar process in the 17th century. However, they produced steel in smaller quantities only. It was Henry Bessemer, who converted this process into a large scale industrial production process and we therefore know it as the Bessemer process.

The invention of the Bessemer process was due to a lucky accident. The Crimean war had started and Henry Bessemer happened to meet King Napolean III in 1854 in Vicennes, France and had a short conversation with him, where the King said that what the world needed was for someone to invent a better and cheaper way to produce steel in quantity, so it could be used for guns (both firearms and cannon were largely made of wrought iron at this time). Henry Bessemer started working on the problem in 1855 and patented the process in 1856. A lucky discovery by him actually gave him an insight into the process. He was working with a puddling furnace and by chance, some of the wrought iron pieces ended up on the side of the puddling chamber and were exposed to the furnace's heat for a while. When he went to push those pieces back to the middle, he discovered that the pieces had been converted to steel. This gave him the idea to rework the furnace to push high pressure air via pumps through the iron. "But wait a minute", the reader asks. "Won't blowing air on top of an object cool it down? People blow air via their mouths to cool down hot coffee or hot soup, so why doesn't blowing air cool down the iron?" Well, hot coffee or hot soup don't contain impurities that burn, whereas cast iron does. The oxygen in the air causes the impurities to burn, which increases the temperature of the vessel, which in turn burns more impurities and increases the temperature of the vessel even more, until the iron melts completely. The first impurities to burn are the silicon and carbon in the pig iron, followed by the rest of the impurities.

In order to make the process more popular, Bessemer licensed his process out to four different vendors in different geographic areas, with the plan of gaining market share for his method. He sold the process to the four vendors for a total of £27,000, but none of them could make it work successfully and he ended up getting sued in court! In the end, he bought back his patent licenses for £32,000 and built his own factory. In his initial process, his method consisted of burning off just enough impurities to reduce the carbon content to the required amount to make the grade of steel desired and then stopping the flow of air. Well, that was the theory anyway, but it didn't work so well in practice and he spent large sums of money unsuccessfully trying to figure out how to determine when to stop blowing the air. Another issue was that certain impurities in the steel also react with nitrogen gas, which happens to be a large part of air as well.

It was left to another British metallurgist called Robert Mushet to provide the solution. Before the Bessemer process was invented, Robert Mushet had discovered in 1848, that adding a small amount of spiegeleisen (an alloy that is rich in carbonates of iron and manganese mainly, with a little carbon and silicon as well) to steel made it much easier to work with when heated. The sample of spiegeleisen was brought back to him by a friend who had returned from a tour of the Rhineland area in Germany and thought that he might like to look at the shiny mineral (spiegeleisen is very shiny and the name literally means "mirror iron" in German). We now know that adding manganese to steel has the effect of increasing the malleability of steel, as we saw earlier when we first started this series.

A sample of Spiegeleisen. Click on the image to enlarge. Public domain image.

Shortly after the Bessemer process was invented, another friend, Thomas Brown, knowing of Robert Mushet's interest in metallurgical problems, brought him a sample of poor quality Bessemer steel and challenged him to improve it. His solution was very simple and was overlooked by everyone else, including Henry Bessemer. Instead of trying to determine when the level of carbon content in the steel had reached the required level and then stopping the flow of air, he instead kept pumping in more air until the entire content of carbon and other impurities had burned off. After all the impurities had been burned off, that's when he stopped the flow of air and added a precise amount of spiegeleisen back into the molten iron, to add back the required amount of carbon and manganese and form high quality steel. This improvement made it much easier to produce steel rails and bars. He also invented other processes to improve the casting of steel (his method is still used today) and also developed the first true modern tool steel. Robert Mushet dreamed that he and Bessemer would become rich men by his inventions, but he didn't manage to profit by them at all, whereas other people did. By 1866, he was bankrupt and ill and his 16 year old daughter went to London alone and angrily confronted Henry Bessemer in his private office and told him that he wouldn't have become rich without her father's invention. Henry Bessemer saw the logic in her argument and paid Mushet a pension of £300 annually (which was a big sum of money in those days) until he died in 1891.

There was also another problem with Henry Bessemer's process. Well, it really wasn't a problem for him, because he was in England and English iron was low in phosphorus content. Remember the section above, where we mentioned that the lining of the Bessemer converter vessel also plays a role in removing some impurities from cast iron. Bessemer lined his vessel with clay and it worked very well with cast iron with low phosphorus content. The process using a clay lining is called acid Bessemer. The trouble is that in the rest of Europe, their cast iron contained a larger amount of phosphorus and this impurity wasn't removed by the clay lining, which resulted in low-grade steel being produced (phosphorus weakens steel). A British chemist by the name of Sidney Gilchrist Thomas solved this problem in 1876, with the help of his cousin, Percy Gilchrist. His solution to the problem was to coat the inside of the vessel with a lining of dolomite or limestone, which removes the phosphorus impurities. This process is called the basic Bessemer process, as the lining is alkaline in nature (as opposed to the acid nature of the clay lining). It is also called the Gilchrist-Thomas process, after its inventor. The process actually generates more slag than the acid Bessemer process. As an extra bonus, the high phosphorus content of the slag meant that it could be sold to farmers as a fertilizer, thereby increasing the profit of the factory! The invention of the basic Bessemer process was very valuable to European countries like Germany and Belgium, where the iron had high phosphorus content and Thomas' name became much more well-known in those countries than in his native England! In the United States, even though more iron ore is low in phosphorus, his method still found lots of supporters here too.

The Bessemer process quickly made Sheffield a major producer of steel. In America, a team of investors went over to England in 1863, to license the technology, with a view to using it to improve shipbuilding, armor and armaments. They built their first factory in Troy, New York, in 1865, to manufacture steel rails for trains. The main American engineer involved, Alexander Holley, continued to improve the Bessemer process and built or consulted for about a dozen different steel plants between 1866 and 1877, including the first Pennsylvania Steel plant for the Pennsylvania railroad company. An early investor who saw great potential in the improvements made by Holley was Andrew Carnegie. who hired Holley to build the Edgar Thomson Steel Works in 1873, located in Pittsburgh. This was one of the largest steel plants in the country at that time and helped make the United States a world leader in steel production, overtaking Britain by 1890 or so. Manufacturing steel made Andrew Carnegie one of the richest men in America and towards the end of his life, he donated his vast fortune to various causes, including funding thousands of public libraries and some universities (he's well known for his contributions to Carnegie Mellon University, but what is not as well known is that he also donated large sums of money to the Tuskegee Institute in Alabama and the University of Birmingham in England).  The Edgar Thomson plant is still in service, now part of US Steel, and this factory currently produces about 28% of US Steel's production in America. About 900 people work in here, many of whom had fathers, grandfathers and great-grandfathers working in the same factory as well.

With the invention of the Bessemer process, not only did the time taken to produce steel from pig iron drop significantly (it was faster to produce than even wrought iron!), it was more efficient and could work with larger volumes of cast iron as well. The cost of producing good-quality steel dropped from about £60 per ton to about £7 per ton, shortly after Bessemer started his first factory. With improvements to the process made by others, the prices dropped even more. For instance, an invention by William Jones, while working in the Edgar Thomson steel plant, improved the Bessemer process to become a continuous process. flowing molten iron directly from the blast furnace to the bessemer converter, without waiting for the cast iron to harden first. As a result of this, steel began to replace wrought iron in many applications, as it was now cheaper to produce, as well as being tougher and stronger than wrought iron. The Bessemer process started declining in England around 1895, but it continued in other places in the world for a lot longer. Germany produced most of its steel in the 1950s and 1960s using this process, and in America, the last factory using the Bessemer process closed in 1968. One of its issues was actually its speed of production -- it ran too fast! Given that the steel could be produced in under 20 minutes, this gave little time to analyze the steel and make sure that it has the alloying elements in the correct proportions and to adjust the percentages as needed. One of the later improved Bessemer processes (the oxygen lance process) replaced the Bessemer process in many places. The oxygen lance process blows pure oxygen instead of air, over the molten metal, to better improve oxidation. Interestingly, the oxygen lance method was actually patented by Henry Bessemer in the 19th century, but he could never build it with the available 19th century technology, because of the difficulty of obtaining large quantities of oxygen.

We will study some more improvements in steel making in the next few posts.

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Metals Used in Firearms - XV

November 17, 2014, 11:50 pm
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In our last post, we studied how the Bessemer process made it possible for the first time for steel to become as cheap or cheaper than cast iron. The quality of steel wasn't as high as the crucible process, but the price of steel was now much more affordable, which meant there were more manufacturers offering steel barrels in their firearms.

One of the problems with manufacturing steel with the Bessemer process was that passing air through the molten metal sometimes formed gas bubbles and these would form internal blow holes in the steel ingot as it cooled down. Also, a steel ingot shrinks as it cools and this shrinkage could also cause gaps and hairline cracks to form in the ingot. These flaws in the ingot could weaken the final product, if not properly eliminated.

The solution to this problem was found by the famous British engineer, Sir Joseph Whitworth. We had studied about his inventions when we studied the Whitworth rifle many months ago. This innovative rifle was much more accurate than its competition, but was more expensive to manufacture, which is why it was rejected by the British military. However, it was used by other people who had need for accurate rifles, such as confederate snipers, during the US Civil War. Sir Whitworth continued to make improvements to his rifle and discovered a way to remove most of the flaws in steel ingots. His method of manufacturing steel was called fluid compressed steel, and we will study the process in today's post.

We know that Whitworth first discovered his method in 1865, because he registered a patent during that year, but it wasn't until 1869 that he finished building the machinery to manufacture the steel in quantity.

The steel is manufactured using the process we studied in our last post, but when the steel is poured out into a mold, instead of allowing it to cool by itself, a hydraulic press is used to apply pressure to the ingot while it is still in a liquid or semi solid state. This pressure causes most of the generated gas bubbles and cracks in the ingot to collapse or move towards the ends of the ingot. To give some idea of the pressures involved, an ingot measuring about 15 feet (4.5 meters) in length before compression,decreases about 12 inches (30.5 cm) in length after compression.  After the steel solidifies, the two ends of the ingot are cut off (about 20% of the length) and discarded and the remaining bar contains far less bubbles and cracks. This bar can now be used for various applications, such as making quality steel barrels.

A Whitworth Hydraulic Press at the Armstrong-Whitworth company. Notice the man standing on the left side of the press.
Click on the image to enlarge. Public domain image


Whitworth fluid compressed steel was generally acknowledged by many gunmakers, to be of excellent quality and Whitworth's name became a selling point. Therefore, many firearm manufacturers of that era would often stamp Whitworth's name and trademark (a sheaf of wheat) on their barrels, alongside their own names, to show that these barrels were made of superior quality steel.

A high quality LC Smith shotgun featuring Whitworth Fluid Compressed Steel barrels. Click on the image to enlarge.

Another double-barreled gun showing the Whitworth trademark (a sheaf of wheat). Click on the image to enlarge.

Whitworth Fluid Compressed Steel was used by many high end manufacturers, such as Purdey and W.W Greener in England, and Parker, Remington and LC Smith in the United States.

Whitworth's patent for fluid compressed steel expired in 1879, but a special committee of the British government extended his patent for 5 more years. After the patents finally expired in 1884, many other manufacturers started making their own versions of fluid compressed steel, The most famous competitor of Whitworth was Krupp Steel works from Essen, Germany, who made their own Krupp fluid compressed steel. Like Whitworth, many manufacturers began to advertise that they used Krupp steel in their barrels.

A pair of barrels made of Krupp Fluid Steel. Click on the image to enlarge.

Some famous American manufacturers like Lefever, Stevens and Ithaca were known to use Krupp's steel, as well as German manufacturers, such as JP Sauer & Son.

Krupp and Whitworth were the two famous manufacturers of fluid compressed steel, but there were also other manufacturers such as Jessop, Sterlingworth, Chromox etc. Fluid compressed steel continued to be used in barrels till about 1925 or so, while other ways of manufacturing steel to eliminate gas bubbles were discovered. We will study some of these other methods in a couple of posts.

In the next post, we will study the open hearth process to manufacture steel and then move on to more modern methods.

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Metals Used in Firearms - XVI

November 19, 2014, 12:23 am
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Two posts ago, we studied details of the Bessemer process, which revolutionized the production of steel and dropped the price of steel to be comparable to that of iron. Today, we will study another process that was developed to complement the Bessemer process. This process is called the Open Hearth Process, or the Siemens-Martin Process.

As we noted two posts ago, the Bessemer process is a very fast process and converts iron to steel in around 20 minutes or so. One problem with this is that it is very hard to control the carbon content precisely, because it is not possible to sample the molten metal at an intermediate stage. Also, the output is less homogenous and could have blowholes and cracks in it (and we saw one way to fix this problem in our last post on fluid compressed steel). The Open Hearth process fixes some of these issues.

The origins of the Open Hearth process go back to the work of Carl Wilhelm Siemens, a German engineer, who moved to England and changed his name to a more British sounding name: Charles William Siemens (and later, he was knighted and called Sir William Siemens). Carl Siemens was the younger brother of Ernst Werner Siemens, the famous German inventor of numerous electrical technologies and later, a co-founder of Siemens AG, the German telecommunications and electrical company. They came from a large family of 14 children and when both parents died, the older brothers took responsibility to support their younger siblings education. Ernst Siemens had shown early interest in electricity and worked on improving existing technologies. One of his early inventions was a better method of electroplating gold and silver onto metal items. Meanwhile Carl Siemens had just finished graduating as a mechanical engineer and the brothers were wondering how to support their younger siblings, so they decided to earn money by licensing Ernst Siemens invention to a British company. Since Carl Siemens had studied English in school and spoke it better than his brother, it was decided to send him to England to act as his brother's agent and market his patent there. Carl Siemens moved to England in March 1843 and liked it so much that he made England his new home. Since he was also an engineer at heart, he liked to devote his spare time to various researches.

One of Carl Siemens early subjects of research was how to improve the efficiency of furnaces and he came up with the concept of a regenerative furnace in 1857. The idea is to use some of the heat from the exhaust gases to preheat fresh air coming into the furnace. This allows the furnace to use less fuel overall. The initial furnaces that Siemens built were used for glass-making and his idea saved about 70-80% of the fuel that was previously used. By the early 1860s, he had built a small factory to produce wrought iron from iron ore and pig iron using his regenerative furnace. In 1865, a French engineer named Pierre-Emile Martin, licensed the Siemens regenerative furnace patent and modified it to be used to make steel and he started a small factory in France. After this, Siemens used Martin's modifications and set up a small steel manufacturing plant in Birmingham in 1866 and later, a larger factory in Swansea in 1869,that produced about 75 tons of steel a week. By 1870, the open-hearth process was perfected and called the Siemens-Martin process after its inventors.

To understand the process, we must first understand the principle of a regenerative furnace. The furnace has at least two chambers, one on either side of the main hearth. The furnace has dampers to regulate the direction of the flow of air and flammable gas. The air and gas are allowed to flow in one direction through one of the chambers, then they are mixed together, ignited and allowed to pass over the hearth, thereby transferring some of the heat to the iron ore to be melted. As the hot gases leave the hearth and move towards the chimney, they are transferred to another chamber lined with fire bricks, where some of the heat of the exhaust gases are transferrred to the bricks. After about 20 minutes, the flow of air is reversed by turning the dampers, therefore the air comes in through the hot chamber and is preheated before it is mixed with the flammable gas. This allows more heat to be produced in the hearth and the hot exhaust gases are piped through the first chamber, heating the bricks in it as well. Every twenty minutes or so, the direction of the air and flammable gas are reversed. This method allows the temperature of the furnace to be hot enough to melt steel.

A regenerative furnace. Click on the image to enlarge. Public domain image.

In the above image, we see a regenerative furnace. The gas enters from the center of the image and the air enters from the bottom. In the above figure, we see the dampers are initially set to allow the air and gas to enter the left most chambers and then combine together and ignite over the hearth. The hot exhaust gases are then led over to the two chambers on the right, where they heat up the bricks in the chamber. The hot exhaust gases are then discharged via the chimney. After twenty minutes or so, the dampers are moved so that the flow of air and gas are reversed. Now, the air and gas flow through the right most chambers first. Since the bricks in the chambers were heated earlier, they now transfer their heat to the incoming air and gas. The burnt exhaust gases are led through the two left chambers to heat up the bricks in there and the process continues until the contents of the hearth are melted fully.

The contents of the hearth may be loaded with scrap steel, sheet metal, pig iron, construction steel, iron oxide (rust) etc.  Once the steel is melted, slag forming agents such as limestone can be added to remove the impurities. The slag floats on top and can be removed when the furnace is tapped. The oxygen in the air burns off the excess carbon in the steel. If more carbon or other elements are needed, they can be added after the molten steel is tapped from the furnace. In the early days, this was not an easy process. According to one US worker from 1919, he described this process as follows: "You lift a large sack of coal to your shoulders, run towards the white hot steel in a hundred-ton ladle, must get close enough without burning your face off to hurl the sack, using every ounce of strength, into the ladle and run, as flames leap to roof and the heat blasts everything to the roof. Then you rush out to the ladle and madly shovel manganese into it, as hot a job as can be imagined!"

Unlike the Bessemer process that can only work with pig iron, this process can use scrap iron, scrap steel and waste metal, as well as pig iron, in the hearth. The Siemens-Martin process is much slower than the Bessemer process and takes about 8-10 hours to complete, but it has some advantages as well. For one, a small sample of the molten metal can be taken out of the furnace and allowed to cool and then taken to a lab for testing, to make sure the carbon content is perfect. The long heating process makes the content of the steel more homogenous than steel produced by the Bessemer process. It is not necessary to remove all the carbon at first, like the Bessemer process, as the longer time of the Siemens process allows the operators to precisely control the carbon content and they can stop it when the required amount of carbon has burned off. This process also allows recycling of scrap steel. One more nice thing is that steel from different sources (with different amounts of carbon content) can all be combined into a single furnace and converted to a new steel with the given amount of other elements in it. Since scrap steel and old sheet metal are often obtained for cheap, the cost savings is considerable. Also, this process allows people to recycle old worn out steel, such as old rails, construction steel beams, scrap metal from junkyard car bodies etc., instead of paying to dispose of the junk. In the Bessemer basic process, the phosphorus remains in the liquid metal until the carbon is all burned off, but in the open hearth process, much of the phosphorus is removed earlier on in the process. When steel of a uniform character is desired, the open hearth process is preferred, but if large amounts of steel  are required quickly, then the Bessemer process is used.

The Bessemer process and the open hearth process complemented each other and were used in many places in the world up to the late 1980s or so. In America, the last open hearth furnace was shut down in 1992, but some open hearth furnaces are reportedly still in use in India and Russia. In many places, the open hearth furnaces were replaced by oxygen lances and electric arc furnaces, which we will study in the next few posts,

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Metals Used in Firearms - XVII

November 24, 2014, 1:30 am
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In our last post, we studied the invention of the Siemens-Martin process to make steel. In today's post, we will study a type of furnace that was invented in the early 1900s, gained popularity around World War II and is still in use today. We are talking about the electric arc furnace.

To understand this type of furnace, we must understand what an electric arc is. An electric arc is a form of electrical discharge between two electrodes, separated by a small gap (typically, normal air). The best known example of this is lightning. Anyone who has performed arc welding is also familiar with electric arcs: you connect the work piece to the negative side of a DC power source and an electrode to the positive side, touch the electrode to the workpiece momentarily and then draw it a small distance apart from the work piece. A stable electric arc forms between the electrode and the work piece and the heat from this arc is sufficient to melt the electrode and weld the workpieces together. The same idea is used in a larger scale in an electric arc furnace.

The idea of electric arcs was first demonstrated by Sir Humphry Davy in 1810 in England and several people after him tried experiments and patented processes in the 19th century, including Carl Wilhelm Siemens, who we read about in our previous article. However, the first successful electric arc furnace was due to the Frenchman, Paul Heroult, in 1900. He was later invited to the United States in 1905, to set up furnaces for American companies, such as US Steel and Halcomb Steel. The process really gained popularity during World War II and afterwards, because of the low costs associated with setting up an electric arc furnace, compared to a complete integrated steel mill.


The furnace is a kettle made with a dished bottom, mounted so that it can be tilted forward and drained. The kettle is lined with fire brick which can withstand very high temperatures. There are doors on either side to put in raw material and the front has a spout to pour out the molten steel. The roof of the furnace is a dome lined with firebrick and has two or three carbon electrodes in it.

Electric furnaces are typically charged with scrap steel, though they may also be used with hot pig iron directly from a blast furnace. Usually though, scrap steel is used. The scrap is prepared based on the grade of steel to be made and the scrap pieces are arranged so that large heavy pieces of scrap metal don't lie in front of the burner ports. Some lime and carbon may also be added at this stage, although more may be injected at a later stage. After the charge is put in the furnace, the roof is lowered on the furnace and an intermediate amount of electricity is sent through, to start the electric arc, until the electrodes bore into the scrap sufficiently. Usually, light scrap is placed on the top of the pile to accelerate the bore-in process. After a few minutes, the electrodes melt enough of the scrap that they can be pushed deeper in and the high voltage can be fed in without fear of electric arcs hitting the roof of the furnace. As the furnace heats up, the electric arc becomes stable and starts melting the material. At this point, air (or oxygen) may be fed into the furnace to burn up the carbon, silicon, manganese etc. and form steel. More carbon and limestone and other elements may be added at this stage to form the steel.

As we have studied before, phosphorus and sulfur tend to weaken the steel and must be removed. As it turns out, the conditions favorable to remove phosphorus are opposite to those favorable to remove sulfur and vice versa. As a result of this, there is a chance that one of these elements may revert back into the steel from the slag, if proper steps are not taken. Therefore, the phosphorus removal is carried out very early on in the process -- while the temperature is still relatively low, the furnace is tilted to pour out the initial slag formed, which gets rid of much of the phosphorus. If this high phosphorus slag is not removed early on, it will revert back into the steel later on. Then the furnace continues to be heated, and more slag formers are introducted to remove the other elements, such as silicon, sulfur, calcium etc.


The molten metal is analyzed via a spectrometer to make sure that the carbon content and oxygen are correct. Once the correct temperature and chemical contents are achieved, the steel is tapped out as shown in the illustration above. At this point, beneficial alloying elements such as nickel or vanadium may be added to the tapped metal stream.

After all the metal is tapped out, the solid slag is cleaned out of the vessel, the electrodes are checked for damage and the new charge is prepared to be introduced into the vessel. The entire process of preparing a charge, melting it, tapping it, cleaning out the vessel and recharging it, takes about 60 minutes on a medium-sized furnace (capacity of 90 tonnes or so).

Electric arc furnaces can range from really small sizes suitable for research labs to large ones capable of working with 400 tons of metal at a time. The nice thing about them is that they can work with 100% scrap metal, which means they are very handy for recycling old steel, which can be bought for far cheaper than iron ore. They can easily be started and stopped, unlike other furnaces. They are also very energy efficient, compared to methods that make steel from raw iron ore. They can produce very high grade steel from cheap and impure metals and even better than the Siemens-Martin process. Since they run at higher temperatures, they allow the operator to make slags that are normally difficult to melt, but useful to remove small traces of impurities. They can be used for superior stainless steel alloys as well. Nucor, one of the largest steel manufacturers in the US, uses electric arc furnaces a lot, because it allows them to put up smaller mini-mill plants near where the steel is needed and they can vary production quickly, depending on the demand.

Electric arc furnaces are also used as part of the process in vaccuum arc remelting (VAR), which is used to produce specialty steels. In this process, the steel is first melted in an electric-arc furnace and then alloyed in an argon oxygen decarburizing vessel and poured into ingots. Then, the ingots are put into another container and the air is removed from it to form a vacuum. An electric arc is used to remelt the steel, since the arc can form without the need for oxygen. Any dissolved gases (such as nitrogen and oxygen) escape out under the vacuum conditions, as do elements such as sulfur and magnesium, which have high vapor pressure. The molten steel is solidified at a controlled rate, using a water jacket around the vessel to control the cooling rate and ensure uniformity.  It is known that the VAR process is used to produce 9310, 4340, Aermet 100 and maraging steels, which we studied earlier when studying steels used for rifle barrels, bolts and firing pins at the start of this series. The process can also be used to produce titanium, which is also sometimes used in the firearms industry, as we studied before.


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Metals Used in Firearms - XVIII

November 30, 2014, 4:19 pm
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In our last post, we studied one of the modern methods of steel making, the electric arc furnace. In today's post, we will study another method that is commonly used today, the Basic Oxygen Furnace (BOF) otherwise known as the Basic Oxygen Steelmaking (BOS) process.

The interesting thing about the BOS process is that the original concept is actually from the 19th century. Recall that the Bessemer process that we studied earlier, works by blowing air through hot molten metal and the oxygen in the air burns off the impurities in the molten iron. Well, the reader is probably thinking that since air consists of a mixture of nitrogen, oxygen, carbon dioxide and other gases, and since only oxygen is needed in this process, wouldn't the process become more efficient if we directly blew pure oxygen over the molten metal? The same idea occurred to Henry Bessemer (allegedly suggested to him by his father as a joke) and he received a patent on October 5th, 1858 for this concept. Unfortunately for him, this idea was not practical in the 19th century, because oxygen was not available at reasonable cost or in large quantities at that time.

Also recall when we studied the Bessemer process, there are two types: the acid bessemer process and the basic bessemer process. The "basic bessmer process" is called that, because it uses an alkaline (i.e. basic) lining in the vessel (as opposed to an acidic lining). The Basic Oxygen Steelmaking process is also called "basic" because it uses an alkaline lining (usually, Magnesium Oxide (MgO)) in the vessel. The purpose of this alkaline lining is to remove elements such as phosphorus and sulfur from the molten metal, as these elements are harmful to steel's properties.

The idea of using oxygen in the furnace was revisited in the 20th century and made practical during the late 1940s. Interestingly, the modern BOS process was developed, not by any large steel companies, but mainly due to the efforts of one man and the support of a few managers in a small company that he worked for. Our story starts with a Swiss metallurgist, Robert Durrer, who graduated from Aachen university in Germany in 1915 and remained there until 1943. He served as a professor of steelmaking in Berlin's Technishe Hochschule (Berlin Institute of Technology) between 1928 and 1943, where he performed many years of experiments using oxygen for steel refining. In 1943, he returned to Switzerland and joined a small Swiss company called Von Roll AG. Here, nhe continued his experiments in the town of Gerlafingen, with a German colleague, Dr. Heinrich Hellbrugge. In 1947, Durrer bought a small 2.5 ton converter in the US and with it, he reported his first success in the internal plant newspaper in May 1948:

"On the first day of spring, our "oxygen man", Dr. Heinrich Hellbrugge carried out the initial tests and thereby, for the first time in Switzerland, hot metal was converted into steel by blowing with pure oxygen... On Sunday, the 3rd of April 1948 ... results showed that more than half the hot-metal weight could be added in the form of cold scrap ... which is melted through the blast produced heat"

Soon after this, two Austrian steelmakers, VOEST and Alpine Montan AG (OAMG), got interested in these developments and worked with Von Roll to commercialize this process. Theodor Suess of VOEST's plant in Linz and the managers of the Alpine Montan plant in Donawitz organized the actual experiments and worked out all the technical issues and decided to construct two 30-ton furnaces in 1949. On November 27th 1952, the first steel was produced by this new type furnace. Since the VOEST plant in Linz and the Alpine Montan plant in Donawitz were instrumental in commercializing this technology, their version is called the Linz-Donawitz process.

In the beginning, big US steel makers paid no attention to this innovation by a small Central European company, whose total steel making output was less than one third that of a single US Steel factory! A smaller American company, McLouth Steel in Michigan, was the first to install BOS furnaces in the US in 1954. The larger American companies, such as US Steel and Bethlehem Steel only built their first BOS furnaces in 1964. However, the rest of the world quickly adopted this new technology and by 1970, 50% of the world's steel (and 80% of Japan's steel) came from BOS furrnaces. In 2000, about 60% of the world's steel output is still made using this method.

A large container, called a ladle, is lined with refractory materials, such as magnesium oxide (MgO). The ladle is tilted about 45 degrees and is charged with scrap steel and then molten pig iron from a blast furnace is also added. The ratio is about 20-30% of scrap steel to about 70-80% of molten pig iron, based on the requirements of the final steel to be produced. This takes a couple of minutes. After this, fluxes such as magnesium or lime are added to remove sulfur and phosphorus. Then the vessel is turned back to the vertical position and a water-cooled lance with a copper tip is lowered down within a few feet of the bottom of the vessel. Through this lance, pure oxygen (greater than 99% pure) is blown over the hot metal at supersonic speeds (about 2x the speed of sound). The oxygen ignites the carbon in the molten iron, forming carbon monoxide and carbon dioxide. These reactions are exothermic (i.e. they produce heat), so the temperature of the molten iron increases even more. The magnesium burns with the sulfur, forming magnesium sulfide, which is also an exothermic reaction, contributing to the rise in temperature. Silicon combines with the oxygen forming silicon dioxide slag. The blowing of the oxygen also churns the molten metal and fluxes, which helps the refining process. The slag, being lighter than the molten steel, floats on top of it.

Click on the image to enlarge

The temperature of the furnace is closely monitored and after about 15-20 minutes, a small sample of the steel is taken and analyzed to make sure that its chemistry is correct. After that, the furnace is tilted horizontally and the molten steel is tapped out into another ladle. At this point, other alloying elements such as nickel, chromium etc. may be added. Sometimes, an inert gas, such as argon may be bubbled through the ladle, to mix the alloying elements properly into the steel. To prevent slag from being poured out with the steel at the end of the tapping process, various "slag stoppers" are used, but a human eye remains the best device to determine when to stop tapping the steel. After tapping the steel out, the vessel is turned upside down and the remaining slag is poured out into a separate slag pot. The vessel is examined to make sure its refractory lining is intact and more lining material is added if needed and the vessel is prepared for the next batch.

The entire process takes about 40 minutes, which is substantially faster than the 10-12 hours that the Open Hearth Process takes. This is why it quickly replaced the open hearth process in many places around the world. Using pure oxygen instead of air makes the process more efficient and it also avoids piping nitrogen and other undesirable gases in the air through the molten steel. The process can take about 250-350 tons of metal in one charge. Unlike the electric arc furnace, this is a primary steelmaking process (i.e.) it works mostly with pig iron rather than scrap steel. This process increases the productivity of steel making -- in fact, as this process became popular, the labor requirements of steel making went down by a factor of 1000. Instead of taking 3 man-hours per ton of steel produced, it now takes 0.003 man-hours per ton of steel. The only disadvantage of this over the open-hearth process is the reduced flexibility of the charge -- the open hearth process can use up to 80% scrap steel, whereas the BOS process can only use a maximum of about 30% of scrap steel. About 60% of the world's steel today is made by the BOS process.

In our next post, we will look at a factory producing rifle barrels at the beginning of the 20th century.

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Metals Used in Firearms - XIX

December 2, 2014, 10:39 pm
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In our last post, we looked at a modern method of manufacturing steel, Basic Oxygen Steelmaking (a.k.a) the BOS process. As we saw, this is based on the Bessemer process, except that we use oxygen instead of air to burn off impurities. When we studied the Bessemer process, shortly after that, we studied how fluid compressed steel was made from steel made by the Bessemer process. The purpose of compressing the steel was to eliminate gas bubbles and hairline cracks in the ingot. Well, the BOS process also could have these problems for the same reasons as well, so we will study how these problems are tackled in today's post.

The problem is that when steel is manufactured using the BOS process, oxygen is injected over the molten metal to burn off impurities. As it turns out, not all of this oxygen gets used up to burn impurities, some of the excess oxygen gets dissolved in the molten steel as well. When the metal solidifies, this oxygen is released out and can do bad things to the steel. For one, it can combine with the iron in the steel, to form iron oxide (i.e. rust). The second is that the oxygen gas can form gas bubbles (blowholes) in the ingot. Thirdly, it can combine with the carbon in the steel, forming carbon monoxide and carbon dioxide, which reduces the carbon content of the steel and weakens it. Also, the carbon monoxide and carbon dioxide gas can form blowholes in the steel as well. Gas bubbles and blowholes cause the steel to have pores in it. One more problem is that the carbon monoxide tends to form more on the outside of the ingot and escapes out. This causes non-uniform distribution of the carbon in the steel, because the outside of the ingot now becomes relatively pure iron, while the inside of the ingot is carbon steel. Also, steel shrinks considerably as it cools and trapped gas in the metal can cause gaps and hairline cracks in the ingot as well. For firearm applications, the presence of rust, bubbles, cracks and pores is undesirable, as is the non-uniform distribution of carbon in the steel.

So clearly, we must minimize the oxygen in the molten steel before it solidifies and preferably remove it without forming a gas like carbon dioxide, because the gas could cause bubbles and cracks to form. In modern times, this is done right after the molten steel is tapped out of the BOS furnace and poured into molds, by adding deoxydizing agents to the molten steel. Basically, a deoxydizing agent is a chemical that strongly combines with oxygen better than carbon and iron do. Therefore, as the molten steel cools, the dissolved oxygen combines with the deoxydizing agents first, before it has a chance to react with the iron or carbon in the steel. A good deoxydizing agent also forms solid slag rather than a gas, so that there are no gas bubbles or cracks formed as the steel cools. Such a steel is called "Killed Steel".

Typical deoxydizing agents are aluminum, ferrosilicon (an alloy of iron and silicon) or ferromanganese (an alloy of iron and manganese). These combine with the oxygen dissolved in the molten steel to form aluminum oxide (alumina) or silicon dioxide (silica). Deoxydizing agents are added as soon as the steel is poured out from the furnace into molds and may be added individually or together, depending on the type of steel desired.

As the molten killed steel hardens in the mold, there are practically no gas bubbles seen, because most of the dissolved oxygen has been removed by the deoxydizing agents. Since there are no bubbles formed, the steel quietly solidifies in the mold and this is why it is called "killed steel". The ingot is generally free from blowholes and the distribution of carbon and other alloying elements in the steel is more uniform. This ensures that the killed steel ingot has excellent chemical and mechanical properties that are uniform throughout the entire length of the ingot. Killed steel ingots are sometimes marked with the letter "K", to indicate how they were manufactured.

Any steel with carbon content greater than 0.25%, or in general, any steel that is meant to be forged later, is killed, Stainless steel and alloy steels are also killed as part of their manufacturing process. As we saw earlier in the series, 4140 and 4150 steels that are used in firearms have 0.40% or 0.50% carbon content. Stainless steel is also used in the firearms industry.


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Questionable Tactics

December 4, 2014, 10:13 pm
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After that long series about different metals used in firearms manufacturing, might as well take a break from a dry topic and watch something else instead.

Initially, your humble editor thought that someone was parodying the infamous Rex Kwon Do scene from the movie Napoleon Dynamite. (In case you haven't seen the movie, here's clip 1 and clip 2). Unfortunately, what you're about to see is not a joke, there is actually someone trying to train people to use tactics like this. This group is called the "Sulsa Do Corps" (no joke, that's what they call themselves). See for yourself:


This video was originally posted at a youtube channel called God Rock Ministries / Expert Karate, which appears to be some sort of combination of church and karate school (dojo). This school appears to be run by a Mr. David Bateman. They removed the original video from the channel, when word about its unintentional comedy started to spread. Unfortunately for them, the video was saved by someone else and here it is :).

It is a darn good thing they are using pellet guns with no ammunition, instead of real pistols. Questionable tactics? Where do we start. First, we have quite a few instances of them sweeping each other with the muzzles of their pistols (big safety no-no). Then, we have fingers placed on the trigger at all times (bad idea). Next we have a couple of cases of shooting at the ceiling and open door, while moving and rolling around (another safety no-no). What's with the backwards dive to the ground anyway and since when is running backwards without looking at where you're going ever a good idea. Then, we have firing the pistols close to own face/someone else's face (if those were real pistols, guess whose eardrums are getting blasted to hell, not to mention hot cartridge brass getting ejected on someone else). We also have thumbs placed behind the slide in some instances (if that was a real pistol, someone is going to have a broken thumb when the slide moves backwards at high speed after the cartridge is fired). Then, they run in front of each other, with the ones in the back shooting (good chance of getting shot in the back if they were using real pistols). Then, there's the firing between the legs position (guess whose kneecap is going to receive some hot cartridge brass if those were real pistols). Firearm sights are there for a reason, but they don't seem to know how to use them. We even have a few instances of triggers pulled when one of the others was in the line of fire. The funniest part in your humble editor's opinion is around 0:13 of the video, when the gentleman starts his solo run backwards, holds his pistol practically next to his cheek, then shoots one through the open door, then falls over backwards and bangs his head against the wall! There's probably a few more bad things I missed because I was laughing too hard.

The scary part is that they appear to be dead serious and actually imagine that this is good training. If those were real pistols, someone is definitely going to get hurt or worse. In case you're wondering, this David Bateman has a few more videos about his martial arts training academy, including this gem:


Yep, seems he's the second coming of Rex Kwon Do himself.

Enjoy!
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Forging Rifle Barrel Blanks in the 1920s - I

December 9, 2014, 1:11 am
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After all the stuff we studied about metallurgy in the last several posts, we will look at an ancillary subject today, forging of rifle barrel blanks. We have already covered barrel manufacture from barrel blanks in some detail in previous posts many months ago. In today's post, we will study the process of manufacturing the barrel blanks as it was done in a factory in America in the 1920s. In particular, this was a factory belonging to Wheelock, Lovejoy & Company, which was designed to mass-produce rifle barrels designed to meet specifications demanded by some foreign governments. Some of the pictures and information in this post was taken from the book "The Working of Steel" by Fred Colvin and K.A. Juthe, and in addition, K.A. Juthe was the designer of the factory as well.

This factory did not manufacture its own steel: instead, they bought what they needed from a large steel manufacturer. The steel manufacturer made the steel to the required specifications and supplied them in the form of bar stock, but the length of the supplied bars was longer, typically each bar measured something like 30 or 35 feet long (about 9.1 to 10.7 meters long).

Cutting the bar stock to size.

Therefore, the first step in the process was to cut each steel bar into smaller lengths to make barrels. The bars came on trucks and were fed through the cutting-off shear, where they were cut into pieces of the proper length. The pieces were actually a little longer than the final barrel lengths, to allow for trimming during the machining process.

A close up of the details of the cutting off is shown in the next image.


A is the stock stop bolted to the side of the frame and the ledge formed by the strip bolted to the stop, keeps the bar stock level during the cutting process. The hold-down B prevents the back end of the steel bar from flying up when the bar is cut. The knife C has several notched edges with which the barrels can be cut, so that it need not be taken out for resharpening, until all the notches are dull.

The cut barrel pieces then passed into the next room, where there was a forging or upsetting press.

Upsetting (or more properly, upset forging) is a process of increasing the diameter of the end of a work piece, by compressing it along its length inside a die. The images below show the process.





The barrel pieces were heated in a furnace to soften them, before being sent through the upsetting press. The press could handle the barrels from all the heating furnaces shown in the room. The men changed work at frequent intervals, to avoid excessive fatigue.

The barrels were then sent through a continuous heating furnace to be reheated and then straightened out as much as possible before being tested for straightness.

A Continuous Heating Furnace



In the above machine, each barrel was tested for straightness by placing it on the rollers as shown in the image above. The screw on the press was used to apply pressure and straighten out the barrel as needed.

We will continue our study of the process in the next post or two.

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Forging Rifle Barrel Blanks in the 1920s - II

December 14, 2014, 11:40 pm
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In our last post, we studied some parts of a factory designed to produce rifle barrel blanks. In today's post, we will continue studying the process. As noted before, some of the details come from a book, "The Working of Steel" by Fred Colvin and K.A. Juthe, and in addition, K.A. Juthe was the designer of the factory as well.

Where we last left off, the barrel blanks were straightened out and tested for straightness. The next process was to heat treat the barrel blanks and increase their hardness. We will discuss this heat treating process shortly.

The last process was to grind the ends of each blank and then grind a spot on the enlarged end of each blank and test the hardness of the blank on a Brinell machine, to ensure that the blanks met the required hardness nunbers

The Brinell hardness test was invented by Swedish engineer Johan Brinell in 1900. It was one of the first standardized hardness tests used in engineering and is still used today. The test is very simple. It uses a steel or tungsten carbide ball of diameter 10 mm. (0.39 inches), which is used as an indenter. The ball is placed on the surface of the object to be tested and a 3000 kg. (or 6600 lbf.) test force is applied to the ball for a specific time (normally 10 to 15 seconds). After this, the ball is removed and it leaves a round indentation on the surface of the object. The diameter of this indentation is measured and the Brinell Hardness Number (BHN) is calculated, based upon the diameter of the ball, diameter of the indentation and the force applied to the ball. For softer materials, such as aluminum, a smaller test force (e.g. 500 kg. (or 1100 lbf.) is used instead.


The above image shows a line drawing of the concept and the formula actually used to calculate the Brinell Hardness Number (HB in the image above).

Returning back to our study of the factory process, the barrel blanks were tested for hardness to make sure that they had a Brinell Hardness Number (BHN) of at least 240.


At this point, the barrel blanks were shipped off to a barrel manufacturer, who would then drill, ream, finish-turn and rifle the blanks into complete barrels.

Now. all through the description of the process so far, we've been talking about heating the blanks for various purposes. We will cover the heat treatments in detail here. There were actually four separate heat treatments done to the blanks.

  1. Heating and soaking the steel above the critical temperature and quenching it in oil, to harden the steel through to the center of the blanks.
  2. Reheating the steel for drawing of temper  for the purpose of meeting the physical specifications of the blank
  3. Reheating the blanks to meet the machineability test for production purposes
  4. Reheating to straighten out the blanks when hot.
We will study each of the four heating processes in detail. 

For the first heating process, the blanks were slowly brought up to the required heat, which is about 150 degrees Fahrenheit (65.5 degrees centigrade) above the critical temperature of the steel. The blanks were then soaked at a high heat for about one hour before quenching in oil. The purpose of this treatment was to eliminate any strains already existing in the bars that may have been put there from milling operations done to the bars. Remember that steel is an elastic substance and working it puts stress on the bars. For instance, during the production of steel, the manufacturer rolls the bars through various rollers to make them the required diameter, which causes the bars to come out stressed. The heat treatment process removed the stress caused by rolling, hammering, cutting etc. It also ensured that the heat treatment applied to the entire cross-section of the bar and not just the surface. In addition, if a blank had seams or slight flaws, these opened up drastically during the quenching process and made it easy to determine if a blank was defective or not.


The oil used for quenching was kept at a temperature of  around 100 degrees fahrenheit (38 degrees centigrade). This is an ideal temperature is to prevent shock to the steel when it is dropped into the quenching oil, otherwise it could develop surface cracks on the piece.

The second heating process (the one for drawomg the temper of the steel) was a very critical operation and had to be done carefully. The steel had to be kept heated within 10 degrees of temperature fluctuation in the process. The degree of heat necessary for this operation depended entirely on analyzing the steel. Even if the steel was purchased from the same manufacturer, there was always some variation in different batches received from the manufacturer.

The third heating process (reheating for machineability) was done at a temperature of around 100 degrees Fahrenheit (38 degrees centigrade) less than the drawing temperature used for the second heating process. However, the time of soaking was almost double that of the second process.

For both the second and third heating process, after the heating was done. the blanks were buried in lime so that they would be out of contact with air, until their temperature had dropped down to room temperature.

The fourth heating process was used when straightening the blanks. In this process, the blanks were first heated to about 900-1000 degrees Fahrenheit (482-538 degrees centigrade) in an automatic furnace for 2 hours before straightening them. The purpose of heating before the straightening was to prevent any stresses being put into the blanks during the straightening operation. This is necessary because when later processes such as drilling, turning and rifling are done to the blanks, they have a tendency to spring back into the shape they were in when they left the quenching bath. By heating before straightening, the blanks are prevented from doing this.

Another method was later found to produce an even better barrel blank. The blanks were first rough-turned to the final barrel diameter and then heated to about 1000 degrees Fahrenheit (538 degrees centigrade) for about 4 hours before sending them to the barrel manufacturer. Blanks produced with this method remained practically straight during the different barrel making operations (drilling, reaming, finish-turning and rifling). This meant that the barrel manufacturers didn't need to straighten barrels after they were finished (which was a much more expensive operation). This method was tested out with one of the largest barrel manufacturers in the US and it proved to be very effective.

As the reader might be wondering, all this heat-treating needed a large amount of oil for cooling and one of the problems was how to keep all this oil at the proper temperature. After much study, a cooling system was developed for the factory. The next two images show the cooling system as seen on the roof from the outside of the factory.


Click on the images to enlarge. Public domain images.


The next image shows the details of the cooling system:


The hot oil is pumped up from the quenching tanks through the pipe A into the tank B, From here, the oil runs down onto the separators C, which break the oil up into fine particles, that are blown upwards by the fans D. The spray of oil particles is blown up into the cooling tower E, which contains banks of cooling pipes and baffles F. Cold water is pumped through the inside of the pipes. The spray of oil particles collects on the outside of the cold pipes and forms larger drops, which fall downwards onto the curved plates G and then run back to the oil-storage tank below ground. The water pumped through the cooling pipes comes from 10 natural artesian wells at a rate of 60 gallons per minute and this serves to cool about 90 gallons of oil per minute, lowering it from a temperature of about 130-140 degrees Fahrenheit to about 100 degrees Fahrenheit. The water comes out of the wells at an average temperature of 52 degrees Fahrenheit. The pump is driven by a 7.5 HP motor and the speed can be varied to suit the amount of oil to be cooled. The plant was designed to handle up to 300 gallons of oil per minute.

The finished blanks from this factory were sent to different barrel manufacturers to drill, ream, rifle etc. to their requirements.


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What's the deal with Barrel Shrouds?

December 16, 2014, 12:59 am
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So what is a barrel shroud? It is simply a hollow covering tube that surrounds a barrel (either partially or fully). What does it do? Well, it protects the user of the firearm from accidentally burning himself or herself with the hot barrel.

A typical barrel shroud

A barrel shroud typically has many holes throughout its length. The holes serve to reduce its weight and also dissipate heat by venting out hot air. The next picture shows a barrel shroud attached to a rifle.


As you can see in the above image, the barrel shroud is simply that tube with holes that surrounds the barrel. In the above example, the user has also attached an extra hand guard to the barrel shroud. Since much of the barrel shroud is not in contact with the hot barrel, if the user was to accidentally touch the front of the firearm, the user will not get burned by the barrel.



The curious reader is probably thinking now, "isn't that what the stock of a firearm is designed to do?", Yes, the stock and the receiver do protect the user's hands as well, but they are not considered as barrel shrouds, because they serve other purposes as well, whereas the barrel shroud is a separate component that is screwed on around the barrel and explicitly designed to protect the user's hands (or other body parts) from heat.

Barrel shrouds are generally commonly seen with air-cooled machine guns, but they are also optional components for many semi-automatic models. Some shotguns also feature barrel shrouds. There are many third party component makers that make barrel shrouds for various rifle and shotgun models. In general, they are useful to have with weapons that fire rapidly, because the barrel can heat up quickly after a few shots.

If a barrel shroud is simply a covering tube to protect a user from touching a hotter part of the gun, then what's the big deal about them? Well, for a while, barrel shrouds were targets of legislative restrictions in the United States. The now expired Federal Assault Weapons Ban explicitly included barrel shrouds in its list of features for which a semi-automatic pistol could be banned (if a firearm had two features in the list, it could be banned under this law). After the law expired, proposals were made to renew the ban, including this provision, but have not been successful so far.

Amusingly, during an interview on MSNBC in 2007, Representative Carolyn McCarthy was asked about her gun control legislation and why it prohibited people from purchasing firearms that have barrel shrouds and if she even knew what a barrel shroud was. After attempting to avoid the questions twice, she finally admitted, "I don't know what it is, I think it is a shoulder thing that goes up!"


It is amazing that she was trying to introduce a law to ban something without even knowing what it was!

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