Metals Used in Firearms - XII

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 Army from 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.

Metals Used in Firearms - XI

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. (about 3300 lbs.) 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. (1100 lbs)  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.

Metals Used in Firearms - X

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.

Metals Used in Firearms - IX

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.

Metals Used in Firearms - VIII

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). Also, a bloomery can only process a small quantity of ore each time, so producing a lot of iron in a bloomery takes a long time. 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. Unlike a bloomery which works with small batches of ore, a blast furnace can be used continuously, because more ore, fuel and flux can be added from the top as needed. 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 local sources 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.

Metals Used in Firearms - VII

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 the last name "Collier" and another American colleague with the last name "Kohler" (the German name for "Collier"). Perhaps they both had ancestors that 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.

Metals Used in Firearms - VI

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.

Metals Used in Firearms - V

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.

Metals Used in Firearms - IV

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.