Metals Used in Firearms - XIV

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.

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, 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 carbon and impurities had been burned off, the flames would no longer shoot out of the front of the furnace thus indicating that they were all 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. 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. The flame produced by burning the impurities is large and spectacular and while it is burning, people cannot approach the vessel to take samples, therefore the amounts of various elements in the steel cannot be adjusted midway through the process. 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.

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 - 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.