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 - XIII

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

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

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

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

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

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

A teemer at work. Public domain image.

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

Click on the image to enlarge.

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

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

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

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

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

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

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