Monday, November 24, 2014

Metals Used in Firearms - XVII

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

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

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

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

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

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

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

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

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

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

Wednesday, November 19, 2014

Metals Used in Firearms - XVI

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

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

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

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

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

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

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

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

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

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

Monday, November 17, 2014

Metals Used in Firearms - XV

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Saturday, November 8, 2014

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 ore.

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

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

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

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

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

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

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

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

With the invention of the Bessemer process, not only did the time taken to produce steel from pig iron drop significantly (it was faster to produce than even wrought iron!), it was more efficient and could work with larger volumes of cast iron as well. The cost of producing good-quality steel dropped from about £60 per ton to about £7 per ton, shortly after Bessemer started his first factory. With improvements to the process made by others, the prices dropped even more. For instance, an invention by William Jones, while working in the Edgar Thomson steel plant, improved the Bessemer process to become a continuous process. flowing molten iron directly from the blast furnace to the bessemer converter, without waiting for the cast iron to harden first. As a result of this, steel began to replace wrought iron in many applications, as it was now cheaper to produce, as well as being tougher and stronger than wrought iron. The Bessemer process started declining in England around 1895, but it continued in other places in the world for a lot longer. Germany produced most of its steel in the 1950s and 1960s using this process, and in America, the last factory using the Bessemer process closed in 1968. One of its issues was actually its speed of production -- it ran too fast! Given that the steel could be produced in under 20 minutes, this gave little time to analyze the steel and make sure that it has the alloying elements in the correct proportions and to adjust the percentages as needed. 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.

Sunday, November 2, 2014

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

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

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

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

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

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

Sunday, October 26, 2014

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.

Wednesday, October 22, 2014

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.