Historical Manufacture of Charcoal - II

In our last post, we looked at the charcoal manufacturing process, as it was done from the 14th to the early 20th century. In today's post, we will look at some variations of the process.

As we saw in the last post, charcoal manufactured for the purposes of gunpowder had to be of a higher quality with uniform charring. Therefore it was manufactured in smaller batches using iron cylinders to heat the wood, instead of heating up large heaps. Our last post also described the process in England, where they used small iron cylinders, each holding about 80 lbs. of wood, being placed inside a furnace and heated. In today's post, we will study some variants of this basic method.

Instead of using fixed carbonizing cylinders, many black powder factories in England started switching towards using movable cylinders in the 19th century. Each furnace was provided with two cylinders, so that one could be refilled while the contents of the other were being carbonized. Each filled cylinder would be run into the furnace on rails, with the rails supporting them over the fire. An elaborate system of pipes and valves was used to distribute the gases and the wood distillation byproducts (wood gas, tar, volatile chemicals etc.), so that they could be redirected back to any one of the furnaces, or allowed to escape through the chimney.

The advantages of this process were:

1. Uniformity of the charcoal being produced.
2. The gases produced by distilling the wood could be reused to additionally heat the furnace, thereby saving on fuel costs
3. The charcoal was cooled down out of contact with the air, which took away the possibility of the charcoal catching fire.

In some British factories, vertical movable cylinders were used instead. The advantages of this were that more cylinders could be fired at the same time and the moving of the cylinders to the cooling room was easier.

In Sweden, some factories used rotating cylinders, with each cylinder being rotated 90 degrees on its horizontal axis every 30 minutes. This allowed the heat to act upon each side evenly and this process gave a more uniform carbonization and saved fuel as well.

Another method of carbonizing wood used superheated steam to do the job. Pressurized steam was produced by passing water through a coil of wrought iron heated by a fire. For the production (from dogwood) of charbon roux (brown charcoal) containing 70% carbon, the temperature of the steam had to be around 280° Fahrenheit; by using steam heated to about 350°, charcoal containing about 77% carbon could be produced, and by heating both the cylinders and the steam to about 450° fahrenheit, charcoal of about 89% carbon content could be produced. The charcoal produced by this method was very uniform in composition, but the method was later abandoned because it gave a larger yield of charbon roux, but not so much black charcoal, as the ordinary method of carbonization using iron cylinders; and the lightly-burnt charcoal was only required for sporting powders. Also, the cost of production of charcoal using superheated steam apparatus was greater.

In 1887, one Mr. H. Guttler of Reichenstein, Germany, invented a process of carbonizing wood (he received British patent # 8929 on June 22nd, 1887 for his idea "Improvement in the Manufacture of Charcoal for Explosives and other Purposes, and Apparatus for that Purpose"). His idea consisted of putting the material to be carbonized into a suitable air-tight cylinder fitted with a pressure gauge and pyrometer and using an arrangement of two furnaces to heat it. One is a normal charring furnace and the other is a producer-furnace, which produces carbon dioxide gas by blowing air through burning coke using a fan. The heated carbon dioxide gas is then piped into the cylinder (similar to the superheated steam process we saw above) and the carbonization takes place. The pressure of the carbon dioxide in the cylinder can be varied as needed. The temperature is regulated by admitting cold air to the muffle and by varying the supply of heated gas into the cylinder. After the charring is completed, cooled carbon dioxide is passed through the charcoal, which rapidly cools and absorbs the carbon dioxide in its pores. The advantage of this process over using superheated steam was that it didn't leave the charcoal produced in a moist state, which the steam process did. It could also be used to produce charcoal from cheaper materials such as wood cuttings, pulp, straw, peat etc. Another advantage was that since it used carbon dioxide instead of air, the charcoal produced could not spontaneously ignite.

In the next post, we will look at the historical production methods of the third ingredient of gunpowder: sulfur.

Historical Manufacture of Charcoal

After spending the last few weeks studying the history of saltpeter manufacturing around the world, we will spend some time today studying another component of gunpowder: charcoal. We actually studied this topic briefly when studying about black powder many months ago. We will revisit this topic in more detail today.

Strictly speaking, the chemical element that is used for gunpowder is carbon, which is supplied by the charcoal. The carbon acts as a fuel in the gunpowder.

Since the very early days of firearms, it was found that the quality of charcoal is a pretty important factor in the quality of the gunpowder produced. Therefore the process of manufacturing high quality charcoal was regarded as a closely-guarded state secret. Charcoal made by burning wood in heaps or kilns is not very suitable for gunpowder. Instead, to make high quality charcoal, the wood must be selected very carefully and burned uniformly in ovens or iron vessels. The procedure to do so hadn't changed very much from the fourteenth to the twentieth century. We will look at the process used at Waltham Abbey in England, during the early 20th century.

First, the choice of wood for making charcoal for gunpowder: Soft and light woods are preferred, as they leave less ash. At one time, the charcoal for black powder in England was exclusively made from alder wood, but later other soft woods were also used. In England, dogwood was extensively used, especially for small grain powders, and for larger grain powders, alder and willow wood were used, with straw charcoal being used for brown powders used in heavy ordnance; In America, cottonwood, redwood, soft pine and western cedar trees were used; In Germany, alder and willow were mostly used; in Austria, hazel and alder; in Switzerland, hazel trees; in France, dogwood was exclusively used in military and sporting powders, but as it became more difficult to procure, alder, poplar and lime were tried out; in Russia, alder was commonly used; in Spain, yew, oleander, willow, hemp stems and vine; in Italy, hemp stems were used mostly.

The trees chosen were usually between two and  ten years old. The trees were generally cut down in spring for a few reasons. First, this is the time when trees are the most full with sap, which means the sap is very watery and contains less dissolved salts in it, thereby producing less ash. The second reason is that trees cut down in spring are easiest to separate the wood from the bark, which is good because the tree bark contains a large portion of the ash produced. The wood was seasoned for at least 1.5 to 3 years, to allow most of the sap in it to dry out. The method of doing this varied by location and type of wood. For instance, in Germany, it was customary to keep the wood inside sheds in Dresden, but at Spandau, they kept the wood out in the open. In England, dogwood was covered with thatch, but the willow and alder woods were dried out in the open.

After drying, the wood was split into pieces about 3 feet long by 1 inch thick. These pieces were placed in iron cylindrical cases called slips. Each cylinder was about 2 feet in diameter and 3.5 feet long. The lid was fastened to each slip, with two openings (about 4 inches diameter) being left in the bottom of each slip. The slips were then placed in horizontal cylinders, the end of the slip with the openings going to the further end of the cylinder, in which end there were openings corresponding to those in the slips. The cylinders were then lifted with pulleys into a furnace, where they could be heated as uniformly as possible. The cylinders were placed such that the furnace flames surrounded the cylinder entirely, so the heat acted upon the whole surface as much as possible. The higher the temperature and the longer the heating time, the lower was the percentage of hydrogen and oxygen in the charcoal, which made it harder and more difficult to ignite. Therefore the type of wood and the type of gunpowder that the charcoal was meant to be used for, determined how long the cylinders were heated. For instance, to make R.F.G (Rifle Fine Grain) powder or M.G. (Machine Gun powder for Nordenfelt guns) powder in Waltham Abbey, dogwood was heated for abuot 4 hours. Alder and willow for R.L.G (Rifle Large Grain) powder was heated for 3.5 to 4 hours and for P grade gunpowder, it was heated for 6 hours. Smaller cylinders were used, to make the composition of the charcoal more uniform, since high temperature is not needed to carry the heat to the center of the wood pile in each cylinder. However, the use of small cylinders reduces the efficiency and raises the cost of production. In England, most cylinders were only large enough to hold about 80 lbs. of wood. Incidentally, for a given temperature, slow carbonization produces much more charcoal than quick carbonization at the same temperature.  Also, the lower the temperature used for carbonization, the lower the temperature at which the charcoal burns. Therefore charcoal made at 260-280 degrees centigrade burns at around 340-360 degrees centigrade, whereas charcoal made at 950 degrees burns at around 1900 degrees.

As the cylinders were heated, the volatile chemicals and tar in the wood would be released by the decomposition of the wood. Normally, these gases could be condensed by using a condenser and used to make useful chemicals like lime acetate and wood spirit. However, charcoal produced by gunpowder mills were generally on a much smaller scale that the charcoal used for metallurgy, therefore it was not considered to be worth the effort to do so. Instead, the gases were removed via a pipe and fed back into the furnace, where they could be burnt. Doing this saved a considerable amount of fuel, thereby reducing costs. When the wood was sufficiently charred, the color of the flame would change to a bluish violet, indicating the formation of carbonic oxides. At this point, the furnace is opened and the cylinder is taken out using pulleys and replaced by a fresh cylinder. The cylinder taken out was placed in a larger cylinder with a tight fitting lid and allowed to cool for about 4 hours, until all the fire in the wood could be extinguished. It is necessary to do this cooling out of contact with the air, otherwise the charcoal could catch fire. The charcoal was then emptied into smaller cylinders and carefully picked by hand to ensure that it is properly and evenly burnt. It was cooled in the smaller cylinders for about one to two weeks, to reduce the danger of spontaneous combustion (caused by the charcoal absorbing oxygen from the air), before being sent to be ground.

Charcoal intended for firearms use was generally jet black in color and so soft that it could not even scratch a copper plate. The following table shows an analysis of the charcoal produced for different powder grades:

Note that the Spanish Hemp Charcoal has a higher percentage of ash than the others. This is because it was manufactured by burning the charcoal in pits holding about 0.5 to 1 ton of wood each (unlike in England, where each cylinder only held about 80 lbs of wood). Also in the Spanish method, when the wood had carbonized enough, the pit was covered with a woolen cloth upon which earth was placed, which accounts for the higher percentage of ash produced.

Inferior quality charcoal was generally used for powders shipped to Africa and Brazil, since the locals there seemed to value the shiny quality of the black powder rather than its shooting properties.

In our next post, we will look at some variations of this process used by factories in England, Sweden and elsewhere.

Propellants: Black Powder - II

In our last post, we studied the basic components of black powder and how they were obtained throughout history. In this post, we will study the history of its manufacture.

The three ingredients, namely potassium nitrate (or sometimes sodium nitrate), charcoal and sulfur are ground up into powder and mixed together. In the earliest processes, the dry ingredients were all put into a grinding apparatus (usually a mortar and pestle) and ground together into a powder mixture called a "serpentine". The exact proportions of the three components in the middle ages were varied from country to country. It is known that England was using a 6:2:1 ratio by weight in the 1350s, while the Germans were using a ratio of 4:1:1 during the same period. The French also had black powder, but it is not known what ratio they were using. In fact, the various proportions of the ingredients and the technique of making high quality charcoal were both closely guarded state secrets throughout most European kingdoms. There were a few issues with black powder manufacturing during this period. One was that the mixing process was highly dangerous since the materials are highly inflammable. The second was that there wasn't much consistency in the final product, so one batch of powder would have better shooting properties than another batch. The third issue was that since the powders of the three ingredients were not the same sized particles, if the powder was transported by cart to a battlefield, chances were very good that the vibrations would make the smaller ingredient particles settle in the bottom of the box. This meant that the serpentine powder would need to be mixed again thoroughly, just before use.

One of the major inventions to improve this situation came out of Europe in the late 1400s or so -- a process called "corning". The powder manufacturers of medieval Europe had realized early on that the way to reduce a large portion of the risks of the manufacturing process was to wet the ingredients with water or wine first and grind them separately and then mix the wet ingredients together. Then the resulting black-powder could be dried in the sun before use. They also realized that the ingredients could be mixed wet and pressed into cakes of a given uniform density (1.7 gm/cc is ideal). The cakes could then be dried in the sun where they become hard and brittle.

Black powder pressed into cakes of uniform density

The cakes could then be broken into grains or corns and these grains could then be sorted into standardized sizes by passing them through various sieves. This whole process was a major improvement, because it fixes all the problems enumerated earlier. In fact, all black powder manufactured to this day is still corned.

Different containers of corned black powder sorted by grain size

The Europeans experimented with manufacturing various grain sizes and determined that larger grains are more suitable for larger guns and cannons and smaller grains, which are quick burning, are more suitable for pistols. Hence, they were the first to produce different types of gunpowder, each of uniform grain sizes. Because of this uniformity and consistency, European gunpowder was generally regarded as higher quality than gunpowder manufactured in America or Asia. However, not all European countries were producing quality black powder. W.W. Greener's book, The Gun and its Development from the mid 1850s, mentions that powder in England is of various grades and makers like Curtis & Harvey and Pigou, Wilks & Lawrence make some quality powders. He says that a grain size classified as "African" is very good for export purposes, but cautions to buy a powder marked "Brazil" designed for export to South America saying that it looks very attractive because it is highly glazed, but is actually a very poor quality powder. Apparently, the South Americans valued its shiny looks more than its shooting properties and the author mentions that the only reason to buy this is for trading purposes. He says that Spanish powder quality varies upon locality and that the Swedes and Norwegians make very good quality powder, but unfortunately in limited quantities for their own markets only. He also says that German powders also vary depending on locality, but some are very high quality, sometimes even exceeding the finest English powders of the time. He classifies the powders made by the French (and all the French possessions at the time) to be the worst quality and full of dust, due to the fact that the manufacture of black powder was a monopoly held by the French government at that time and they didn't allow imports of black powder from other countries either.

By the 1750s, the ratio of 15:3:2 by weight (i.e. 75% potassium nitrate, 15% charcoal and 10% sulfur) was becoming common in most of Europe and by 1800, it was the ratio used around the world and is still the standard today.

The process of manufacturing corned black powder is still the same, except that since the 1800s, the process of grinding and mixing the materials has been largely automated. In the early 1800s, the common type of mill used for grinding was the edge runner mill. This is an ancient type of mill that was invented in China in the 5th century AD and spread to Europe about 800 years later. It was commonly used for hulling rice and crushing ore. It consists of one or more heavy disks set on their edges and a circular lower milling trough or tray. The disks roll around the trough in a circular path and crush everything in their way.

Public domain image courtesy wikipedia.com

These mills were usually driven by water power and therefore the powder factories were located close to rivers. Edge runner mills are still used in food processing today and are seen in some chocolate factories.

Most modern processes use a ball mill to do the job. It consists of a hollow cylinder into which the raw materials are put in. There are also a number of balls made of lead, brass or bronze put into the cylinder. These materials are chosen because they are non-sparking in nature. The materials are put in the cylinder and water or alcohol is added to keep it wet. The cylinder is then closed and rotated about its axis for about 3 hours.

The balls rub among themselves and grind up the raw materials between them into a fine powder. After some time, the cylinder is opened and the ground up material is extracted. In some cases, the three ingredients are ground up separately and then combined later. In other cases, the three ingredients are put into the mill simultaneously and ground up together. The three powdered ingredients are mixed together when wet and then pressed into cakes, which are dried and then broken up into grains which are sorted by sieves into various standardized sizes.

Black powder was the only propellant used between the 1200s to the late 1800s or so. One of the problems with black powder is that it leaves behind a lot of residue, which means that guns need to be cleaned every few shots or so. The invention of smokeless powders in the late 1800s reduced the need for black powder, since smokeless powders burn cleaner and with more power than black powder. These days, the only users of black powder are those that wish to hunt with weapons similar to what their ancestors used.

Propellants: Black Powder - I

The invention of black powder (the first true "gunpowder") is credited to several sources, the Chinese, the Indians, the Arabs, the Germans and the English. Certainly, the original source seems to have been the Chinese by 900 AD or so, but they seem to have used it for medicinal and alchemical purposes initially, rather than use them for firearms. The Arabs mention a formula for gunpowder by the late 1200s. Gunpowder was used in firearms in India and Arabia by the early 1300s. Roger Bacon, an English monk, published a description of gunpowder back in 1242, although he did not claim to invent it. In 1268, he published a more exact formula, listing the proportions of the various ingredients to be used. Even though Roger Bacon was the first to describe an exact formula in the western world, curiously his notes (written in Latin) begin with what would be roughly translated in English as "As everyone knows, you can make ...". Black Berthold (or Berthold Schwartz), a medieval German monk, also conducted research on black powder in the early 1300s. It is possible that the secrets of black powder were brought back to Europe by the crusaders arriving back from the middle east. Black powder was used as a propellant from the 1300s all the way to the mid 1870s or so, and is still used by many avid black-powder hunters who wish to hunt using the same technologies that were available to their hallowed ancestors.

The primary three components of black powder are a fuel, an oxidizer and a stabilizer, mixed in various proportions. The fuel is usually charcoal or sugar, the oxidizer is usually potassium nitrate (KNO3) (or sometimes sodium nitrate, NaNO3) and the stabilizer is generally sulfur (S). The burning of the carbon (C) in the charcoal produces carbon dioxide and energy in the form of heat and light. Normally, the charcoal would burn at a normal rate when burnt in atmospheric air, but with an oxidizer that supplies extra oxygen, it burns much faster than usual. The final reaction produces nitrogen and carbon-dioxide gases and potassium sulfide. Gunpowder may be made using just potassium nitrate and charcoal, but the force is not as much as when sulfur is added. Sulfur also reduces the temperature at which the ignition takes place.

The proportions of the various ingredients of black powder have varied over time. Sir Francis Bacon's formula of 1268 called for 7 parts by weight of potassium nitrate, 5 parts of charcoal and 5 parts of sulfur, though some scholars maintain that he'd invented the modern ratio of 15:3:2 as well. By 1312, the records of the Battle of Crecy and the Battle of Agincourt show that the English had settled on a formula in the ratio 6:2:1 of KNO3, C and S, while the Germans were using 4:1:1 ratio. By the 1750s, the standard ratio for gunpowder used was 15:3:2 (i.e.) 75% potassium nitrate, 15% charcoal, 10% sulfur by weight and this ratio has stayed pretty much the same since. Other ratios were used for black powder not suitable for use for firearms (for instance, blasting powder used different ratios of the ame materials).

The charcoal used in the manufacture of black powder is generally manufactured from the wood of softwood trees. Softwood trees are preferred because the charcoal from hardwood trees leave too much ash behind after combustion. According to W.W. Greener's, The Gun and Its Development, Second Edition, trees such as willow, black dogwood, alder etc are/were traditionally used in England to manufacture charcoal. In India, the woods of the locally available Grambush plant (Cythus Cajan), Parkinsonia and Milk Edge (Euphorbia Tiraculli) are used. In America, cottonwood, soft pine, redwood and western cedar are the trees of choice. The trees are generally felled in springtime, because the bark is easier to remove from the tree trunks during this time, though winter wood may also be used. The removal of bark is a necessity because it prevents the scintillation of gunpowder. The wood is cut into chips or smaller pieces about 1-4 inches in diameter and put into a vessel with a tight fitting lid. There is a small hole on top of the vessel to allow the escape of other gases. The vessel is then placed on a hot fire and heated, until organic gases begin to escape from the wood through the small hole. The gas is called wood-gas and is primarily composed of methane. This gas may be ignited with a match as it is escaping through the hole. When the gas stops issuing out of the hole, the flame goes out and this indicates that the wood has been converted to charcoal. The time taken for charring depends on the thickness of the wood pieces, as well as the heat of the furnace. Charcoal made at 240 degrees centigrade will readily ignite at 330 degrees centigrade, whereas charcoal made at 950 degrees centigrade will take nearly 1900 degrees to ignite. Hence, the best charcoal for gunpowder is made at lower temperatures. The vessel is allowed to cool and then the charcoal is removed and ground up into a powder. For uniform results, the vessels used to make charcoal are all kept at the same temperature. After the powder is ground up, it is allowed to sit for a couple of weeks. The reason for this is that freshly ground-up charcoal is highly prone to spontaneous combustion, but if it is allowed to sit for 10-12 days, it loses this property and can now be used to make gunpowder more safely.

The second ingredient of black powder is Potassium Nitrate (KNO3), commonly known as saltpeter. This occurs naturally as an efflorescence on the ground in some parts of the world, such as India and Arabia and the Andalusia region of Spain, due to the weather conditions. In parts of Europe, it was manufactured by preparing beds of manure mixed with wood ashes and leaching with urine for a period of time. In France and Sweden, the mortar from old farm walls and stables were a source for saltpeter Another source was bat dung from caves. Sodium nitrate was also used as an alternative for a while, as it was available in the Chilean desert. In the 1600s, most of Europe was importing saltpeter from ports in the Gujarat region of India. By the late 1700s and early 1800s, England's source of potassium nitrate was entirely from the Gangetic plains of India, especially from the Bengal and Oudh regions where it was naturally occurring. The salt was collected here off the ground, mixed with water and boiled and the solution then placed in shallow troughs and allowed to evaporate in the sun, leaving behind impure saltpeter crystals (called "grough saltpeter"). These were then packed into gunny bags and shipped off to England for refining. On arrival at the Royal Waltham mills in England, about two tons of the grough saltpeter were put in a large vat and dissolved in 275 gallons of water. The mixture was heated for about two hours to allow the contents to boil, the specific gravity being 1.49 and the temperature of water getting close to 230 F. Scum rising to the surface was skimmed off until no more scum was generated. Then more cold water was added and the solution was heated and then allowed to cool to 220 F. The solution was then pumped into shallow trays and allowed to cool. The cooling would crystallize the excess potassium nitrate, while the solution would contain the impurities of sulphates, chlorides etc. The solution was gently agitated to prevent formation of large crystals and form a flour instead. The flour was then washed three times and a small sample was tested to make sure it was pure enough, before the batch was used.

These days, most potassium nitrate is generally mass-produced using the Haber process, which was invented by Fritz Haber shortly before WW-I. This consists of combining nitrogen with hydrogen in the presence of a catalyst, to produce ammonia (NH3). This ammonia is then oxidized to produce nitrates. The advantage of this process is that the raw ingredients are all abundantly available in nature (nitrogen and oxygen from the air and hydrogen and oxygen from water) and thus cannot be embargoed. During WW-I, the allies had access to large naturally occurring deposits of nitrates from Chile, but the Germans were cut off from this supply and had to produce their own. It was strongly suggested that without Haber's process, the Germans could not have gone to war or would have had to surrender much earlier. Haber received a Nobel prize for his discovery. Ironically, he was forced to leave Germany by the Nazis in the 1930s simply because he was Jewish.

Sulfur is also obtained naturally around the world, mostly around hot springs and volcanic regions. It is found naturally in Sicily, Japan, Chile, Indonesia etc. It was known in ancient China and India as well, where it was extracted from pyrite ores. In fact, the word "Sulfur" is actually of Sanskrit origin (sulvari). It is also found around petroleum deposits. Sulfur is generally refined by using distillation or sublimation. Historically, the two methods used for purifying sulfur were the Sicilian process (from ancient times) and the Frasch process (used from 1890s onward till about 2002). The Claus process extracts sulfur from hydrogen sulfide gas and is the process of choice in modern times.

In the next post, we will discuss how these ingredients are combined to make black powder.