Propellants: Smokeless Powders

In the last couple of posts, we studied the manufacturing techniques of black powder. In this post, we will study the next development of propellants, namely smokeless powders. First, let's get a couple of common misconceptions out of the way:

1. There is no single smokeless powder. Instead, the term applies to a number of different powders, all made of different ingredients.
2. Smokeless powders are not truly smokeless. It is true that during combustion, most smokeless powders burn up most of their mass into gaseous products, unlike black powder, which leaves behind 55% of its weight as solid residue. However, there is still some smoke produced.

Smokeless powders provide much more propellant force than the same amount of black powder, which made it possible for weapon ranges to increase. Since smokeless powders don't leave behind as much residue as black powder does, the weapons require less cleaning after use. The development of semi and full-auto weapons was also made possible because of the fact that there is very little residue and thus cannot easily jam the many moving parts of an automatic weapon.

While we noted in our post about propellant basics, that materials classified as "low explosives" are suited for propellants, smokeless powders generally contain a good percentage of high explosive materials such as nitroglycerine (go read the post on propellant basics to understand the difference between low explosives and high explosives and why low explosives are used with firearms). The way it works is that even though the propellant has a high explosive substance, a stabilizing chemical is also added to slow down the burn rate of the high-explosive so that it behaves more like a low-explosive.

The first smokeless powders were made in 1846, when both nitrocellulose (gun cotton) and nitroglycerine were first developed in Europe. The invention of gun cotton was actually the result of a happy accident. A Swiss scientist named Christian Schonbein was forbidden by his wife from conducting chemistry experiments at home, but he didn't always obey her. One day in 1845 when she was away, he accidentally spilled a mixture of strong nitric and sulfuric acids in the kitchen. He quickly wiped the mess up with his wife's cotton apron and then hung it over the stove to dry. To his surprise, the apron ignited and disappeared almost instantly, leaving behind almost no ashes. What Schonbein had done was accidentally manufacture nitrocellulose. Soon, with the help of another professor, he came up with the recipe of one part of fine cotton fibers, combined with fifteen parts of an equal blend of sulfuric and nitric acids. The cotton is dipped in the acid mixture for two minutes and then washed in cold water to remove any acids. Then the cotton is dried at moderate climate temperatures to form nitrocellulose. This material provides less heat and smoke and upto six times the explosive force of the same volume of black powder. However, guncotton was notoriously unstable and therefore, the British, French and Prussian governments stopped using it soon after. However, the French continued to perform experiments to improve its stability.

In 1884, a French chemist named Paul Viellie succeeded in improving guncotton's stability issues. He discovered that by treating guncotton with a mixture of alcohol and ether, it could be gelatinized. The material could then be rolled into sheets, cut into small squares or flakes and then stabilized with a 2% solution of diphenylamine. This formulation was codenamed Poudre B by the French government and it was a closely guarded secret. This formula produces a substance that is much more stable than guncotton and it will not detonate unless it is compressed. Unlike black powder, Poudre B also burns when wet and produces about three times the force for the same volume. This was the first "single-base" powder. The French developed the 8 mm. Lebel cartridge (the first smokeless military cartridge) and a new rifle, the Lebel Model 1886 to use this new technology.

In 1888, Alfred Nobel (the same person who started the Nobel prizes) discovered that he could gelatinize nitrocellulose by using nitroglycerine. The chemical formula was 45% nitroglycerine, 45% guncotton (nitrocellulose) and 10% camphor. He started to market his invention as "Ballistite" and it was the first "double-base" powder. The presence of nitroglycerine gave ballistite an even greater range than Poudre B. Nobel Industries set up a plant in Scotland to manufacture ballistite.

In the very next year, the British government appointed an "Explosives Committee" to monitor and study developments in other countries. They obtained samples of Poudre B and Ballistite, but decided that neither was suitable for adoption in UK. Two of the committee members, Frederick Abel and James Dewar, discovered that by combining 58% nitroglycerine, 37% guncotton (nitrocellulose) and 5% vaseline and dissolving the mixture in an acetone solvent, they could produce a paste which could be squeezed through a die to form a long thin string or cord of 1 to 5 mm. diameter depending on the application. From the cord, small pieces and shavings could be cut mechanically using a knife or a file, or it could be ground up using a device similar to a coffee grinder. In most cases, it was cut into small strings and packed into a cartridge case like spaghetti. This material was naturally given the name "cordite". It is also a double-based explosive like ballistite and later, a triple-base cordite was also invented. Abel and Dewar were the target of a lawsuit by Alfred Nobel, who felt that they had merely modified Ballistite slightly. The case took several years to be resolved and eventually reached the House of Lords, where the court ruled in favor of Abel and Dewar.

Disassembled cartridge. Note the light brown strings of cordite which were packed inside it.

An interesting feature of cordite (and some other smokeless powders as well) is that if the strings are burnt outside the cartridge, then they burn rather slowly with a yellow flame and no explosion. Cordite only explodes if it is lit in a confined space (such as a cartridge packed with cordite). It is also very resistant to shock. For example, it is possible to shoot cordite with a rifle bullet and still not explode it.

The first version of cordite was labelled Cordite Mk-1. The original version was the cause of early gun barrel erosion and so a new version was invented. This version had 65% guncotton, 30% nitroglycerine and 5% vaseline and was dissolved in acetone and was dubbed Cordite MD (MD for Modified). This version did not damage the barrels as much, but exploded with lesser force than Cordite Mk-1. Hence the cartridges were made to contain 15% more material to maintain the same force as Cordite Mk-1.

During WW-I, due to a shortage of acetone, Chaim Weizmann (later, the first president of Israel) invented another formula for use by the Royal Navy. This formula was called Cordite RDB (for Research Department Formula B). It was made by combining 52% guncotton, 42% nitroglycerine and 6% vaseline. Cordite SC (SC = Solventless Cordite) was invented before WW-II and used for larger guns (such as anti-aircraft). Another development in WW-II was Cordite N. This was made by combining cordite with nitroguanine, which is another explosive. Cordite N was the first triple-base explosive.

Despite all the improvements, cordite started to lose popularity around the middle of WW-II when newer propellants were invented. By the end of the 20th century, the last cordite manufacturing plant closed down.

One series of powders developed during WW-I to replace cordite was called IMR (Improved Military Rifle), which was developed by Dupont to replace the older MR (Military Rifle) series of powders. There were a number of IMR powders developed between the two World Wars. They are made of nitrocellulose, but contain dinitrotoluene (DNT) to slow down the burn rate of nitrocellulose to a low explosive. Graphite is also added to minimize static electricity and a small amount (0.6%) of diphenylamine is used as a stabilizer. A small amount (1%) of potassium sulfate is added to decrease the amount of muzzle flash. The powder is extruded out in the form of sticks. Different IMR powders were used to manufacture such famous cartridges as the .30-06 used by Enfield and the Mauser 7.92x57 mm. IMR powders are still used to this present day and are sometimes known as "stick powder" because the process of extrusion creates sticks of the propellant.

In 1933, another invention was the ball-powder propellant. This is made by dissolving guncotton in ethyl acetate and then forming the round grains under water. This process is similar to how round oil droplets are formed when mixing oil in water and shaking the contents of the bottle. Nitroglycerine is added to the grains to increase the explosive force and dinitrotoluene or a similar substance is added to slow down the burn rate. Like IMR powders, there are a number of ball powders as well using slightly different proportions and different substances to slow down the burn rate. Ball powders started to gain popularity in the 1950s. For instance, the ball powder WC 844 is currently used in the NATO 5.56x45 mm. cartridges.

The advantages of ball powder over other types of smokeless powder are many. For one, it takes a lot less time to manufacture than other types. Most other smokeless powders take a few months to manufacture. Dupont did manage to get one IMR powder type to be manufactured in 2 weeks. In contrast, one production lot of ball powder could be made in under two days. Ball powder can also be stored longer than other types. Excess acids during the manufacture of smokeless powder cause the powder to deteriorate more quickly. The ball powder manufacturing process is more efficient in eliminating most of the excess acid and it doesn't produce much acid as it ages either. The manufacturing process is also safer as it happens under water and also requires much less specialized equipment to set up a manufacturing line.

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.

Propellants: Basics

After our study of barrel making technologies, we will devote some time in the study of propellants. A propellant is the substance that when burned, generates pressurized gas to push the bullet out of the barrel of the gun and onto its target. For most of firearms history, black powder was the main propellant of choice until about the late 1800s or so when it began to be replaced by smokeless powders and cordite.

The main requirements of propellants are:

• Stability: The propellant should not easily ignite if it is shaken or accidentally dropped.
• Long shelf life
• Less residue: Burning black powder leaves a lot of residue behind in the barrel, which means that the user has to spend time cleaning the barrel often. Newer powders leave very little residue behind.
• Uniformity: The propellant should be manufactured so that it provides a similar amount of propulsive force, even if one batch of propellant is manufactured at a different time from another batch, or by another manufacturer. This way, the firearm's range and accuracy are not compromised when firing cartridges manufactured at different times and places.
• Low explosive power: This one is a bit hard for newbies to understand. Basically, the propellant should burn pretty violently when lit, but not quickly enough for the speed of the flame front to exceed the speed of sound (otherwise, it is classified as a "high explosive".) The idea is that the burning propellant generates pressurized gas, which expands out and pushes the bullet out at speed. The expansive forces are also acting on the inside of the barrel (which is made of iron or steel), which has a certain elasticity. The forces acting on the barrel shouldn't apply force at a rate greater than what the barrel's elasticity can handle, otherwise the barrel will explode. Therefore, a good propellant should be explosive enough so that the maximum speed of the bullet can be achieved, but not explosive enough to convert the firearm into a hand-grenade! This is why people don't use TNT or RDX in modern propellants! In some cases, people use high-explosives (e.g. nitroglycerine) for propellants, but they add enough stabilizers to slow down the burn time of the high-explosive so that it can be safely used as a propellant.

In the next few posts, we will study the history of propellants and their manufacturing techniques.