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.

The History of Saltpeter - XIX

In the second half of the nineteenth century, people began to look for other sources of natural nitrates besides the plains of India. Today, we will study natural nitrates from South America, called Chile Saltpeter or Peru Saltpeter or soda niter.

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

Unlike the saltpeter we've been studying so far, which is potassium nitrate, chile saltpeter is sodium nitrate. It occurs naturally in the Atacama desert, which lies between Peru and Chile. The first shipment of Chile saltpeter to Europe arrived in England in 1820 or 1825, but they could not find any buyers and therefore dumped it at sea, so that they didn't have to pay a customs toll. However, by the time of the Crimean war (1854-1855), the demand for saltpeter was so much that the existing sources from Europe and India couldn't keep up and it became profitable to ship South American saltpeter to Europe. There is a good reason why this stuff was called "white gold".

There are a few theories about how the saltpeter forms there. The classic explanation is that in the Tarapaca region in the Atacama desert, which is a dry desert that experiences a little rainfall every six or seven years, when the rain falls, it floods the plain. However, the plain slopes gently towards the coast hills and as there is no outlet for the water, it collects there and evaporates in the desert's arid conditions, and all the nitrate that was dissolved from the entire plain is deposited in a relatively narrow area. A more modern theory states that the Andes climate was warmer and wetter about 20 million years ago and nitrates, iodine and chromium deposits were leached from the ground water. About 10 million years ago, as the Andes and the coast mountains began to grow higher and higher, the climate shifted to desert and forced the groundwater to evaporate, leaving behind the nitrates. Sea spray and fog from the ocean also drop small amounts of nitrates on the surface.

Originally, the areas where most of the deposits were concentrated belonged to Peru and Bolvia: specifically Peru's Tarapaca region and Bolivia's coastal region of Antofagasta. Mining concessions were granted to Chilean and the British companies to mine these areas. Then the Peruvians and the Bolivians attempted to control the British and Chilean companies by imposing new taxes on them. This led to the War of the Pacific (1879-1883) which ended with Chile capturing both the Tarapaca and Antofagasta regions. By the 1890s, Chile was supplying a whole lot of Chile saltpeter to the world. 

Sodium nitrate can be converted to potassium nitrate by interacting with the chloride of potassium, which could be made from kelp, wood ash, or later, from carnallite, which is a mineral that occurs in huge deposits in Europe and America, particularly in Carlsbad, New Mexico, Paradox Basin in Colorado and Utah, Stassfurt in Germany and the Perm Basin in Russia.

In a heated and concentrated mother liquor, sodium nitrate (about 95% pure from the chile saltpeter) and potassium chloride (greater than 80% purity, from the carnallite) are dissolved. Due to the chemical reactions, the solution can now contain four possible salts: sodium chloride, potassium chloride, sodium nitrate and potassium nitrate. As it happens, when the temperature of the solution is high, potassium nitrate has the highest rate of solubility (i.e. it dissolves easily in water), but sodium chloride has the least solubility. At low temperatures, potassium nitrate has the least solubility. This fact is exploited to filter out the crystals of the other salts out. The liquid is boiled for about half an hour to complete the reaction as much as possible and convert most of the salts to sodium chloride and potassium nitrate. Then it is run through a filter into shallow cooling tanks. Since sodium chloride crystals don't dissolve as well in hot water, a good amount of it gets filtered out, while the potassium nitrate (which dissolves well in hot water) passes through in the solution. The solution is kept stirred while it cools, so that the potassium nitrate may form smaller crystals. The crystals are then drained and washed with the liquors from the next crystallization using a centrifuge. The crystals still contain a fair amount of sodium chloride, so they are purified by washing in cold water, which dissolves most of the sodium chloride, but leaves most of the potassium nitrate undissolved, which reduces the percentage of sodium chloride to below 0.05 percent. Then the remaining potassium nitrate crystals are dried and ready to be used for gunpowder production.

During the First World War, demand for Chilean nitrate exports skyrocketed. Before 1914, only one-fifth of Chilean nitrates were used for explosives, but after the war started, almost four-fifth of all nitrate exports were used for military purposes. Before the war, Germany was the largest market for Chilean saltpeter. For example, in 1912, Germany imported 37.9 % of all Chile saltpeter exports, England had 5.7% and the rest of Europe had 31.6%, which means that Germany alone imported more than all of Europe combined! The United States imported about 23.6% of Chile saltpeter exports during the same year. At the beginning of World War I, German Admiral Maximilian Reichsgraf von Spee commanded a fleet of ships off the coast of Chile in an attempt to disrupt Chile saltpeter supplies to everyone else. After the German fleet was destroyed in the Battle of the Falkland Islands, the exports to Britain resumed. Germany was blockaded from accessing Chilean saltpeter from 1915 onwards, leading to the United States and the UK becoming the largest markets for Chilean saltpeter. Because of this, Germany was forced to discover a method to produce synthetic nitrates. The first breakthrough came from Fritz Haber, who discovered a way to produce ammonia from nitrogen and hydrogen in the air and water (neither of which could be blockaded by the allies). Then Wilhelm Ostwald discovered a method to convert ammonia to nitric acid and from then on, Germany's military industry was free from its dependency on Chilean saltpeter. In fact, if it weren't for these two discoveries, the shortage of munitions would have forced Germany to end the war by 1915, instead of prolonging it till 1918. 

Ultimately though, the German innovations forced the decline of the Chilean saltpeter mining industry, as more and more countries began to produce synthetic nitrates using the techniques that they pioneered. At one time, Chile saltpeter accounted for 50% of Chile's Gross National Product, but by the time the 1940s rolled around, it had fallen to practically 0. Of the 170 or so "nitrate towns" that were formed in the Atacama desert to mine for nitrates, only one of them remains open today, the town of Maria Elena.

The History of Saltpeter - XVIII

In our last post, we saw how the British secured their supply of saltpeter from the largest source in the world. In today's post, we will look at how they refined saltpeter over in England. We will study the refining process at the The Royal Gunpower Mills at Waltham Abbey.

We had actually studied a bit of this process earlier, when we studied black powder several months ago. In today's post, we will look at the process in more detail.

Waltham Abbey is a small town north east of London and fairly close to it. The river Lea flows through this area and there is a large church in town. There was once a large monastery here and the monks had diverted some of the waters of the river towards a watermill that they built to process wool for cloth production (a procedure called fulling). In the early part of the 1600s, the mill was converted into an mill for producing vegetable oils. Around 1665, the Second Anglo-Dutch war started and there was a shortage of gunpowder in England, therefore the oil mill was converted to gunpowder production. At around the same time, it was acquired by a man named Ralph Hudson. At the end of the 1600s, the Hudson family sold it to William Walton and the Walton family ran it successfully as a private enterprise for almost 100 years. In 1787, it was decided that the British government should acquire the mill from the Walton family, to ensure the supply of gunpowder and establish standards for quality, at which point it became the Royal Gunpowder Mills.

Our description of the process at Waltham Abbey comes from a book written in 1915 by Arthur Marshall, who served as a Chemical Inspector, Indian Ordnance Department.

The Royal Waltham Gunpowder Mills acquired its saltpeter exclusively from India. The crude saltpeter (also called "grough saltpeter") was produced in India and then packed in jute bags and shipped over to England. At Waltham Abbey, the process was as follows:

The crude or "grough" saltpeter is dissolved in a larger pot A, which has a capacity of 500 gallons and is fitted with a perforated false bottom, which prevents the saltpeter adhering to the vessel. For each batch, about 25 cwts. (2800 lbs. or 1270 kg.) of grough saltpeter are taken, and 5 cwts. (560 lbs. or 254 kg.) of crystals recovered from liquors, and 5 cwts. of crystals left in the crystallizing cisterns. This is all dissolved in about 280 gallons (1060 liters) of the washings of the purified saltpeter, which also contains a considerable amount of the salt. The fire is lit under the pot, and in about two hours the saltpeter is dissolved and the liquid boiling. Just before it boils, a thick scum rises to the surface, consisting mostly of impurities. This is skimmed off and the false bottom is removed, and cold water is added from time to time to induce fresh scum to form, if it will. The fire is then withdrawn and the the liquid is allowed to settle for about two hours. Then a hand pump is lowered into the pot and the liquid is pumped into filters B, where it passes through linen cloth. From here it runs to shallow copper crystallizing troughs C. As it cools down, the liquid is stirred by a workman in order to make the saltpeter separate into small crystals, and the saltpeter "flour" as it forms is drawn up on to an inclined draining platform D, and from there  is passed to a washing vat E. After the temperature has fallen to about 32 °C. (90 °F.), the solution is no longer stirred and any crystals that form after that are treated as grough niter.

The washing vat E is about 6 feet long, by 4 feet wide, by 3.5 feet deep, and is fitted with a false bottom made of wood with small  holes bored in it. Below the false bottom is a plug which can be removed to allow the washings to flow away. First, the charge is washed with 70 gallons (264 liters) of water sprinkled over it by means of a rose, the plug being left out so that the washings can drain away into liquor tank F. After draining half an hour, the plug is inserted and the saltpeter is covered with fresh water, which after standing for half an hour is also allowed to drain into F. Finally the salt is washed by sprinkling 100 gallons (378.5 liters) of water, the plug remaining out. The saltpeter is now allowed to drain all night and is then removed to the store house where it is allowed to dry spontaneously. In about three days, the moisture has fallen to 3 or 5 percent.

The mother liquors and other impure solutions are boiled down to about a quarter of their original volume. Any scum or deposit that forms during the boiling should be removed and water then be added. The solution is now filtered and allowed to crystallize. The crystals are treated as grough saltpeter and the mother liquor returned to the evaporating pots.

The author mentions that the methods of refining adopted in France, Germany and other countries are substantially the same as that of Waltham Abbey. He also mentions that a small amount of size (a gelatinous solution) is often added to the pot to assist in the formation of scum.

In our next post, we will look at the production of charcoal in some detail.