In our last post, we studied the composition of different kinds of black powder as manufactured in various countries. In today's post, we will study some of the physical and mechanical properties of black powder. Gaining some knowledge of this will help understand the reasoning behind the processes of manufacturing the powder when we study that later on.
The first thing we should note about black powder is that it is a mixture and not a compound. Your humble editor will explain what that means:
A compound is formed when different substances combine with each other at a molecular level. The compound will often have properties different from its component substances. For instance, hydrogen and oxygen atoms can combine together to form water (a compound substance), which is a liquid at room temperature, whereas hydrogen and oxygen are gases at the same temperature. Oxygen can help substances burn rapidly, whereas water can be used to stop fires. So you can see that a compound (in this case, water) has quite different properties than its original ingredients (in this case, hydrogen and oxygen).
On the other hand, a mixture is when multiple substances are physically mixed with each other, but do not react at a molecular level. This means that they may be separated from each other by some physical means and mixtures often retain the physical properties of their separate ingredients. For example, you can make a mixture of iron filings, sand and sugar crystals. However, the iron filings can easily be removed from the mixture by passing a magnet over it, while the sugar can be separated out by dumping the mixture in water and letting the sand settle at the bottom while the sugar dissolves in water. Another example could be sand and glass marbles, which can be mixed together easily, but trivially separated by passing the mixture through a sieve, which will allow the sand to pass through, but retain the glass marbles. Black powder is a mixture of potassium nitrate (saltpeter), sulfur and carbon (charcoal). The three substances do not chemically react with each other at room temperature and therefore it is a mixture. Only when the powder starts to burn do the three substances react with each other and form multiple compounds.
Since it is a mixture, the various ingredients of black powder must be ground into particles of roughly the same size as each other to stay mixed together (especially before corning of black powder was invented). Otherwise, the mixture could separate out where the ingredient with the smallest size particles ends up at the bottom of the box, given enough vibration to the box. This is because the smaller particles fit in easily between the gaps of the other particles and fall to the bottom, thereby pushing the bigger particles up. The same phenomenon can be observed with a bag of potato chips (it doesn't matter what flavor of chips!). Notice that when you buy a bag of potato chips, the smallest broken chips are always at the bottom of the bag, whereas the larger pieces end up on top. This is because the bag is shaken during transport from the factory to the grocery store and from the grocery store to your home and the smaller chips end up fitting into the gaps between the larger chips, making their way to the bottom of the bag eventually and thereby pushing the larger pieces upwards. The same principle used to apply to gunpowder before they learned to cake the grains and manufacture them to the same uniform particle sizes. In fact, one of the problems of early black powders (also called serpentine powders) was that when they transported the powder to the battlefield via carts drawn by horses or oxen, the bad roads would cause the barrels of gunpowder to shake heavily, thereby moving the smaller particles to the bottom of the barrel. Therefore, if the ingredients were ground up into particles of different sizes, the ingredients would separate out into three separate layers by the time the barrel got to the battlefield, with the sulfur ending up at the bottom of the barrel and charcoal rising to the top. This is why they would remix the ingredients right there in the field before the battle commenced, which was a somewhat hazardous procedure that produced clouds of potentially explosive dust.
Black powder can be ignited in three different ways: the first method is by contacting it with sparks or open flame, the second method is by a sharp blow and the third method is by increasing its temperature rapidly beyond a certain point.
The first method (exposing it to open flame or sparks) is the principle that different ignitions systems such as matchlocks, wheel locks, flintlocks, percussion locks etc. use. However, the source of the flame or sparks must be hot for the powder to ignite. It is possible for a shower of lower temperature sparks to fall upon black powder without igniting it, whereas a single spark of great intensity can start combustion.
The second method (striking it between two objects) is because black powder is somewhat impact sensitive. Experiments by Aubert, Lingke and Lampadius verified that black powder can be ignited by striking iron on iron, iron on brass, brass on brass, and less easily by a blow of iron on copper, or copper on copper. Of course, some of this might be explained away by the impact causing sparks which ignite the powder. Experiments in 19th century England showed that black powder is also ignited by striking brass on copper, iron on marble, quartz on quartz, lead on lead and lead on wood (a lead bullet was shot against a wooden pendulum covered with powder). Mining accidents over the years showed that striking copper on stone or even wood on stone could occasionally cause ignitions of black powder. One Dr. Dupre even showed that there is hardly any explosive, which, when laid in a thin layer on a wooden floor, will not explode, when it receives a glancing blow with a wooden broom-stick.
The third method (heating it beyond a certain temperature) has some interesting effects. Black powder may be ignited when heated rapidly above a certain temperature, even without the presence of an open flame. The temperature at which this happens depends on the nature of the powder and the proportions of its ingredients and grain size. An experiment by Horsley in the 1800s showed that black powder could be ignited by heating it to around 600 °F (about 315 °C) by heating a saucer in an oil-bath, with the temperature of the oil being taken by a thermometer dipped into it. Experiments by Leygue and Champion in 1871 used a more precise method to determine ignition temperatures and the found that a common sporting powder ignited around 550 °F (about 288 °C), while cannon powder ignited around 563 °F (about 295 °C). However, note that we said that the powder should be heated rapidly for it to ignite. What if it is heated slowly?? Leygue and Champion detail some interesting issues here: They discovered that the grains of corned black powder cake together on account of the sulfur they contain. However, note that black powder before ignition is a mixture, which means it retains many of the physical properties of its separate ingredients. When the temperature of black powder is slowly increased beyond 212 °F (about 100 °C, the temperature of boiling water), the sulfur begins to volatilize and turn into vapor. The volatilization of sulfur rapidly increases with temperature and if the temperature is slowly increased upwards, but kept below the boiling point of sulfur, then the sulfur can be completely driven out of the powder without any ignition taking place. When the sulfur is completely eliminated from the mixture, the temperature can be further increased, so that even the saltpeter melts, and the charcoal ends up floating on top of it, thereby separating out the two ingredients from each other. If, on the other hand, the temperature is rapidly increased before the sulfur is completely volatilized, then the sulfur vapor is ignited and causes the powder to explode. The shape and size of the grains of black powder have considerable influence on the temperature of ignition as well.
If a small quantity of black powder is ignited in open air, it merely burns, but if larger quantities are ignited, or if the powder is ignited under higher pressure or in a closed space, then it explodes. The larger the grain size, the slower the combustion rate. We will study more about this in the next post when we study more about grain sizes.
If good quality black powder is ignited over a sheet of white paper, it will burn rapidly and leave no residue on the paper. If black spots are found, then this indicates that either the mixture contains too much charcoal or the powder is badly mixed. The same can be said for sulfur if yellow spots are left behind. If unburned grains are found, this indicates that the saltpeter is impure. The powder should not burn holes into the paper, as only moist or otherwise bad black powder does so.
As early as 1765, Papacino d'Antoni found that lower air pressures make it more difficult for black powder to ignite. Later experiments by Munke, Hearder, Bianchi, Heeren and Sir Frederick Abel showed that gunpowder didn't explode in a vacuum tube, even in the presence of a platinum wire glowing white hot. Heeren tried to explain this phenomenon by suggesting that at normal pressures, the hot gas escaping from an exploding body would communicate the flame to neighboring particles, but under low pressure, the gas expands so rapidly on account of the lack of resistance of the surrounding air, that it cools down below the ignition temperature of neighboring particles.
On burning gunpowder under normal or high pressures, the various ingredients of the mixture combine with each other chemically and produce gases and solid residue. While this was known from the day that gunpowder was invented, the nature of the gases and solid residue was not. In fact, given the primitive state of chemistry for centuries, it was not known if the products of combustion was just one or several gases. For instance, in 1705, the great Issac Newton thought that sulfuric acid formed by the combustion of sulfur drove out the spirit of niter from the saltpeter and burned it. The same view with slight modifications, was held in 1771 by Majow, who thought a mysterious substance called "phlogiston" (thought to exist in all flammable substances) combined with the nitric acid. It was left to the famous French chemists, Joseph Louis Gay-Lussac and Michel Chevreul, to determine exactly what gases and solid residues were produced. Their experiments showed that among the gases produced were carbonic acid, nitrogen and carbonic oxide, while the solid residues were potassium sulfate, potassium carbonate, potassium sulfide, potassium thio-sulfate etc. Incidentally, Gay-Lussac was the first to prove that water is made of hydrogen and oxygen and also worked on alcohol-water mixtures, the results of which are still used to today to measure alcoholic beverages in many countries around the world (a fact that drinkers will surely appreciate!)
In our next post, we will look into the effects of grain sizes of black powder and how/why different grain sizes were used for different applications.