Evolution of Ballistic Propellants
The exact date that gunpowder was developed is not clear, but historians know that a compound similar to black powder was used to propel rockets in combat as early as the 13th century AD. Potassium nitrate (common name saltpeter), charcoal, and sulfur were mechanically blended to form a combustible, even explosive, compound. There are indications that the Chinese were experimenting with saltpeter compounds as early as 1000 AD.
The trick to making a suitable ballistic propellant was in the correct ratio of the three chemicals. Although many combinations have been used, the best formula for small arms applications is 75 percent potassium nitrate, 15 percent charcoal, and 10 percent sulfur. This recipe makes the propellant we know today as black powder. Blasting powders often used a different ratio of components.
Black powder that is merely blended dry was known as serpentine powder. An early problem was that this simple mechanical mixture separated in transport. The power of the propellant varied as a result. A "work-about" to this was to transport the three components separately to a battle site, and then blend them in the correct ratio just before loading. You can imagine the hazards of blending thousands of pounds of black powder just behind a line of active cannon! We won't even bother to enumerate the problems associated with potassium nitrate's habit of absorbing moisture from the air. Early cannoneers were a fatalistic lot.
Nearly all saltpeter at that time was used just like it was mined. Purity was not a concern. Some scientists knew that the potassium nitrate could be refined to produce a serpentine powder of greater energy, but such added energy was often more than poor-quality metal in cannon barrels could safely handle. Therefore, serpentine powder was to survive a long time.
In an effort to prevent the separation that plagued serpentine powder, corned powder (also corn powder) was invented in the mid-1400's AD. The basic components were dampened and then ground together while wet under heavy stone wheels. This arranged the components into intimate contact with each other for greater energy release, and prevented separation of the components. The resulting damp mass was allowed to dry to a cake, and then broken into small pieces. The crumbled residue was sieved to sort grains by size—the smallest sizes were used for flintlock priming and the largest for cannon. By the time corned powder was available, metallurgy had improved the strength of gun barrels so that they could safely fire this more energetic powder. This also allowed the purer forms of potassium nitrate to be incorporated for even greater strength.
The final refinement was glazing. After sieving for size, the raw granules were tumbled in graphite. This had several positive effects. The graphite glaze reduced the tendency of the granules to absorb moisture, so the powder better retained its original strength in storage. Glazing rounded the sharp, irregular edges of the granules so the powder flowed better from containers. This had the added effect of reducing the potentially disastrous build-up of static electricity when the powder was handled. Last but not least, glazed powder could be stored without clumping or caking, an old problem with serpentine powder.
With these improvements, glazed corned powder remained the propellant of choice until the end of the 19th century. It was simple to make, easy to ignite, and put a lot of projectiles on target.
In spite of its long service record, black powder has its drawbacks. On combustion in a firearm, less than half the weight of propellant goes into producing gas to propel the bullet. The remaining solids create a big cloud of white smoke and a heavy residue in the barrel. That residue absorbs moisture. Adding moisture to black powder residue creates weak acids that are corrosive to steel.
Another drawback is that, to get more velocity, you have to add more propellant. There is a practical limit to how much propellant will fit in a cartridge case. The really powerful black powder rifle cartridges had huge cases up to 3 1/4 inches long. These were fine for single-shot rifles, but unsuited for the new repeating rifles that were becoming so popular. The need arose for an efficient propellant that could release more energy from a smaller volume.
Science Marches On
While black powder was still the primary firearms propellant, scientists were discovering new energetic materials. Around 1845, they discovered that treating cellulose (wood fibers or raw cotton) with nitric acid resulted in a material that burned cleanly with little visible residue. On combustion, nearly all the solid material converted to gas. The chemical name for this material is nitrocellulose; commonly named guncotton. Its clean combustion was due to the nitration process adding additional oxygen that made burning very efficient.
Being nearly impossible to control the burning rate of raw guncotton, little relevance was found for ballistic propulsion. However, in making the first plastics around 1880, Vielle of France discovered that, by partially dissolving guncotton in alcohol and ether, he could shape the resulting gelatinous mass to almost any form. Rolled into a thin sheet and then cut into flakes, celluloid could be a controllable ballistives called stabilizers improve storage life.
The shape of the propellant is important to burn rate control. The four major propellant groups by shape are flake, cylindrical, ball or spherical, and cut sheet. The latter is more common in European propellants. Cylindrical powders are mistakenly called extruded. This is technically incorrect because nearly all propellants are extruded at some point in their manufacture.
Burning rate is a relative term that addresses the rate at which energy is released during burning. Slow-burning powders release their energy (in the form of pressure) more slowly than fast-burning powders. Fast-burning powders are commonly used for light loads with light bullets. The slow-burning powders generate their force over a longer time increment. This gives a heavy bullet a gentle start but keeps pushing the bullet longer, allowing high velocities.
Burning rates are determined in the lab using a devise called a calorimeter bomb. A known amount of the propellant is burned in a closed vessel and the heat production is measured in calories. The relative burning rates are just that—relative. The relationships seen in calorimeter tests will not be the same when c material. However, as the volatile alcohol and ether escaped in storage, control becomes more difficult.
Further control and energy came with Alfred Nobel's discovery of nitroglycerin. This explosive liquid could dissolve guncotton just like alcohol and ether, provided its own energy to the total, yet did not evaporate with time. Nitroglycerin-based propellants were very stable in storage.
Combining Vielle's and Nobel's work, and adding intelligent granulation technology, resulted in high-energy, efficient propellants suitable for a wide variety of cartridges. Today, both alcohol/ether-based and nitroglycerine-based propellants are used. Alcohol/ether-based propellants are called single-base, and those with nitroglycerin are called double-base. Because either type produces very little smoke compared to black powder, these modern propellants are classed as smokeless. Residue is minimal, and is not corrosive in most cases.
In addition to granulation, modern nitrocellulose propellants gain additional burn rate control through the application of deterrents. These chemicals, often coatings, change the rate at which each granule converts to gas by delaying the ignition of the outer surface. These coatings are often compared to a "temporary fireproofing." Other additional propellants are tested in a real cartridge. For this reason you must never attempt to substitute propellants based on published burning rate charts. True rates depend on the geometry of the cartridge, the weight of the bullet, and the pressure range in which the cartridge operates. Always use published ballistic lab data when choosing a propellant and charge weight.
Tips for Propellant Success and Safety
- Always weigh charges on a proper scale. A volumetric powder measure must be calibrated with a scale intended for cartridge loading and checked often during the loading session.
- Do not interchange IMR 4831 and Hodgdon H4831. Although they bear the same number, they have different burning rates. Any data published prior to 1973 (when IMR 4831 appeared) refers the the older Hodgdon ("H") version.
- Store propellants in an area with stable temperature and humidity. Store only in accordance with applicable ordinances. This also applies to the quantity stored in a residence.
- Keep all propellants in their original factory canisters with all labeling intact. Any propellant that lost the labeling and identification should be considered defective and properly scrapped.
- Never mix or blend propellants.
Suggestions for Additional Readings on Propellants
Speer Reloading Manual
Cartridges of the World, 8th Edition, pp 417-425. DBI Books, 1997.
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