Manufacturing Gunpowder on the Brandywine

IMG_4831The history of the du Pont family and of Delaware itself is closely tied to the manufacture of gunpowder (see posting of 11/2/2012, How the du Ponts Came to Delaware).  It was the only product of the DuPont company for almost their first hundred years, and that early history is still preserved in the world-class Hagley Museum along the Brandywine River.  (The original powder mills were expanded onto the former Hagley farm.)

The gunpowder we are talking about is what we saw in old cowboy movies—those small kegs of black powder that blew up the bad guys. The Spanish-American War was the last time it was seriously used in guns, and it was called “black powder” after that.  For a short while It continued to be sold as a blasting agent in mines until the miners become familiar with the superior, new products.  Today, gunpowder is only found in firecrackers, fireworks, and at historic battle reenactments.

It is one of the easiest products to make, as long as you do not blow yourself up in the process.  Gunpowder is simply a physical blend (no chemistry involved) of three naturally occurring ingredients: charcoal, sulfur, and saltpeter.  The usual formulation contained 75% saltpeter and this was the most difficult ingredient to obtain.  Deposits of saltpeter, or “potassium nitrate,” were only found in India, but it forms naturally at the bottom of manure piles, and European governments had extensive teams of “petermen” scattered over the countryside busily collecting it from barnyards.  (As a chemist, I suspect what the petermen were collecting was mostly ammonium nitrate, but the properties of ammonium nitrate and potassium nitrate are very similar, and they probably could not tell the difference.)  It was not a choice assignment.  Today, it is made synthetically, and cheaply,  but is no longer in demand.

Normal combustion gets its oxygen from the air that is quickly depleted and the gases that remain suffocates the fire until fresh air drifts in. Gunpowder removes this limitation by getting its oxygen from the heated saltpeter.  Charcoal is the material that gets burned, and the small amount of sulfur lowers the ignition temperature of the mixture.  This combination is essentially the same as that developed in China by trial-and-error a thousand years ago.

The quality of the final gunpowder depends on grinding these three ingredients as fine as possible and blending them evenly to get them in the most intimate contact with each other.  Grinding and mixing them is not as dangerous as it may seem.  Gunpowder is not set off by impact.  It can only be set off by high temperature, but that can come from the tiniest spark, so manufacturers were scrupulous in keeping any iron metal from the process.

The ground and blended gunpowder is a fine dust, too fine to use.  A slight breeze would blow it away.  It must be formed into granules that also significantly increases its explosive power.  That was done on the Brandywine by dampening the powder and compressing it into slabs as hard as slate, then breaking them up and sifting them into a variety of sizes.  All of this could be done safely as long as any chance of sparks was eliminated, but this was not easy in the days before electricity when all lighting and heating came from fires.

As a final step, the granules were tumbled together to knock off the sharp edges and coated with a small amount of graphite to improve their resistance to moisture.  Gunpowder readily draws moisture from the air, and it has been estimated that more gunpowder was ruined by moisture than actually got used.  It would suck the moisture from the wood staves in the kegs, opening cracks that allowed more moisture in.

When gunpowder is ignited, only half is typically converted to explosive gases.  The other half is left as solid residue that remains in the breech, fouls the gun barrel, and blows off as smoke.  A smokeless powder was developed prior to WWI in an entirely new technology by reacting nitric acid with various substances.  React the acid with glycerin and you have nitroglycerin.  React it with cellulose and you have nitrocellulose, or gun cotton.  React it with toluene and you have trinitrotoluene (TNT).  The nitro group that supplies the oxygen is now part of the molecule that gets burned, rather than just physically close to it, and almost no  reside is left.

The familiar explosives are basically materials that combust very rapidly, and for them to explode they must be confined in some way.  In a firecracker, the cardboard container is enough.  For larger amounts, just the weight of the unburned material can turn a rapid combustion into an explosion.  The nitrated explosives burn faster than the speed of sound, and, because a shock wave cannot move away faster than the speed of sound, this alone provides confinement.   Nothing else is needed.  These products are designated “high explosives” and their combustion is “detonation.”  The old gunpowder burns slower than the speed of sound and is a “low explosive.”  Its combustion is “deflagration.”   Put a match to a small amount of unconfined low explosive and the flash of fire can singe your eyebrows.  The same amount of high explosive can blow your head off.

All of the nitrated explosives are smokeless and more powerful, but far more dangerous.  The unformulated products are sensitive to shock.  The slightest jostling can set off pure nitroglycerin.  Alfred Nobel’s great discovery was that absorbing nitroglycerin on diatomaceous earth, an inert mineral, will remove its sensitivity to shock, making it practical to transport and use.  He named it “dynamite.”  The nitroglycerin in tablets placed under the tongue to relieve angina pains are also safely absorbed on an inert material.

The nitrating reaction used to manufacture these explosives is extremely dangerous.  The reaction generates a lot of heat and runs faster as the temperature rises.  If not kept cool, the reaction heats up, runs faster, produces even more heat, and runs still faster.  The whole reaction can quickly spiral out of control and explode.  The reactions are performed next to large tubs of water where they can be quickly dumped in an emergency.  The large volume of water stops the reaction by cooling and dilution.

For a time, the DuPont management resisted manufacturing the new explosives as being too dangerous.  Competition eventually forced them to change, and when Pierre du Pont, of Longwood Gardens fame, was only 14 years old, his father, Lammot du Pont, was killed trying to stop just such a runaway reaction in their Repauno, New Jersey, facility, located across the Delaware River from the south end of today’s Philadelphia Airport.  He dismissed the workers and opened the valve to dump the reaction.  He turned to leave and got about ten feet when it blew.  The workers had gotten about thirty feet, but they, too, were killed.  The head chemist was just coming out of the nearby laboratory building and was killed by a broken neck from the force of the blast.  It was a bad day in New Jersey.

RWalck@Verizon.net

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About Roger Walck

My reasons for writing this blog are spelled out in the posting of 10/1/2012, Montaigne's Essays. They are probably not what you think.
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