v1.1.0 / chapter 15 of 15 / 01 sep 10 / greg goebel / public domain
* In addition to explosives, there's a wide range of other pyrotechnics, including incendiaries, a wide range of miscellaneous specialized pyrotechnic devices, and last but not least the diverse and entertaining field of fireworks. This chapter completes the survey of pyrotechnics.
* Incendiary weapons have long been used in combat, for example, the "flaming arrows" used by Apaches to set wagons on fire in Western movies. In the 7th century CE, Byzantine Greek alchemists found that a mix of pitch, naphtha, sulfur, and petroleum would burst violently when ignited, and named the mixture "Greek Fire". The empire's military used it to defend Constantinople from invading Saracens. Later, in naval fighting during the age of sail, cannonballs were often heated red-hot before firing in hopes of setting an enemy vessel on ablaze.
Modern military incendiary munitions consist of "napalm", "fuel-air explosives (FAE)", and metallic compositions. Napalm is simply gasoline to which a thickener has been added to make the burning fluid viscous and sticky. The original World War II form of napalm used a thickener named "sodium palmitrate", leading to the name "na-palm". Modern "napalm B" uses polystyrene plastic beads as a thickener. Homemade napalm can use liquid or powder soap, or styrofoam packing peanuts, as a thickener. FAEs spray out an aerosol cloud of a hydrocarbon liquid, and then ignite it to create a flaming explosion over a wide area.
Aluminum has already been mentioned as an incendiary metal. Other incendiary
metals include zirconium, magnesium, titanium, and depleted uranium. They
all burn at very high temperatures. A particularly useful metallic
incendiary is "thermite", which is a mix of ferrous oxide (Fe2O3, essentially
rust) and aluminum. The thermite reaction is as shown below:
Fe2O3 + 2Al -> Al2O3 + 2Fe
The reaction burns very hot and releases a tremendous amount of energy.
Thermite is often used in demolition grenades to burn or melt down
military gear that has to be abandoned to an enemy.
One new scheme uses a "combustible foil" based on pyrotechnic metals to perform emergency welds. The foil contains ultrathin alternating layers of metals such as nickel and aluminum. The foil is ignited by a match or a 9 volt battery, and instantly ignites over its entire surface. Varying the thickness and composition of the layers provides control over the speed, temperature, and total energy of the reaction. It works in a vacuum or underwater, and can be used by soldiers for emergency field repairs. The combustible foil could also be used for detonators and heating devices.
* White phosphorus was also once used as a military incendiary. Elemental phosphorus comes in two forms, a "red" amorphous form, and a "white" form arranged as tetrahedral units of four atoms. Red phosphorus is relatively easy to handle, but white phosphorus ignites spontaneously at room temperature. White phosphorus is now mainly used to generate smoke.
White phosphorus also serves a role in the most common pyrotechnic device, the safety match. The match was invented by an English chemist named John Walker (1781:1859) in 1826, when he was mixing chemicals with a small stick and accidentally scraped the stick on a rough surface. It caught fire. Walker followed up the lucky accident by developing and selling the first matches.
The basic design of the match remains much the same as Walker's original invention. The head still consists of an oxidizer such as potassium chlorate (KClO3), a fuel such as sulfur or rosin, and a binder such as glue. Modern safety matches, however, have a tip of phosphorus trisulfide (P4S3). The stem is dipped in a fireproofing agent to keep it from burning too easily, and the head is coated with paraffin (in the US meaning of "candle wax") to keep it dry. The striking surface on the package of matches is coated with powdered glass and red phosphorus mixed in a binder. When a match is scratched over the striking surface, the red phosphorus in the surface is converted to white phosphorus by heat of friction, and the phosphorus trisulfide burns in the air. This ignites the fuel and oxidizer, which creates a sustained flame. The match will not easily ignite if scratched on any other abrasive surface.
Other pyrotechnic materials are used as heating fuels, in fuzes or flares, to create smoke, fill up automotive airbags, or propel rockets. Some common pyrotechnic devices include:
Military aircraft often carry thermal flares to distract heat-seeking missiles. At first, ordinary signal flares were used in this role, but missiles became more sophisticated and able to see through such decoys, and so flares became more sophisticated as well. Modern decoy flares consist of teflon plastic mixed with a fluorine compounds as an "oxidizer". They may contain two "stages" that burn at different temperatures at different times. The latest "pyrophoric" flares are made from ribbons of metal that oxidize at a rapid rate just short of burning in order to simulate the moderate temperatures of a jet exhaust. Missiles have become so smart that aircraft are now moving towards laser systems to confuse them, since they can't be fooled by flares any more; besides, flares are a nuisance to stockpile and handle, and when an aircraft has expended its supply, it is more or less defenseless.
After the war, this scheme evolved to modern solid rocket fuels based on certain forms of synthetic rubber, mixed with ammonium perchlorate oxidizer and a high concentration of aluminum powder. The synthetic rubber not only provided a fuel source, but also acted as a "binder" that could be cast in a huge solid block, without voids or cracks that would cause uneven combustion, that could be safely stored for a long period of time without degradation. Later improvements involved the addition of powdered iron oxide to promote a thermite reaction. "High energy" solid-fuel formulations were also developed that incorporated a proportion of high explosives, such as a nitroglycerine / nitrocellulose mix or HMX, but these rockets are unsurprisingly more dangerous to handle and have been only used for small upper stages.
One interesting application of modern solid-rocket fuel is for mine disposal. Flares filled with solid-rocket fuel have been designed so they can be set up over a mine on little pop-out legs. The hot exhaust burns through the mine casing and sets the explosive filling on fire.
* While fireworks may not seem like high technology, they are a highly refined art. There are two basic schools for fireworks fabrication, the Oriental and the Italian. In the US, fireworks are manufactured by a few concerns, most of which run in Italian families, such as the Zambellis and the Gruccis. Nearly all pyrotechnic materials except for high explosives are used in fireworks. The basic constituent of many fireworks is, as mentioned earlier, black powder, but flash powders and smoke-generating pyrotechnics are used as well.
Simple firecrackers and rockets are made from black powder in paper cases. Sparklers are made from a thick slurry consisting of fuels, binders, and oxidizers into which wires are dipped. Whistling fireworks use gas-generating pyrotechnics that are packed into narrow tubes that create the whistle when the gas escapes. Roman candles consist of a set of bright "stars" that generate light and color, packed into a paper tube in layers of black powder. As the black powder burns down from the top of the tube, it ignites each layer of black powder in turn, spitting out a star.
The stars are made of mixes of pyrotechnic metals, salts, and binders such as resin and gum. Stars in oriental fireworks are rolled into shape, while Italian stars are generally made from cakes and cut into cubes. The round Oriental stars can have multiple layers, causing their appearance to change as they burn. Early stars and other illuminating elements could only obtain white and gold effects, using saltpeter. Modern stars obtain a wide range of color effects, such as:
Clever chemistry has to be employed to get the desired effects. The strontium and barium compounds aren't stable in storage, for example, and have to be synthesized through chemical reactions during the pyrotechnic process. Copper compounds will break down if the pyrotechnic process is too hot, destroying the color, so the firework has to be carefully designed to burn at a low temperature.
A "setpiece" is an obscure form of fireworks display that is undergoing something of a revival. It consists of hundreds of tubes of color-generating fireworks mounted on a wooden frame in a graphics pattern and linked with a fast-burning black powder fuse taped to the frame. The fuze sets off all the tubes quickly to generate a vivid display, and can also set off pinwheels and other fireworks attached to the display. Centuries ago, setpieces could be very elaborate, including such things as fire-spouting dragons "flying" on wires, but such tricks are expensive, troublesome, potentially hazardous, and not seen much today.
Large skyrockets are also used in public displays. They use a black powder propellant, sometimes mixed with other pyrotechnic materials so they leave a spectacular trail, and have a payload consisting of stars or other pyrotechnic elements dispersed by a black powder bursting charge. However, the main firework for public spectaculars is the "shell". Shells can be built to produce a variety of effects:
Shells consist of a payload and a "lift charge" of black powder that lifts it into the sky. The shell is stuffed down a PVC pipe mounted in a sandbox, and lit off by an incendiary fuze or, more commonly in big fireworks displays, by an electric spark from a "squib". When the shell is fired, a time-delay fuze, or "spegette", inside the shell is lit, and burns down to set off a black-powder charge that bursts the shell and disperses the stars.
Oriental shells are spherical, while Italian shells are cylindrical. Bursting charges in Oriental shells may consist of rice hulls impregnated with black powder to increase the flash of the burst. Oriental shells are appropriate for generating symmetrical displays, such as chrysanthemums.
Italian shells burst in a more irregular fashion, but they can be designed with multiple firework stages or "breaks", connected by spegettes, that detonate consecutively. For example, a three-break Italian shell might consecutively disperse a burst of red, white, and blue stars.
Multiple-break shells can be very elaborate. The blast charge may ignite a set of stars so the shell's launch is suitably spectacular, and the stages may contain such elements as whistling pyrotechnics as well as stars.
Sophisticated fireworks displays often use elaborate control systems to sequence the ignition of fireworks, and synchronize them with sound and laser effects. One of the more interesting fields in modern fireworks are indoor fireworks displays. Such displays are used in rock concerts and other entertainments, and use conventional fireworks technology, modified with strict safety standards in mind to ensure no toxic emissions and appropriate safety for performers and audience.
* Although I enjoy the sciences, I wasn't fond of my chemistry classes in high school and college. Possibly the emphasis on rote learning turned me off; I recall teaching assistants in my college classes expressing a certain derision for that approach as well. However, once I decided to build a series of documents on the physical sciences, there was simply no way to avoid writing a document on chemistry.
As I worked on this document I began to realize that I didn't want to write something that was much like a conventional chemistry text. There's nothing wrong with such things, of course, but they do tend to be overkill for someone who "doesn't do chemistry for a living", for example myself. A long survey of, say, organic chemistry is pretty useless for non-chemists, involving a lot of wading through factoids that are promptly forgotten. Does anyone except a chemistry professional have the slightest need to know the IUPAC naming schemes for hydrocarbons?
The end result was that I tried to focus on history and applications, making brief references to topics such as chemical equations and organic chemistry, so vital to a formal chemistry class, in footnotes. I generally followed the line of least resistance in writing this document, taking as minimalist as possible an attitude towards topics that seemed laborious and focusing on stuff that was more like fun: "When in doubt, throw it out." It turned out that being lazy was a useful strategy, since it helped keep a focus on a readable end product for nonspecialists, instead of a dry formal text.
I don't claim that this document is a replacement for, much less superior to, a conventional chemistry text. It is targeted for readers who would find a formal text too much to be useful. However, I would claim it also provides a useful background supplement for those who are working on a formal chemistry course. I will add more to it in the future as I learn new things and figure out places where I went wrong -- not being a chemist, I am certain there are a few naive statements here and there.
* Sources include:
I surfed quite a few websites for data, too many and too trivial to mention, and even found some information in TV shows on explosives and fireworks broadcast on the HISTORY CHANNEL and the DISCOVERY CHANNEL.
Somewhat to my surprise, different sources gave different, sometimes seriously different, numbers for the properties of the elements, which left me in a quandary over who to believe. I finally decided to buy a CRC HANDBOOK OF CHEMISTRY & PHYSICS to get the data -- I bought a used one, the 70th edition from 1989:1990, there being no reason to shell out the stiff price of a new one when all I wanted was simple basic data that's been nailed down for decades.
* The chapters on plastics, batteries & fuel cells, and explosives & pyrotechnics were originally released as stand-alone documents:
* Revision history:
v1.0.0 / 01 sep 07 / gvg
v1.0.1 / 01 oct 08 / gvg / Minor cosmetic update.
v1.1.0 / 01 sep 10 / gvg / Minor cosmetic update.
BACK_TO_TOP
