from: scientific american july 90                                 
                                 Pyrotechnics               
   The secrets behind dazzling fireworks displays are yielding to scientific
snooping. Similar principles are at work in devices ranging from the space shuttle to safety matches

                              by John A. Conkling

    A  distant, muffled "whoomp" resounds, and a trail of yellow-orange  sparks tumbles  into the night sky, culminating in a circular burst of brilliant  blue and  green streaks. Another explosion shoots out a ragged arc or red streamers. followed  by  a  shower of white and gold sparks. A third firework  produces  a staccato barrage of bright flashes or white light and thunderous noise.

   These effects have been a familiar part of major celebrations for centuries. For most of that time, the design and composition of fireworks was a craft, not a  science.  Only  in  recent decades have researchers  begun  to  unravel  the physical processes that underlie the production of dramatic colors and  special effects.   As   a  result  of  these  investigations,  a  true  discipline   of pyrotechnics, the "science of fire" has emerged. Pyrotechnics embraces not only fireworks  but  a whole range of devices that use similar materials,  including hazard  flares, safety matches and even the solid-fuel rocket boosters  of  the space shuttle.

    One  of the oldest pyrotechnics compositions, black powder, serves as  both the  propellant  and  explosive charge in modern firework shells.  The  Chinese developed black powder (the original gunpowder) more than 1,000 years  ago  for use in crude missiles and firecrackers. Awareness of black powder traveled west during  the Middles Ages. The English monk Roger Bacon disclosed a formula  for the  explosive  mixture in 1242 as part of his defense against  accusations  of witchcraft. He considered it such a dangerous material that he wrote  about  it in  code.  As the formula became more widely known, black powder revolutionized quarrying  and  construction. Weapons such as muskets  and  cannons,  developed during the 14th century, exploited black powder as a propellant.

    The  basic  formula  for black powder has persisted  essentially  unchanged throughout  the  centuries:  an intimate blend of potassium  nitrate  (commonly known as saltpeter), charcoal and sulfur in a 75:15:10 ratio by weight. It  may in  fact  be  the only chemical product that is produced today using  the  same ingredients,  the same proportions and the same manufacturing process  as  were used  in  the  time of Columbus. This constancy reflects the  fact  that  black powder  is  a  nearly  ideal pyrotechnic substance. It  consists  of  abundant, inexpensive  chemicals that are relatively non-toxic and environmentally  safe. The   mixture  is  so  stable  that  it  can  be  stored  for  decades  without deteriorating,  if  kept dry. Black powder is easily  ignited  by  means  of  a moderate jolt of energy, such as a spark or a small burning fuse.

    Historically,  only  a  handful of families have  dominated  the  fireworks industry  in  the West. Details such as chemical recipes and mixing  procedures were  cloaked  in  secrecy  and passed down from one generation  to  the  next. Families  remain an important force in the industry. In the U.S., for instance, there are the Gruccis of Bellport, N.Y., the Zambellis of New Castle, Pa.,  the Rozzis of Loveland, Calif. One effect of familiar secretiveness is that,  until recent decades, basic pyrotechnic research was rarely performed, and even  when it was, the results were not generally reported in scientific journals.

    In  principle, the pyrotechnic process is not unlike normal  combustion.  A pyrotechnic  composition contains an oxygen source (oxidizer)  and  a  reducing agent  (fuel).  They  are  usually  separate,  solid  chemicals  that  must  be physically  mixed  together.  When heat is applied,  an electron-transfer,  or oxidation-reduction (redox), reaction takes place.

    Atoms  in the fuel lose electrons to atoms in the oxidizer. In the  process
the  fuel  atoms bond to the oxygen atoms that are liberated from the  oxidizer and  form stable reaction productions. The new chemical bonds are more  stable, and  so  energy  is  released in the form of heat; the same process  occurs  in combustion,  however, the oxygen comes from the air. In a  pyrotechnic  mixture the oxygen is self contained, and the heat is much more closely confined.

   As long as a pyrotechnic mixture remains cool and dry, it is generally quite
stable.  A  solid  mixture  experiences  only  a  very  slow  surface  reaction
controlled by diffusion. When the composition is ignited, it begins to  liquefy
and  vaporize  in  the resulting pyrotechnic flame, and the fuel  and  oxidizer
closely intermingle. This proximity leads to faster chemical reactions and,  in turn, still more rapid energy release.

    Pyrotechnics  make  use  of a variety of fuels. Many  mixtures  incorporate organic (carbon-containing) materials, such as charcoal (used in fireworks  and gunpowder) or sugar (in smoke grenades). Other common fuels include nonmetallic elements, such as sulfur, silicon and boron. Silicon and boron release a  large amount  of  energy when oxidized, and they do not produce gas in  the  process.  They  are  used in delay fuses to ignite other compositions at a desired  time.

Chemically  active metal fuels - most often aluminum, magnesium and titanium  - burn  at  high temperatures and emit bright light. They came into use the  19th century and dramatically improved the brilliance of pyrotechnic explosions.

    The spectacular splashes of light produced by fireworks are the most famous pyrotechnic  phenomenon.  The  color of the light consists  of  electromagnetic radiation having wavelengths between 380 and 780 nanometers (a nanometer is one billionth  of  a  meter). The longest visible rays appear  red,  the  shortest, violet.  A  glowing object appears white if it radiates throughout the  visible spectrum. If most of the light emitted in a narrow portion of the spectrum,  it takes on the color of that portion.

    Pyrotechnic compositions emit light by three basic processes: incandescence (blackbody  radiation),  atomic emission and molecular emission.  Incandescence occurs when solid or liquid particles in the pyrotechnic flame are heated to  a high  temperature. The hot particles emit a broad spectrum of radiation as they attempt  to  shed their excess energy. The higher the temperature, the  shorter the  wavelength  at which the most radiation is emitted. The intensity  of  the emission  is  proportional to the fourth power of the flame temperature,  so  a moderate increase in temperature drastically brightens the flame.

   White  light flares contain a reactive metal, such as magnesium, as a  fuel.
Solid  metal oxide particles, created when the fuel is oxidized, are heated  to more  than 3000 degrees Celsius; at these temperature, their incandescent  glow is white hot. A mixture of potassium perchlorate and fine aluminum or magnesium powder  produces a powerful explosion along with a burst of white  light.  Such "photoflash" or "flash and sound" compositions have a wide range of uses,  from firecrackers  to  special effects for rock concerts  to  bursts  of  light  for nighttime  photography.  These compositions  produce  the  bright  flash  that traditionally terminates a firework explosion.

    Larger  metal  particles  retain their heat longer  than  powders  and  can
continue to burn by drawing on the oxygen in the air. They create white  sparks rather  than an instantaneous flash. The bigger the particles, the  longer  the sparks  last. Charcoal and iron particles do not become as hot as active  metal particles  -  only  about  1500 degrees C - and so they  produce  dimmer,  gold colored sparks.

    The brilliant colors seen in modern fireworks displays are generated either by  atoms or by molecules present in a vapor form in the pyrotechnic flame.  In the former case, the heat of the flame excites an electron in an atom and bumps it  from  its normal, ground state orbital to a higher energy one. The electron rapidly returns to its ground state and emits the excess energy as a photon  (a single particle, or unit, of radiation) of a specific wavelength.

    Sodium is one of the most potent atomic light emitters. Sodium atoms heated above  1800 degrees C give off yellow-orange light having a wavelength  of  589 nanometers.  The process is so efficient that it tends to overwhelm  any  other atom  or molecular light sources in a pyrotechnic flame. Even small amounts  of sodium  containing impurities can ruin efforts to produce a flame of any  other color.

    In  other applications, sodium's prodigious light emission can be  helpful.
Sodium  nitrate  oxidizer combined with magnesium metal fuel is  the  principle composition  used by the U.S. military to illuminate nighttime operations.  The magnesium  is  oxidized by the sodium nitrate when the mixture is ignited;  the resulting  hot magnesium oxide particles shine with a white incandescent  glow. High  temperatures  (3600 degrees C) in the magnesium flame  also  broaden  the range  of  wavelengths emitted by the sodium atoms. The result  is  an  intense white light.

    As  with atomic emission, molecular emission involves a transition  from  a ground  state to an excited one. The molecule must be in gaseous  form  in  the pyrotechnic flame, and it must be heated to a temperature high enough to  reach the  excited state that causes it to radiate. If the flame is too hot, however, the  molecule disintegrates into its constituent atoms and no light is emitted. Moreover, the molecules must be sufficiently intensely colored light,  but  the production of solid or liquid particles must be kept to a minimum because  they give off incandescent radiation that washes out the color.

    In  the  absence  of theoretical understanding, colors  were  generated  in fireworks  by a trial and error process. Over the past several decades  Bernard E. Douda and Henry A. Webster III of the naval weapons support center in Crane, Ind., and David R. Dillehay of the longhorn division of the thiokol corporation in  Marshall,  Tex.,  have constructed research that has  helped  identify  the principal  colored emitters in pyrotechnics. Takeo Shimizu of the Koa Fireworks Company in Japan also has contributed to this area.

    A  few  groups  of molecules are responsible for nearly all the  colors  in fireworks.  Compounds  of  the element strontium produce  the  reds:  strontium hydroxide  (SrOH) and strontium chloride (SrCl) emit red light  at  wavelengths between  605 and 682 nanometers. Molecules containing barium create the greens.

Barium  chloride (BaCl), for instance, emits green light at wavelengths between 507 and 532 nanometers.

    These  molecules are so fragile that they are unstable at room temperature; consequently, they cannot be packed directly into a firework. Instead they  are synthesized  in rapid reactions in the flame. Manufacturers add such  compounds as  chlorinated rubber, polyvinyl chloride (a chlorine containing  plastic)  or perchlorate or chlorate oxidizers (containing a chlorine atom and four or three atoms  of oxygen, respectively). These compounds decompose at high temperatures and release free chlorine. The chlorine atoms combine with barium or strontium, briefly creating the desired light producing molecules.

    A  rich blue flame is perhaps the ultimate challenge to the pyrotechnician. The  best  blue emitter yet identified, copper chloride (CuCl), is unstable  at
the  elevated temperatures needed to produce intense light in fireworks. If the flame  temperature exceeds that necessary for optimal molecular  emission,  the molecules  disintegrate  rapidly. Distinctly blue  fireworks  therefore  demand especially precise control over the relative proportions and particle  size  of the necessary chemicals. The same holds true for purple or violet colors, which are  created  by combined emission from strontium chloride and copper  chloride formed  in  the  flame. I pay close attention to flame colors  when  I  view  a fireworks  display; if a decent blue color appears, I am always  impressed  and curious  to  know  what  chemical mixture was used. Color generating  compounds combined  with the appropriate fuels and oxidizers can produce special effects.

Red  sparklers  derive their color from the combination of strontium  carbonate (which emits red light) and aluminum granules (which provide the sparks). These ingredients  are mixed with fuels, binders and an oxidizer to  create  a  thick slurry;  wires  are  dipped into the slurry and allowed  to  harden  to  create sparklers.  Another  strontium compounds, strontium nitrate,  is  blended  with potassium  perchlorate (an oxidizer and chlorine source) and various  fuels  to create the distinctive red glow of roadside hazard flares.

    The  structure  of  a  firework is also an  intricate  brew  of  craft  and engineering.  There  are two kinds of firework "shells". Cylindrical  American-European  style shells, typically seven to thirty centimeters in diameter,  are launched  from  metal, cardboard, or plastic mortar tubes. A portion  of  black powder  in  the bottom of the shell is ignited, which propels the  tube  a  few hundred meters into the air. A time delay fuse begins burning when the shell is set off; some seconds later, when the shell is far above the ground, a bursting charge  of  black  powder breaks the shell open and ignites  pellets  of  color composition  (called stars), which are irregularly packed into the  shell.  The stars  are  expelled  in  a random pattern of light and  color.  This  type  of firework may also contain several ounces of flash and sound powder rather  than stars and a bursting charge. Such a shell, called a salute, produces a flash of light and a loud boom instead of a burst of color.

    Round  Japanese-style  chrysanthemum shells  are  similar  in  diameter  to American-European  shells, and they, too, are launched from  mortar  tubes.  In chrysanthemum  shells the stars are arranged in a sphere about a central  black powder bursting charge. When the charge explodes, it ignites the numerous stars and distributes them in a round, symmetrical pattern. Depending on the size and chemical  composition of the stars, result can vary from a quick  flash  to  an extended trail. The trail may even change color if the stars contain more  than one layer of color producing composition.

    Some  American-European shells contain several compartments, each with  its own bursting charge and stars (or flash and sound powder). When one compartment explodes,  it ignites a time delay fuse that leads to the next compartment.  In this  way,  a  single  shell  can  produce  multiple  bursts.  Incredibly,  the protective  barriers  that separate the explosive compartments  are  fabricated from nothing more exotic than cardboard.

    In addition to light, pyrotechnics is often exploited for its production of heat. The best known heat generating pyrotechnic, the safety match, contains an energetic blend of potassium chlorate oxidizer and sulfur, along with  a  glue-like fuel and binder.

    Calcium silicide fuel mixed with iron oxide generates a moderate amount  of heat  but no gas. During World War II small pyrotechnic devices containing this composition  and a fuse were built into cans of rations so that they  could  be warmed  in the absence of a stove. Time delay mixtures, usually pressed columns containing boron, tungsten, or silicon fuel, produce a controlled dose of  heat for  a  specific  length of time that can then set off a larger reaction.  Such mixtures  are used to control the timing sequence in various aerospace devices, including the exploding bolts that rapidly jettison emergency exit hatches  and spent  rocket stages; similar time delays prevent hand grenades from  exploding as  soon as the pin is pulled and lever released. Decoy compositions have  been developed  to  protect  aircraft  from  enemy  heat  seeking  missiles.   These compositions  emit infrared radiation the emulates the thermal signature  of  a jet engine.

    Heat  production is often associated with the emission of  smoke  and  gas. Colored smoke grenades, used for signaling and for daytime displays, contain  a mixture  of  potassium chlorate oxidizer and sugar fuel that,  when  activated, vaporizes  organic  dyes to create a richly hued smoke  cloud.  Sugar  is  used because  it  burns at a low temperature; a hotter flame would disintegrate  the dyes.

    Solid  fuel  rockets are in essence giant pyrotechnic devices  designed  to optimize  gas  production. Each space shuttle booster rocket  contains  half  a million  kilograms of a propellant consisting of energetic pulverized  aluminum fuel  and  ammonium perchlorate oxidizer; the mixture also includes  a  special fuel  and  binder  called poly-butadiene-acrylic acid-acrylonitrile  terpolymer (PBAN). When oxidized, PBAN releases copious quantities of carbon monoxide  and carbon  dioxide  gas and steam that help loft the shuttle into space.  Ammonium perchlorate  is  well  suited for this application  because  its  decomposition products are all gasses, and so it enhances the rockets' thrust.

     Gas generation on a smaller scale creates the whistle effect heard in some fireworks.  Compositions  containing  potassium  perchlorate  oxidizer  and  an organic salt (such as sodium salicylate, a chemical cousin of aspirin) burn one layer  at a time and emit gas in spurts. When such compositions are pressed  in narrow tubes, the rapid pulses of escaping gas create a whistling sound.

    The  most  appropriate application for a particular pyrotechnic mixture  is largely  determined  by  the  reactivity of its  oxidizer  and  its  fuel.  The reactivity  of  the fuel is closely related to the amount of  energy  (heat  of combustion)  liberated  when  it combines with  oxygen.  Metals  release  large amounts of energy when oxidized; sugar releases relatively little. Charcoal and natural  materials  such as red gum, a tree secretion,  produce  moderate  heat needed to activate the color producing compounds in a firework.

    The  reactivity  of an oxidizer depends on two main factors:  decomposition temperature  and  heat  of  decomposition. At  decomposition  temperature,  the oxidizer begins to release oxygen at a significant rate. Heat of decomposition, as  the  term implies, is the amount of heat required to decompose the oxidizer in  order to release the oxygen. This amount can be positive (endothermic),  in which case decomposition absorbs heat, or negative (exothermic), in which  case it generates heat.

    Potassium chlorate decomposes at a low 360 degrees C and is exothermic;  it is  used  in  smoke grenades and household matches because it is energetic  and easily  activated. At the other extreme, iron oxide decomposes at  nearly  1500 degrees C and is strongly endothermic. It can be activated only by an energetic metallic fuel such as aluminum.

    Packaging  and  the homogeneity of a pyrotechnic mixture  also  affect  its reaction  rate. As every maker of a pipe bomb knows, confinement  significantly speeds  up the pyrotechnic process by concentrating heat and hot gas  near  the reaction  site.  A  mixture that burns at a controlled rate  in  the  open  can explode violently if confined. In general, the greater the homogeneity  of  the fuel and oxidizer, the faster the burning rate.

    Once,  while conducting a seminar, I was asked why liquids are  not  widely used as pyrotechnics, since they should mix more thoroughly and so produce more reactive  compositions  than solids. The answer, I realized,  is  that  liquids intermingle  too  well. Liquid compositions would be extremely homogeneous  and therefore highly reactive and sensitive to ignition. Liquids also could  settle out  during storage, thereby upsetting the chemical balance. Early versions  of dynamite  of porous materials (such as sawdust) soaked in liquid nitroglycerin, were extremely unstable precisely for this reason.

    Reactivity is greatest when the oxidizer and fuel are blended at the atomic level  and  the electron accepting oxidizer is located immediately adjacent  to the  atom  or  ion  of  fuel that donates the electrons  when  the  pyrotechnic reaction  is initiated. Such energetic atomic mixtures are, strictly  speaking, explosives  not pyrotechnics, but the principles underlying their  behavior  is similar.  Nitroglycerine, for instance has the molecular  formula  C3N3H5O9.  A small  disturbance (heat or impact, for instance) causes it to  decompose  into carbon  dioxide  (CO2), water (H2O), nitrogen (N2) and a little  excess  oxygen (O2).  In  the process, nitrogen-oxygen atomic bonds are replaced by  far  more stable carbon-oxygen, hydrogen-oxygen and nitrogen-nitrogen bonds; the  result is a violent release of energy.

   A less familiar, but increasingly important, material of this kind is sodium azide,  the  active component in automotive airbags. This compound consists  of interpenetrating  lattices  of ions of sodium  and  azide  (a  group  of  three chemically  bond  nitrogen  atoms). An energetic impact  disrupts  the  lattice structure.  The  sodium combines with oxygen while the nitrogen  atoms  regroup into pairs to form a large quantity of nitrogen gas.

    The history of the pyrotechnics industry, both in the U.S. and abroad,  has been  filled  with tragic accidents that have occurred during the manufacturing process,  such as the devastating explosion that destroyed the Grucci plant  in Long  Island,  N.Y., in 1983. Improving safety demands a detailed understanding of the phenomenon of ignition.

    Ignition  begins  when energy from some source; a flame, friction, impact, spark, elevated temperature or even laser beam, breaks the chemical bonds in  a pyrotechnic  mixture. As a result, more stable bonds are formed and  energy  is released. If the energy is adequate to activate the next layer of the  mixture, the  reaction continues; if the energy is absorbed by the surrounding  material or is insufficient to activate the next layer, the reaction dies out.

     Fred  L.  McIntyre and his coworkers at the John C. Stennis Space Center's Hazard  Test  Range  in  Mississippi  have analyzed  a  series  of  pyrotechnic compositions  to determine how sensitive each is to ignition by various  energy sources.   These   studies  demonstrated  that  critical  factors   controlling sensitivity  are the amount of heat generated by the reaction and the  ignition temperature,  the  minimum  temperature necessary  to  induce  a  rapid,  self-propagating  reaction. Ignition sensitivity is also affected  by  the  particle size  of  the  chemical  components  and by  the  grain  size  of  the  blended composition; fine grains ignite more readily than large ones. Ease of  ignition by  friction  depends  on the presence of abrasive materials  in  the  mixture. Adding  a  lubricant, such as wax, can significantly reduce the  likelihood  of friction induced ignition.

    Safety  is  also  important for the users of pyrotechnics,  often  children celebrating  Guy  Fawkes  Day,  New  Year's  or  other  holidays  traditionally accompanied  by fireworks. In 1976 the U.S. consumer Product Safety  Commission enacted strict federal standards for consumer fireworks. The European Community nations  have  a  wide range of safety standards that they  are  attempting  to incorporate into a single code as part of their legal and economic unification.

    There  is  no  question, based on numerous conversations I  have  had  with researchers in many fields, that many science careers were stimulated by  early experiments  in  pyrotechnics. The need to come up with a quick explanation  to parents  concerning  a basement filled with smoke from a successful  experiment probably has also sparked more than a few legal careers.

    The  family business nature of the civilian pyrotechnics industry  and  the classified  nature of much defense related work have made academic training  in the  field difficult to acquire. The only academic pyrotechnics courses in  the U.S.  that  I  am aware of are several annual, one week seminars at  Washington College in Maryland, taught by several colleagues and me. Fortunately, a number of  organizations  actively encourage treating pyrotechnics as  a  science  and publish   research  in  the  field.  These  groups  include  the  International Pyrotechnics   Society  and  the  more  amateur  oriented  Pyrotechnics   Guild International  Inc.  An  occasional journal called Pyrotechnica  also  contains articles on current work.

     The  most  active supporter of the civilian industry continues to  be  the viewing  public. Since 1976 the annual consumption of fireworks in the U.S.  by private  individuals for family and neighborhood celebrations has doubled.  The centuries  old  tradition of fireworks displays is still  fascinating,  despite competition  from  rock concerts, music videos and other eye dazzling  and  ear filling forms of entertainment. Evidently modern technology has yet to  find  a match  for  the  excitement one fe