I NTRODUCTION
This book is an introduction to the basic principles and theory of pyrotechnics. Much of the material is also applicable to the closely-related areas of propellants and explosives. The term "high-energy chemistry" will be used to refer to all of these fields. Explosives rapidly release large amounts of energy, and engineers take advantage of this energy and the associated shock and pressure to do work. Pyrotechnic mixtures react more slowly, producing light, color, smoke, heat, noise, and motion.
The chemical reactions involved are of the electron-transfer, or oxidation-reduction, type. The compounds and mixtures to be studied are almost always solids and are designed to function in the absence of external oxygen. The reaction rates to be dealt with range along a continuum from very slow burning to "instantaneous" detonations with rates greater than a kilometer per second (Table 1. 1).
It is important to recognize early on that the same material may vary dramatically in its reactivity depending on its method of preparation and the conditions under which it is used. Black powder is an excellent example of this variability, and it is quite fitting that it serve as the first example of a "high-energy material" due to its historical significance. Black powder is an intimate mixture of potassium nitrate (75% by weight), charcoal (15%), and sulfur (10%). A reactive black powder is no simple material to prepare. If one merely mixes the three components briefly, a powder is produced that is difficult to light and burns quite slowly. The same ingredients in the same proportions, when thoroughly mixed, moistened, and ground with a heavy stone wheel to achieve a high degree of homogeneity, readily I
Introduction
3
1
2
Chemistry of Pyrotechnics
black powder as a propellant and delay mixture in many applica-TABLE 1. 1 Classes of "High-Energy" Reactions tions, there is still a sizeable demand for black powder in both the military and civilian pyrotechnic industries. How many black Approximate
powder factories are operating in the United States today? Ex-Class
reaction velocity
Example
actly one. The remainder have been destroyed by explosions or closed because of the probability of one occurring. In spite of Burning
Millimeters/second
Delay mixtures, colored
a demand for the product, manufacturers are reluctant to engage smoke composition
in the production of the material because of the history of prob-Deflagration
Meters /second
Rocket propellants, con-
lems with accidental ignition during the manufacturing process.
fined black powder
Why is black powder so sensitive to ignition? What can the chemist do to minimize the hazard? Can one alter the performance of Detonation
> 1 Kilometer /second
Dynamite, TNT
black powder by varying the ingredients and their percentages, using theory as the approach rather than trial-and-error? It is this type of problem and its analysis that I hope can be addressed a bit more scientifically with an understanding of the fundamental concepts presented in this book. If one accident can be prevented ignite and burn rapidly. Particle size, purity of starting materias a consequence of someone's better insight into the chemical nature of high-energy materials, achieved through study of this als, mixing time, and a variety of other factors are all critical in producing high-performance black powder. Also, deviations from book, then the effort that went into its preparation was worth-the 75/15/10 ratio of ingredients will lead to substantial changes while.
in performance. Much of the history of modern Europe is related to the availability of high-quality black powder for use in rifles and cannons. A good powder-maker was essential to military suc-BRIEF HISTORY
cess, although he usually received far less recognition and dec-oration than the generals who relied upon his product.
The use of chemicals to produce heat, light, smoke, noise, and The burning behavior of black powder illustrates how a pyro-motion has existed for several thousand years, originating most technic mixture can vary in performance depending on the condi-likely in China or India. India has been cited as a particularly good possibility due to the natural deposits of saltpeter (potas-tions of its use. A small pile of loose black powder can be readily ignited by the flame from a match, producing an orange flash and sium nitrate, KNO 3) found there [1].
a puff of smoke, but almost no noise. The same powder, sealed Much of the early use of chemical energy involved military ap-in a paper tube but still in loose condition, will explode upon ig-plications. "Greek fire," first reported in the 7th century A.D. , nition, rupturing the container with an audible noise. Black pow-was probably a blend of sulfur, organic fuels, and saltpeter that der spread in a thin trail will quickly burn along the trail, a generated flames and dense fumes when ignited. It was used in property used in making fuses. Finally, if the powder is com-a variety of incendiary ways in both sea and land battles and pressed in a tube, one end is left open, and that end is then added a new dimension to military science [2].
constricted to partially confine the hot gases produced when the At some early time, prior to 1000 A.D. , an observant scientist powder is ignited, a rocket-type device is produced. This varied recognized the unique properties of a blend of potassium nitrate, behavior is quite typical of pyrotechnic mixtures and illustrates sulfur, and charcoal; and black powder was developed as the first why one must be quite specific in giving instructions for pre-
"modern" high-energy composition. A formula quite similar to the paring and using the materials discussed in this book.
one used today was reported by Marcus Graecus ("Mark the Greek") Why should someone working in pyrotechnics and related areas in an 8th century work "Book of Fires for Burning the Enemy"
bother to study the basic chemistry involved? Throughout the
[ 2]. Greek fire and rocket-type devices were also discussed in 400-year "modern" history of the United States many black pow-these writings.
der factories have been constructed and put into operation. Al-The Chinese were involved in pyrotechnics at an early date though smokeless powder and other new materials have replaced and had developed rockets by the 10th century [1]. Fireworks
4
Chemistry of Pyrotechnics
Introduction
5
11
were available in China around 1200 A.D. , when a Spring Festi-then be produced. Strontium, barium, and copper compounds val reportedly used over 100 pyrotechnic sets, with accompany-capable of producing vivid red, green, and blue flames also being music, blazing candle lights, and merriment. The cost of came commercially available during the 19th century, and mod-such a display was placed at several thousand Bangs of silver ern pyrotechnic technology really took off.
(one Hang = 31. 2 grams) [ 3] . Chinese firecrackers became a Simultaneously, the discovery of nitroglycerine in 1846 by popular item in the United States when trade was begun in the Sobrero in Italy, and Nobel's work with dynamite, led to the de-1800's. Chinese fireworks remain popular in the United States velopment of a new generation of true high explosives that were today, accounting for well over half of the annual sales in this far superior to black powder for many blasting and explosives country. The Japanese also produce beautiful fireworks, but, applications. The development of modern smokeless powder -
curiously, they do not appear to have developed the necessary using nitrocellulose and nitroglycerine - led to the demise of technology until fireworks were brought to Japan around 1600
black powder as the main propellant for guns of all types and A.D. by an English visitor [4]. Many of the advances in fire-sizes.