Explosives

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Flash! Bang! Whiz!

An introduction to propellants, explosives, = pyrotechnics and=20 fireworks

Introduction

This article reviews the technological use of exothermic chemical = reactions=20 that release their energy in a very short time interval. There are three = primary=20 fields of application for these effects: propellants, explosives and=20 pyrotechnics. Propellants create a high gas pressure for driving = projectiles or=20 rockets and for similar uses. Explosives create a disruption of solid or = liquid=20 bodies, as in construction, mining or warfare. Pyrotechnics have effects = that=20 are mainly sound and light, but include many other varied applications, = mainly=20 on a small scale. Fireworks is an application that is entertainment--a = show of=20 light, noise and motion. "Fireworks" is an almost exact translation of = the Greek=20 roots of "pyrotechnics." Perhaps "pyrotechnics" should be reserved for = the=20 serious applications and "fireworks" used for the entertainment = side.

The chemical reactions we shall consider are reactions between an=20 oxidizer that supplies oxygen or receives electrons, and a = fuel=20 that combines with the oxygen or releases electrons, and is a = reducing=20 agent. These two functions may reside in the same molecule, or in = different=20 molecules. Some constituents, such as sulphur, may serve as either a = fuel or an=20 oxidizer. In any case, both functions are present in every explosive (a = general=20 term for all three kinds of applications), and the oxygen of the = atmosphere=20 plays no role as an oxidizer, as it does in combustion or respiration, = two other=20 chemical sources of energy. For rapid reaction, the oxidizer and the = fuel must=20 be intimately mixed before the reaction occurs. In some cases, = atmospheric=20 oxidation may play a minor role.

The reactions we use must certainly be spontaneous, yet must not = begin until=20 the proper moment, even though all necessary ingredients are in close = contact.=20 This means that there must be some energy barrier to initiation of the = reaction,=20 which will not occur until this energy is supplied. This is only meant = in a=20 general sense; no such unique barrier can be identified. The agents will = exhibit=20 a range of sensitivity from the exquisitely sensitive that will be set = off by=20 the slightest shock, such as the decomposition of NCl₃, to an = almost=20 total insensitivity, like that of TNT. In most cases, we search for a = degree of=20 sensitivity that will not respond to the usual shocks of handling and = transport,=20 but will detonate reliably when a definite stimulus is applied.

The energy to start a reaction may be supplied by impact, friction, = heat,=20 flame, spark, radiation, shock wave or deformation. Each explosive agent = has its=20 own set of sensitivities to the various stimuli, so there is no single=20 detonation energy that can be supplied by multiple means. The device = supplying=20 the initiating reaction is called the detonator, initiator, primer, = first fire=20 or some other descriptive name. The reaction in this device then = initiates the=20 main charge. A match is a simple example. The head of the match is the = first=20 fire, lighting by friction. Its heat then ignites the wood of the match, = which=20 represents the main charge. Often the "first fire" is not the initiator, = but is=20 ignited by it and strengthens its effect.

Nuclear explosions will not be included here, but they are precisely = analgous=20 to chemical explosions, in that the energy is liberated in a very short = time=20 interval by a very exothermic reaction. There are also mechanical = "explosions"=20 where the energy transfers occur in brief intervals. These, also, will = not be=20 included.

Reactions and Heat

A chemical reaction can be described by a balanced chemical equation, = such as=20 C + O₂ → CO₂. This means that one atom of = carbon reacts=20 with one diatomic molecule of oxygen to produce one molecule of carbon = dioxide.=20 In masses, 12 g of carbon combines with 32 g of oxygen to make 44 g of = carbon=20 dioxide. Of course, here we are taking the weight of a fixed number of = atoms or=20 molecules, 6.02 x 10²³ of them. The ratio of 12 g of carbon = to 32 g=20 of oxygen is called stoichiometric, meaning that nothing is in = deficiency=20 or excess for a reaction according to this equation.

The writing of an equation does not necessarily mean that the = reaction will=20 take place in that way. For example, carbon could react as 2C + = O₂ →=20 2CO, producing carbon monoxide instead. Here, 24 g of carbon react with = 32 g of=20 oxygen to make 56 g of carbon monoxide. Also, the equation may not show = the=20 mechanism of a reaction, or intermediate states. Reactions may = not go to=20 completion, but reach an equilibrium state with all of the reactants and = products present. Furthermore, just because a reaction will go does not = mean it=20 will go rapidly. In fact, it may not proceed at all unless a catalyst is = present, or some other necessary condition is established.

In the first reaction above, 94 kcal of heat are evolved for each = gram-mole=20 of carbon (12 g) that reacts. This is the heat of reaction. In = this case,=20 CO₂ is being formed from its elements, so it is also the = heat of=20 formation of carbon dioxide. We presume that the carbon, oxygen and = carbon=20 dioxide are all in some standard state when we determine the heat. If = they are=20 hotter or colder, or in some other form, corrections must be made to the = heat of=20 reaction. We shall not bother with such refinements here, since they are = rarely=20 necessary for general conclusions, and shall also take rounded values = for the=20 heats. A reaction that evolves heat, as this one does, is called=20 exothermic. In explosives and pyrotechnics, we are concerned = mainly with=20 exothermic reactions.

The heat of reaction Q is the decrease in the enthalpy H of = the=20 system, if the reaction takes place at constant pressure, or Q =3D = -ΔH. Enthalpy=20 changes are what are given in handbooks, so they are the negatives of = the heat=20 of reaction. For the formation of CO₂, then, ΔH =3D -94 = kcal/mol. An=20 exothermic reaction means a negative ΔH, while an endothermic = reaction has a=20 positive ΔH. Some compounds, potentially unstable ones, have = negative heats of=20 formation, or positive enthalpy changes, which are the same thing.

The heat of formation of carbon monoxide, in the second equation, is = 26=20 kcal/mol. Now suppose we are burning carbon monoxide, and want to find = the heat=20 of reaction. The equation is 2CO + O₂ → = 2CO₂. Imagine that=20 we make each side from its elements. To make the left-hand side, we get = 26=20 kcal/mole, as just stated, or 52 kcal. To make the right-hand side, we = get 94=20 kcal/mole, or 188 kcal. By the conservation of energy, we will get the=20 difference, 188 - 52 =3D 136 kcal, when we burn two moles, 56 g, of = carbon=20 monoxide. We can say, then, that burning a mole of carbon monoxide to = the=20 dioxide gives us 68 kcal. This can be done with any equation, so long as = we know=20 the heat of formation of each molecule involved.

Let's take another common reaction, 2H₂ + O₂ = →=20 2H₂O. Per molecule of water, the heat of formation is 58 = kcal/mol if=20 the water is left as a vapor. If we condense the water to liquid, we = recover the=20 heat of condensation as well, so the heat of formation is now 68 = kcal/mol. When=20 water is involved, we always have these two choices of the final state, = and so=20 the heat values will differ by 10 kcal/mol depending on which we = choose.

So far we have worked on a per mole basis, but it may be more = convenient to=20 work on a per kilogram basis. In burning hydrogen to water vapor, we get = 58 kcal=20 for each 2 g of hydrogen. This is 1000/2 =3D 500 mol per kg, so the heat = is 500 x=20 58 =3D 29,000 kcal/kg. In burning carbon to carbon dioxide, 12 g of C = gave us 94=20 kcal. A kg is 83.3 mol, so the heat is 83.3 x 94 =3D 7833 kcal/kg. In = both these=20 cases, we assumed that the oxygen came free from the air. If we include = the=20 weight of oxygen as well, then 18 g of hydrogen and oxygen give us 58 = kcal, so=20 the heat per kg is 3220 kcal/kg. In the case of the carbon, we find 2140 = kcal/kg. From the equation, then, we can figure out the heat evolved in = a=20 reaction per kg of reactants.

It is very handy to use a concept called formal charge to keep = track=20 of electrons in a molecule. The oxygen atom is assigned a formal charge = of -2=20 (as if it gained two electrons, which it would like to do), and hydrogen = a=20 formal charge of +1 as if it had lost its electron. We make the rule = that in a=20 molecule, the net formal charge is zero (if the molecule has remained = neutral).=20 In H₂O, then, we have +2 for the two H, and -2 for the = oxygen, so 2 -=20 2 =3D 0 and all is right. In CO₂, the two oxygens have formal = charge=20 -4, so the carbon must be +4. The carbon has not actually lost four = electrons,=20 and the oxygen has not gained four electrons, but they are shared. In = CO, the=20 carbon must have formal charge +2, since the oxygen is -2. This is just=20 accounting, but is of some help in understanding what goes on in = reactions. In=20 an ionic reaction, the formal charges are actual charges and reflect = valence.=20 For example, in NaCl, the Na has formal charge +1, the Cl -1, and these=20 correspond to the actual ionic charges in the crystal. In = Cl₂, formal=20 charge is meaningless, as it is in CH₄. However, we usually = say that=20 Cl has formal charge 0 in the first case, and C formal charge -4 in the = second=20 case if H has +1, but these are not actual charges. There may be = fractional=20 charges if the electrons are not symmetrically distributed in the = covalent=20 bonds.

Now let us consider a pyrotechnic reaction used to produce heat and = "flash,"=20 KClO₃ + 2Al → KCl + Al₂O₃. = KClO₃ is=20 potassium chlorate, a powerful oxidizing agent, and Al is powdered = aluminium.=20 This is a relatively dangerous mix, that can be set off by heat or = shock. The=20 heat of formation of potassium chlorate is 94 kcal/mol, and the heat of=20 formation of potassium chloride, KCl, is 104 kcal/mol. The heat of = formation of=20 aluminium oxide is 385 kcal/mol. Therefore, the heat of this reaction is = 385 +=20 104 - 94 =3D 395 kcal/mol, which is quite large. The formal charge on = the chlorine=20 goes from +5 to -1, while that on the aluminium goes from 0 to +3, for = two=20 atoms. The chlorine is said to be reduced, while the aluminium is = oxidized, on the basis of the direction of the change of formal=20 charge.

The fact that a reaction evolves heat when it proceeds does not mean = that it=20 is spontaneous. Chemical reactions are driven not by simple energy, but = also by=20 entropy, and seek for a minmum of the free energy U + PV - TS, not just = a minmum=20 of the energy U alone. In fact, the heats of reactions are changes in = the=20 quantities H =3D U + PV, the decrease in H (the enthalpy) when the = reaction takes=20 place. We see that if the entropy S decreases in a reaction, then the = free=20 energy is raised and the reaction is less favorable. A reaction can = proceed=20 spontaneously even if absorbs energy, provided the entropy increase is = large=20 enough. An example is the evaporation of a liquid. It takes energy to = boil off=20 some liquid, but the increase in entropy is very satisfying. With = explosives and=20 pyrotechnics, the entropy increases are usually substantial, so most of = the=20 reactions go easily to completion.

Black Powder

Black powder was the sole propellant, explosive and pyrotechnic agent = for 500=20 years, from 1300 to 1800, and is still in use for certain applications. = It is a=20 unique and fascinating compound chemically, technologically and = socially. It was=20 invented as a pyrotechnic substance, then applied as a propellant in = firearms,=20 and finally used in engineering and mining. The history of black powder = and=20 firearms is treated in Cannon. Some = authors make=20 assertions about the history of black powder that are not supported by = good=20 evidence, and should not be accepted without better proof. An egregious=20 assertion is that Chinese alchemists experienced a black powder = explosion in 220=20 BCE. There is no evidence of "black powder" in China, and this is about = 1200=20 years before nitrates were first discovered and used, according to more = reliable=20 sources. The great Chinese invention was pure nitrates, which they used = in=20 pyrotechnic devices, arrow throwers and rockets. The invention of black = powder=20 is shrouded in mystery; neither Roger Bacon nor Berthold Schwartz = invented it,=20 but high-nitrate powder is probably a European invention. Black powder = is not a=20 simple mixture of nitrate, charcoal and sulphur.

The composition of ordinary black powder is 65-75 KNO₃, = 15-20 C,=20 10-15 S, which is close to the "stoichiometric" ratio of 84:8:8 that = gives the=20 ideal reaction 10KNO₃ + 8C + 3S → = 2K₂CO₃ +=20 3K₂SO₄ + 6CO₂ + 5N₂. The = heat=20 released is 685 kcal/kg, and the volume expansion factor is 5100. The = solid=20 products make the characteristic white smoke. The actual reaction = depends on the=20 exact constitution of the powder, how it is prepared, and how it is = detonated.=20 The density of gunpowder is about 1.04 g/cc. Black powder is the safest = of all=20 explosives. It is insensitive to shock and friction or to electric = spark. It=20 must be initiated by heat or flame. Moisture renders black powder = useless, and=20 drying does not restore its properties.

The nitrogen in KNO₃ has a formal charge of +5, which is = reduced=20 to 0 in N₂ (in such molecules the formal charge is taken as = zero, its=20 average value). The carbon is oxidized from 0 to +4 in CO₂ = and the=20 carbonate, and the sulphur from 0 to +6. KNO₃ is a stable and = safe=20 oxidizing agent, not capable of explosion on its own. Black powder is a = very=20 stable explosive, insensitive to shock or friction, but sensitive to = heat and=20 flame. Like all explosives, it supplies its own oxygen and does not rely = on the=20 atmosphere. Note that it is much less efficient as a heat source than = carbon and=20 oxygen, which gives 2140 kcal/kg. Its utility lies in its ability to = furnish its=20 energy in a very short time, while the carbon will take a good while to=20 burn.

How the powder burns is affected by the grain size. The larger the = grain, the=20 slower the powder burns. Fine powder is used for blasting, small grain = for=20 firearms, and large grain for cannon. A large variety of black powders = are=20 manufactured, and each type has a special designation and use. Black = powder is=20 essentially a propellant that burns at a rapid but finite rate = determined=20 mainly by its temperature. It is often said that gunpowder will only = burn in the=20 open, but explodes when confined. This is much too simple a statement. = When in=20 the open, the unburnt powder never becomes hot enough to burn rapidly. = When=20 confined, as in a firecracker, the powder quickly becomes hot enough to = burn=20 very rapidly, releasing all the energy in a very short time, quickly = enough to=20 make a loud report. Pressure does raise the rate of burning, but = gunpowder has=20 the least pressure effect of any common explosive, and for this reason = is gentle=20 to guns. A thread of gunpowder, wrapped in paper or other covering, = burns at a=20 slow and reliable rate, making a delay element or fuse.

Because of its safety and reliability, pressed black powder is used = as the=20 propellant in small rockets. A powder for this service has less = KNO₃=20 and S, and more C. Its rate of burning can be slowed with chalk, wax or = talc. A=20 typical mix is 91 black powder, 9 chalk. No more than 3% of the powder = can be=20 stopped by a #20 sieve (0.84 mm) and no less than 60% must be stopped on = a #40=20 sieve (0.42 mm). It is compressed to 1.82-1.89 g/cc, and contains = 1.8%-2.5%=20 moisture. This propellant grain is burned in a chamber with a = ceramic=20 choke in army signal rockets, which reach 700 ft. altitude. A bursting = charge=20 expels 5 white stars that free-fall, or else a red star with parachute, = that=20 burns for about 50 sec. and falls at 10-15 ft/s. A model rocket has a = pressed=20 black powder propellent grain, and a granular black powder ejection = charge.=20 There is a delay element between the two charges, so that the rocket = coasts to=20 its maximum altitude before releasing the payload. The fuel for = solid-fuel=20 rockets, though called the "powder grain," is a cast plastic cylinder of = the=20 fuel material. The word "grain" does, in fact, seem to come from the = grains of=20 black powder that are used in a pressed charge, and has been transferred = to the=20 whole fuel assembly of any type.

Black powder is an oxidizer--fuel mixture of the type we shall = discuss at=20 more length under pyrotechnics. The sulphur and charcoal are first = ground=20 together, so that the thixotropic sulphur coats the colloidal charcoal=20 intimately. Then the nitrate is mixed in by wet grinding. The nitrate = produces=20 oxygen to oxidize the sulphur and carbon, catalyzed by the large active = surface=20 of the charcoal, while releasing the nitrogen with the evolution of = heat. The=20 reaction begins at a temperature where there is a change in the crystal=20 structure of the nitrate, which creates lattice defects that encourage = the=20 solid-state reaction.

War rockets were not extensively developed in China, and were used = only=20 incidentally in the West. Rockets for pleasure pyrotechnics did, = however, become=20 widely used, and were the basis for later war rockets. William Congreve=20 developed his war rockets in the late 18th century, but they were only=20 successfully used first in 1807 at Copenhagen. They were difficult to = control,=20 and not very effective. However, they could be fired with a light = launcher=20 instead of a heavy cannon, a principle later extensively applied. The = last major=20 use of Congreve rockets was in the Zulu war of 1879. Rockets appeared = again in=20 the Second World War for use in mass flights from landing ships and to = support=20 infantry. They also were used in the recoilless rifles and antitank = rocket=20 launchers that are still valuable, providing powerful artillery without = the=20 weight and recoil.

Fulminate

In 1799 or 1800, Edward Howard discovered the second explosive = substance to=20 be found, mercuric fulminate. The name comes from Latin fulmen, = fulminis,=20 "lightning." To make fulminate, dissolve 1 part mercury in 13 parts = nitric acid=20 (sp.gr. 1.36), then pour the solution in 8 parts of alcohol. A vigorous = reaction=20 occurs, with frothing and emission of nitrogen oxides. Finally, the = fulminate=20 separates as small white needles. The solution is filtered, and the = precipitate=20 thoroughly washed until all the acid is gone. The salt is dried, but = should=20 remain moist until it is used. Do not, incidentally, do this at home! = The=20 crystalline salt is heavy, 4.42 g/cc, and explodes when exposed to = flame, impact=20 or friction. It produces sufficient flame to initiate black powder. = Percussion=20 caps were made from about 1815 in which a little fulminate, perhaps with = a=20 little chlorate, were sandwiched between thin sheets of copper. When = struck by a=20 firing pin, they released a puff of flame that could discharge a = firearm. The=20 pinfire breech-loading shotgun appeared in France in 1836, and the = "needle gun"=20 in Germany in 1841. Percussion caps displaced the flintlock only = gradually.

Justus Liebig studied the fulminates in 1822, determining that the = formula=20 for mercuric fulminate was Hg(ONC)₂. The fulminate radical=20 ONC^- is an isomer of the cyanate radical, NCO^-, = but very=20 different in behavior. Cyanates are stable (and nonpoisonous), while all = fulminates are explosive. Note that in the cyanate, the carbon is in the = middle,=20 and we can write the valence structure -N=3DC=3DO where all the atoms = show their=20 usual valence. For the fulminate, we must write -O-N=3DC, and the carbon = is left=20 with only two bonds, instead of the usual four. The actual behavior = depends on=20 the electronic structure, of which this is only a crude representation, = but the=20 result is the same. Mercuric fulminate decomposes by Hg(ONC)₂ = → Hg +=20 N₂ + 2CO (or CO₂ + C), so that all the products = are gases=20 or vapors. Mercuric fulminate "puffs off" at 195=B0C in a test. It is = easily=20 detonated by a hot wire.

It is said that the alchemist Johann Kunkel von L=F6wenstein tinkered = with=20 fulminate in the 17th century. This fact was discovered only long after = Howard's=20 discovery had been put to practical use, and Leibig had studied = fulminates.=20 Other discoveries have similar precursors, only found after the fact and = often=20 by enthusiastic misinterpretation of the sources. Sometimes isolated = Russians=20 and Americans, both ingenious peoples, did make earlier discoveries at = least=20 partially like later and better-known ones that were not only conceived, = but put=20 to practical use, but many are only the results of wishful thinking. = William=20 Kelley of Kentucky may have refined pig iron by blowing air through it, = but only=20 Henry Bessemer solved the difficult problems of making the process a = practical=20 one.

Radicals like ONC^- that are associated with explosive = potential=20 are called explosophores. We shall see that = NO₂^-,=20 NO₃^-, N₃^-,=20 ClO₃^- and ClO₄^- are other = examples=20 frequently seen. The nitrogen radicals seem to release active oxygen = when=20 N₂, a very stable molecule, is formed. The chlorates and = perchlorates=20 are also easy sources of oxygen, the chlorates more so than the = perchlorates.=20 Explosive reactions are usually solid-state, or at least liquid-state = reactions,=20 in which the crystal structures and molecular neighborhoods play an = important=20 role. Free radicals (groups with unpaired electrons) and chain reactions = are=20 probably important. The mechanisms of explosive reactions are by no = means well=20 known, and those that are known are often surprising.

The azide radical, N₃^-, with a structure = something like=20 -N=3DN≡N, releases N₂ to give the free radical -N, = which can do further=20 execution. Mercurous azide, HgN₃ decomposes according to=20 2HgN₃ → 2Hg + 3N₂, giving only mercury vapor = and nitrogen.=20 It is one of the rare explosives that contains no oxygen. Lead azide,=20 Pb(N₃)₂, was discovered in 1891 by Curtius, and is = a=20 useful fulminating substance, less sensitive than mercuric fulminate. = Indeed, it=20 cannot be exploded with a firing pin or a safety fuse, and must usually = be mixed=20 with a more sensitive compound. If copper is around, lead azide forms a=20 dangerously sensitive compound. It is even heavier than mercuric = fulminate, 4.8=20 g/cc. Most of the fulminates and azides of heavy metals are sensitive=20 explosives. These materials do not make efficient explosives; their sole = attraction is their sensitivity.

Lead styphnate, the lead salt of 2,4,6-trinitro,1,3-dihydroxybenzene, = or lead=20 2,4,6-trinitroresorcinol, is now used, in combination with other = ingredients (to=20 give it more bang), in electrical detonators, replacing mercuric = fulminate. It=20 is non-hygroscopic and stable, having a positive heat of formation, but = very=20 sensitive to flame or spark. It is unusually sensitive to static = electricity, so=20 it is dangerous to handle. It is insensitive to nuclear radiations.

Detonators, or blasting caps, are given numbers from 1 to 10 = depending on tbe=20 detonating charge. No. 1 is the weakest, with 0.3 g, while No. 10 is the = strongest, with 3.0 g. No. 3 is adequate for gelatin dynamite, No. 6 for = Gelignite, and No. 8-10 for ammonium nitrate. This is only for = orientation;=20 ratings may well be quite different at the present time. Blasting caps = are=20 usually fired electrically, the most effective and reliable method. They = are=20 kept shorted until just before firing. Detonators must not be stored = near=20 explosives.

It was long a mystery why one mass of some explosives would detonate = another=20 some distance away. This effect, called "detonation by influence," was = employed=20 usefully in practice, although it was not understood. It did not occur = with=20 black powder, but only with the newer explosives. Some investigators = thought=20 there might be resonances of molecular frequencies, or other arcane = influences,=20 but it was shown that the detonations were not specific to particular = compounds.=20 Brisant explosives, however, were more effective than less brisant. We = now know=20 that this effect was due to the propagation of shock waves, that = travelled=20 faster than sound, but rapidly decayed to sound waves with = distance.

High Explosives

In 1838 M. P=E9louze treated cotton with concentrated nitric acid, = producing=20 cellulose nitrate, a substance later called guncotton. In 1845, = Sch=F6nbein=20 showed that the nitration could be accelerated by mixed nitric and = sulphuric=20 acids, and dreamed that guncotton could replace black powder. This new = explosive=20 proved very unpredictable. In 1847, a guncotton factory exploded in = England. Von=20 Lenk worked for years in Austria to adapt guncotton to ballistics, but=20 explosions in 1862 and 1865 put an end to his experiments. Not until = 1865 did=20 Sir Frederick Abel at Woolwich Arsenal finally produce a safe guncotton, = by=20 rigorously purifying the raw materials and carefully controlling the=20 manufacturing process. Guncotton was a much more powerful explosive than = black=20 powder, but it was difficult to use. E. O. Brown showed that moist = guncotton=20 (which was relatively safe) could be exploded by a little dry guncotton = (which=20 was sensitive to shock) and a detonator. Guncotton releases about 1100 = kcal/kg,=20 nearly twice the heat of black powder, and two-thirds that of=20 nitroglycerine.

There is no molecule of cellulose, but the formula=20 C₂₄H₄₀O₂₀ describes its composition. It = is, of=20 course a carbohydrate, since H and O are in the ratio 2:1, = consisting of=20 linked sugar molecules. Sugar is rich in -OH groups. Nitration replaces = -OH by=20 -NO₃, so if n hydroxyls have been replaced, the formula = becomes=20 C₂₄H_40-nO_20-n(NO₃)_{n. The=20
percentage of nitrogen is easily worked out as %N =3D 1400n/(648 + 45n). =
The usual=20
range of n for manufactured cellulose nitrate is 8-12. The n =3D 12 =
product cannot=20
be made in reliably stable form, so n =3D 11 is the usual maximum. In =
this case,=20
%N =3D 13.47%, and the result is called guncotton. For n =3D 8-10, the =
nitrogen=20
ranges from 11.11% to 12.76% and the result is called pyrocotton, =
used=20
for making collodion cotton, or pyroxylin, for =
gelatinizing=20
nitroglycerine and for smokeless powder.}

Highly nitrated guncotton is insoluble in 2:1 ether-alcohol mixtures, = but=20 pyroxyline is completely soluble. Pyroxyline, dissolved in ether, makes = films=20 called collodion that were used for early motion picture film. It = can be=20 plasticized by a hot mixture of camphor and alcohol. Fillers and = pigments can be=20 incorporated, and the mixture hardens as celluloid or = xylonite,=20 the first plastic, which filled a long-standing need. It was invented=20 simultaneously in the United States and Britain, which was the reason = for the=20 two different trade names given above. In the United States, it was = invented by=20 J. W. Hyatt (1837-1920), who was trying to win a prize for an = alternative to=20 ivory for billiard balls. The one disadvantage of celluloid was its=20 inflammability.

Meanwhile, in 1846, Antonio Sobrero in Milan synthesized glyceryl = trinitrate=20 by treating glycerol with concentrated nitric and sulphuric acids. = Fortunately=20 for him, he did not synthesize much before discovering that it was a = powerful=20 and sensitive explosive, which was named nitroglycerine. This was = a much=20 purer and more controllable explosive than guncotton, but it was too = sensitive=20 to be generally used, and too powerful for guns. Alfred Nobel, who had = become=20 wealthy through the family oil wells in Russia, became fascinated with = this=20 powerful explosive and looked for ways to employ it. In 1865, he = discovered that=20 nitroglycerine could be detonated by a mercury fulminate primer in a = copper=20 tube. The copper cap containing mercury fulminate for detonating = gunpowder had=20 been invented in 1816. The next year, he found that nitroglycerine could = be=20 rendered insensitive to shock by adsorption in diatomaceous earth, or=20 kieselguhr. 75% nitroglycerine in 25% kieselguhr made an explosive that = you=20 could use as a hammer, but would explode with full power when detonated = by a=20 fulminate primer. This explosive was called dynamite. He went on to = discover=20 that ammonium nitrate could also be detonated, and was a powerful and = useful=20 explosive. In 1875, he discovered that colloidal "collodion cotton" = (11.2%-12.2%=20 N₂) would dissolve in nitroglycerine and form a gel that was = nearly=20 as stable as dynamite. This blasting gel is the most powerful = chemical=20 explosive known. Colloidal guncotton was made by plasticizing it in = ether and=20 alcohol, and then allowing the solvents to evaporate.

=20 Glyceryl trinitrate is the nitric ester of glycerol, a thick liquid with = density=20 1.6 g/cc, melting at 13=B0C and becoming rather volatile above 50=B0C. = It invariably=20 explodes at 200=B0C to 260=B0C, with a propagation velocity of 7450 m/s. = It is=20 difficult to detonate when frozen (below 13=B0C!), which has been the = source of=20 much difficulty with dynamite. Its vapors cause headache. It is used in = small=20 amounts as a medicine to dilate cardiac blood vessels and relieve = angina=20 pectoris. The stoichiometric reaction is=20 4C₃H₅(ONO₂)₃ → = 12CO₂ +=20 10H₂O + 6N₂ + O₂, which provides 330 = kcal/mol,=20 or 1470 kcal/kg. Note that the reaction products are completely gaseous, = and=20 that there is excess oxygen, so it produces little smoke. In the = diagram, the=20 nitrate group is written ONO₂ instead of NO₃ to = show that=20 O is bonded to the carbon, not N. The NO₃'s have replaced the = OH=20 groups of glycerol.

Straight dynamite is made by absorbing 15%-60% nitroglycerine = in wood=20 meal, which is an active ingredient that releases more energy than the = inert=20 kieselguhr, but also renders the nitroglycerine insensitive. = Nitroglycerine=20 produces a little free oxygen on explosion, which burns the carbon in = the wood=20 meal. Some nitrate can be added to give more oxygen. It is sensitive and = shattering. Ammonia dynamite replaces some or all of the nitroglycerine = with=20 ammonium nitrate and sodium nitrate. It is cheaper, but not as = shattering, as=20 straight dynamite. A typical gel dynamite has 62.5% nitroglycerine, 2.5% = collodion cotton, 25.5% NaNO₃, 8.75% wood meal, and 0.5% soda = to=20 prevent acidity and stabilize the nitrate. A famous type of gel dynamite = is=20 Gelignite, 54%-63% nitroglycerine, 3%-5% collodion cotton, = 26%-34%=20 KNO₃, 6%-9% wood meal, and 0.5% chalk. Another composition of = the=20 same name is 70% NH₄NO₃, 29.3% nitroglycerine, and = 0.7%=20 collodion cotton. This gives some idea of the variations in composition = of these=20 explosives. Dynamite is rated on the equivalent nitroglycerine = percentage in=20 straight dynamite. A 30% dynamite is as powerful as 30% straight = dynamite.

Guncotton and nitroglycerine are high explosives, which means = that=20 they decompose at very high rates, and have a property called = brisance, a=20 somewhat foggy concept expressing the shattering power of an explosive. = Brisance=20 is a combination of a fast rise of pressure and rapid projection of = mass,=20 probably equivalent to the creation of a strong shock front. Black = powder is=20 very low in brisance, while guncotton and nitroglycerine are high. This = rules=20 out using high explosives for propellants in guns. These compounds also = contain=20 everything necessary in a single molecule, both oxidizer and fuel, = unlike most=20 pyrotechnic mixtures. They are relatively insensitive to heat and flame, = but=20 respond to shock and friction. For this reason, they must be detonated = by=20 detonators instead of by powder fuses.

The new explosives required nitric and sulphuric acids for their = manufacture,=20 and both acids had been expensive and available only in small = quantities,=20 especially the nitric. The chamber process for making sulphuric acid = rendered=20 that raw material plentiful and cheap. Once you have sulphuric acid, you = can=20 make any of the strong inorganic acids. In 1850, the great reserves of = Chile=20 saltpeter, NaNO₃, were discovered, and in 1863 the first = nitric acid=20 was made from it, solving the greatest bottleneck in the manufacture of = all=20 explosives, including black powder. Especially in Germany, the = manufacture of=20 organic chemicals from the distillation of coal, notably the aromatic = compounds,=20 became a major industry. This not only began the manufacture of = synthetic dyes,=20 so much superior to the natural product, but also supported the = explosives=20 industry. At the time of the First World War, the Haber process for = fixing=20 atmospheric nitrogen freed Germany and Europe from dependence on the = Chilean=20 nitrates, and brought down their cost steeply.

The high melting point of nitroglycerine has been a source of = difficulty.=20 Miners are instructed not to use frozen dynamite for blasting, since it = may not=20 detonate reliably and cause "hung shots" and other dangerous situations. = To thaw=20 the dynamite quickly, miners sometimes heated it in warm water. This is = the=20 wrong thing to do, because nitroglycerine is released from the absorbent = in=20 preference to water, and may run out and collect in the bucket holding = the warm=20 water. When the water is thrown out, the nitroglycerine explodes to the=20 detriment of the miners present. Leaking dynamite presents the same = hazard.=20 Dynamite will burn on an open fire.

In the 1920's and 1930's, liquid nitroglycerine was used for = "shooting" oil=20 wells to stimulate production. The productive formation might have a = large=20 porosity, so it held a lot of oil, but might be relatively impermeable, = or=20 "tight," so the oil would not flow into the small hole with sufficient = speed. In=20 limestone, hydrochloric acid was often used, but this was not useful in=20 sandstones. By exploding from 2 to 200 quarts of nitroglycerine, the = rock could=20 be fractured for a considerable distance, greatly enlarging the surface = through=20 with the oil would flow, equivalent to making a much larger hole. The = "shooter"=20 drove alone in a Ford coupe, with the "soup" in the back where the = rumble seat=20 used to be, from his source of supply. Nitroglycerine could not be = commercially=20 shipped, of course. He poured the "soup" into tin "torpedoes" and = lowered them=20 one by one, each fitting into the top of the one below. On the top went = a time=20 fuze that ticked away and exploded the charge at a reasonable interval. = Then=20 everyone filtered back to the well from their places of refuge. = Occasionally,=20 all did not go well, but the "shooters" were well paid and their widows = had=20 insurance. There are few graves of "shooters."

Shooting should not be confused with perforating, which also used = explosives.=20 When the hole had been "cased" with pipe and the section at the = productive=20 horizon cemented in solidly, a device (a gun perforator) was lowered = with=20 bullets aimed radially in short guns. These guns were set off, and the = bullets=20 punched holes in the casing for the oil to enter. This is still common = practice.=20 Shooting was later mostly replaced by a process known as "hydrofrac" in = which=20 high pressure water was used to fracture the rock around the base of the = hole. A=20 sort of apotheosis of shooting was the use of nuclear explosives in = Project=20 Rulison in an attempt to loosen up very tight gas formations in Western=20 Colorado. It was unsuccessful, since it largely destroyed the hole in = the=20 process.

Sometimes a burning well was attacked by drilling another hole at = some=20 distance that was deflected to some point close to the hole of the = burning well.=20 This new hole was then "shot" as described above, which often = extinguished the=20 fire, but at the expense of considerable subsurface damage.

Because nitroglycerine is so hazardous to transport, portable = nitroglycerine=20 plants were popular in the 19th century. These were small nitrating and = washing=20 facilities that produced the "soup" close to where it was used, in = mines,=20 construction projects, oil wells and so forth. The raw materials were = anhydrous=20 glycerol, fuming sulphuric acid, and 100% nitric acid. One model was = produced by=20 a telegraphic equipment manufacturer, one of the few "science-based" = industries=20 of the time, which also supplied electrical detonating apparatus.

Mannitol is a hexahydroxy alcohol, like two glycerols end to = end. It=20 is found in manna (the sweet juice from Tamarix gallica), celery, = rye=20 bread and cane sugar. Mannitol hexanitrate (HNM) is a white powder or = colorless=20 crystals melting at 112=B0C, also called nitromannite. It is less = sensitive=20 than mercuric fulminate, and often replaces fulminate in detonating = caps. Its=20 explosion point is 160=B0C - 170=B0C, about the same as fulminate or = nitroglycerine.=20 It is the most brisant explosive, slightly more than even = nitroglycerine. It is=20 said, however, to be inefficient as an initiator, which is probably why = it is=20 not used alone as a detonator.

Guncotton, nitroglycerine and nitromannite have nitrate groups bound = directly=20 to carbons. The free nitrate group is a symmetrical structure with all = the=20 oxygens equivalent and the whole group with charge -1. This group is = still free=20 to move when bound to a carbon, and there is a very low barrier to = exchange of=20 electrons that are closest to the carbon. When a neighboring nitrate = group tries=20 to form N₂, an oxygen is right there to begin oxidizing the = carbon.=20 The reaction probably proceeds due to free radicals in the liquid = association of=20 the molecules.

Ammonium nitrate, NH₄NO₃, is also an excellent=20 explosive, used in certain dynamite mixes (Nobel, 1879) and as a nitrate = oxidizer in pyrotechnics. It is very hygroscopic and must be protected = against=20 moisture. It decomposes to nitrogen and water, giving very little smoke, = by the=20 ideal reaction 2NH₄NO₃ → 2N₂ + = 4H₂O=20 + O₂. The excess oxygen can be used to oxidize some organic = material=20 mixed with the nitrate, such as wood meal, starch or diesel oil. = Explosives of=20 this type have been widely used since 1867. It is rather insensitive, = and must=20 be strongly detonated, perhaps by Primacord or a similar booster. Its = density is=20 1.725 g/cc. Amatol is a mixture of ammonium nitrate and TNT (see below), = either=20 80:20 or 50:50. The nitrate oxidizes the TNT so that no smoke is = produced. It=20 was a popular shell filling, economizing on the expensive TNT and = stretching out=20 toluene supplies.

Ammonium nitrate is an excellent nitrogen fertilizer, supplying = immediately=20 usable nitrate and time-release ammonia. For this reason, it is readily=20 available in bulk. After World War II, fertilizer-grade ammonium nitrate = (FGAN)=20 was shipped in large quantities from Texas to France to aid the recovery = of=20 European agriculture. On 16 April 1947, SS Grandchamp blew up at Texas = City,=20 followed by the SS Highflyer on the 17th. On 28 July 1947, SS Ocean = Liberty blew=20 up in the harbor of Brest, France. These disastrous explosions = demonstrate the=20 power of ammonium nitrate, and led to more careful handling of this = cargo.

Permissible or permitted explosives, also called safety explosives, = are=20 explosives approved for use in coal mines where there is a hazard of = methane=20 (fire damp) explosions. One modern permitted explosive is ANFO, an = ammonium=20 nitrate-fuel oil mixture. The idea is to minimize the flame on = explosion, and=20 keep it below the temperature that will ignite the methane. These = explosives=20 usually contain mainly ammonium nitrate, sensitized with nitroglycerine = so they=20 can be exploded with normal detonators (No. 6), and cooling salts, such = as=20 sodium nitrate or sodium chloride, and some wood meal or other organic = fuel.=20 Low-velocity grades are specially useful for producing lump coal, since = they=20 will not shatter the coal as much as the more powerful explosives. It is = not a=20 good idea to do blasting in a gassy mine anyway, so it is better to = avoid=20 explosions by adequate ventilation than to rely on permissible = explosives, which=20 might ignite the methane anyway.

Smokeless Powder

Vielle discovered how to make a propellant from cellulose nitrate in = 1886. He=20 started with low-nitrogen guncotton, or pyrocotton, with 11%-12% = of=20 nitrogen, and plasticized it with ether and alcohol. Pyrocotton will = dissolve=20 completely in this solvent, unlike guncotton. The gel was rolled out = into sheets=20 the sheets were broken up into powder, and the powder formed into = grains. These=20 grains, with various additives to control the rate of burning, chemical=20 properties and stability in storage, made a propellant called = smokeless=20 powder that could replace gunpowder, and was more powerful. Because = no=20 solids are produced in the reaction, there is no smoke, which is a great = benefit. Smokeless powder made entirely from pyrocotton is called=20 single-base powder.

Nobel mixed gelatinized the pyrocotton with nitroglycerine to make a=20 smokeless powder of different constitution, called double-base = powder.=20 Smokeless powder has completely replaced black powder in ballistics = because of=20 its superior power and lack of smoke. It is easily detonated by = fulminate caps.=20

The high temperature of the smokeless powder detonation made erosion = of the=20 gun barrel more serious, and the gases resulting could contain flammable = constituents such as CO, CH₄ and H₂. Four = molecules of=20 13.2% guncotton, which is approximately=20 C₂₄H₂₉(NO₂)₁₁O₂₀, = decomposes to 30CO₂ + 71CO + 41H₂ + CH₄ = +=20 35H₂O + 22N₂, where the flammable gases are = evident. On 13=20 April 1904, the U.S.S. Missouri, during gunnery practice off Florida, = suffered=20 an explosion in the aft 12" gun turret that killed 32 men. The = "flareback" when=20 the breech was opened reached a powder magazine, causing a powerful = explosion.=20 Flame can often be seen issuing from the muzzle of a gun that has just=20 fired.

Cordite was a famous British double-base smokeless powder, = British=20 Service Powder, used in everything from small arms to naval guns until = the=20 1930's. It was 30-40% nitroglycerine, 55-65% guncotton and 5% paraffin = grease.=20 It was colloidalized in acetone, and extruded in the form of cords, = hence the=20 name. Cordite MD, with 30% nitroglycerine, was a rifle powder. United = States=20 Service Powder was single-base, in cylindrical perforated grains. The = grains are=20 of different sizes and shapes for different applications. Rifle grains = have one=20 perforation, others seven. Small grains may be coated with graphite to=20 facilitate loading of cartridges (to discharge static electricity as = well as=20 lubrication). Ball powder is a single-base powder manufactured in = spherical=20 grains, for small cannon. Sporting powder is double-base powder that = ignites=20 more easily and burns faster than single-base rifle powder.

Mixtures of explosive compounds with a polymerizing binder, making = PBX=20 (polymerized-binder explosives) materials, are now popular. The = explosives used=20 for these modern material are HMX (cyclotetramethylene tetranitramine, = Octogen),=20 PETN (pentaerythrytol tetranitrate), and RDX (cyclotrimethylene = trinitramine,=20 Hexogen). Some of these compounds are discussed below, or in Organic = Chemistry. The=20 polymer binder acts like the gelling of nitroglycerine, making the = explosive=20 safer, which is important for the use of the explosives mentioned, which = are=20 quite sensitive. The first PBX was developed at Los Alamos in 1952, = using RDX in=20 polystyrene. PTFE (Teflon) and other polymers are also used as active = binders.=20

Aromatic Explosives

What I shall call "aromatic explosives" are not those that smell = good, though=20 they may, but those that contain the benzene ring in their structure. = Such=20 compounds were discovered in the liquids condensed when distilling coal, = of=20 which the archetype is benzene, which has a very pleasant aroma. = Unfortunately,=20 benzene causes liver cancer, so it is best avoided. I do not know if = just=20 smelling it is dangerous, but I suppose it is. Anyway, one whiff to see = what you=20 are missing is probably without hazard.

=20 The formula of benzene is C₆H₆, so it is quite = different=20 from the cycloalkane C₆H₁₂, cyclohexane. It is a = very=20 stable molecule, the motif of graphite, which consists of fused benzene = rings,=20 and no hydrogen at all. The functional groups -OH for an alcohol, -COOH = for a=20 carboxylic acid, -NO₂ for a nitrate, and -NH₂, as = well as=20 hydrocarbons such as -CH₃ for methyl, can be attached to the = ring=20 with little problem, replacing a hydrogen. Some typical aromatic = compounds are=20 shown at the right. The circle in the hexagon represents the delocalized = electrons that confer stability. Hydrogens are not shown.=20 C₆H₅OH is phenol, or carbolic acid.=20 C₆H₅COOH is benzoic acid,=20 C₆H₅NH₂ is aniline, the basis for many = dyes,=20 C₆H₅NO₂ is nitrobenzene, and=20 C₆H₅CH₃ is toluene. All of these, = including=20 benzene, are liquids. More than one functional group may be present.=20 C₆H₄(OH)₂ is resorcinol, hydroquinone, = or=20 pyrocatechin, depending on whether the OH groups are adjacent (ortho), = separated=20 by one C (meta), or by two C's (para).=20 C₆H₃(OH)₃ is gallic acid, or=20 (3,4,5)trihydroxybenzoic acid, with the three OH's on adjacent carbons. = Two=20 CH₃ groups give us ortho-, meta- and paraxylene. All of these = compounds are frequently seen and are useful. Benzene rings may be fused = in=20 pairs to form anthracene, or in triplets to make naphthalene, famous = from moth=20 balls. These are the first steps on the way to graphite. Aromatic = compounds are=20 these days more frequently obtained from petroleum than from coal.

One of the first aromatic explosives was picric acid, or = trinitrophenol,=20 C₆H₂(NO₂)₃OH. It was first = prepared=20 in 1771 by Woulfe as a dye, and was also used in medicine, long before = it was=20 first employed as an explosive in 1830. The name comes from its = extremely sharp=20 or bitter taste, from the Greek pikros, "sharp." = It forms=20 pale yellow crystals of density 1.76 g/cc, melting at 122=B0C and = exploding above=20 300=B0C. It is too sensitive to heat to be poured into shells, and must = be=20 press-loaded. It corrodes metals, forming sensitive picrates. The OH = group makes=20 it easier to nitrate the benzene ring. For picric acid to decompose to=20 N₂, CO₂ and H₂O, 27 oxygens would be = required,=20 but only 14 are available. Even if all the C goes as CO, one O is still = lacking.=20 It is typical of aromatic explosives to be short on oxygen, so they make = black=20 smoke. In 1886, France adopted picric acid as the standard bursting = charge for=20 shells, under the name of Melinite. In Britain, it was called Lyddite. = Picric=20 acid releases 810 kcal/kg on explosion, about half the yield of = nitroglycerine.=20 It is a relatively stable explosive, and of low brisance (about = equivalent to=20 ammonium nitrate, and half that of nitroglycerine). An explosive more = sensitive=20 than picric acid cannot be used in artillery shells.

The most famous aromatic explosive, however, is trinitrotoluene, = called TNT=20 for short. TNT is deficient in oxygen, so makes a cloud of black smoke. = It is a=20 popular bursting charge for shells and bombs, replacing picric acid = after World=20 War I. Picric acid seems still to be used in armor-piercing shells, = however,=20 which must delay before exploding. TNT, like picric acid, forms yellow = crystals,=20 density 1.654 g/cc, melts at 80.8=B0C and explodes at 240-280=B0C. TNT = is very=20 toxic; its dust and vapor must not be inhaled. It is rather insensitive = to=20 shock, and requires considerable energy to detonate, especially when = cast. To=20 avoid the hazards of a large amount of detonator, a booster = charge is=20 used that is first detonated, and then detonates the TNT. TNT cannot be=20 detonated with ordinary blasting caps. TNT has been used in some = smokeless=20 powders.

The standard booster for TNT is Tetryl, or trinitrophenyl = methylnitramine.=20 This molecule is like picric acid, but instead of the OH, has an=20 N(CH₃)(NO₂) group. Tetryl is fairly stable, but is = more=20 sensitive than TNT to shock and friction. It cannot be cast, but must be = pressed=20 into pellets. Like TNT and picric acid, it is toxic. Tetryl caps, used = in the=20 early 20th century, contained mercury fulminate and potassium chlorate = to insure=20 detonation, and are not as safe as ordinary blasting caps, which contain = no=20 chlorate. Tetryl boosters may be used in artillery shells. Tetryl was = first=20 obtained by Mertens in 1877, and its structure determined by Romburgh in = 1883.

Detonating tubes or cordeaux, also called Primacord, = are cords=20 filled with explosives used for detonating charges. They are easily set = off with=20 a blasting cap. Do not confuse Primacord with slow fuse! One type has a = lead=20 covering and is filled with TNT, detonating at 5200 ft/s. Another has a=20 waterproof textile covering and is filled with PETN,=20 pentaerythritoltetranitrate, detonating at 6200 ft/s. PETN is=20 C(CH₂ONO₂)₄, where the four groups are = bonded=20 tetrahedrally to a central C atom. It is very sensitive to impact, and = has a=20 high detonation rate, 8000-8300 m/s. PETN is a very powerful explosive, = and was=20 mixed with TNT for bursting charges in World War II. The 50:50 mixture = was=20 called Pentritol.

The aromatic explosives have NO₂ groups bound to a benzene = ring.=20 If the nitrogen is attracted away by a free radical nitrogen to form=20 N₂, then the oxygens may attack and oxidize the benzene ring, = releasing more free nitrogen radicals that pick apart other nitro = groups. The=20 mechanism does not seem to be known, but lone nitro groups do not appear = to be=20 explosive.

All the explosives we have so far discussed depend on the nitro or = nitrate=20 groups. As mentioned above, nitrates were often scarce and expensive, = especially=20 in time of war, so alternatives to nitrate explosives were sought. The = only real=20 alternative was to mixes containing chlorates, which are widely used in=20 pyrotechnics. French Cheddite was an example of such explosives, = which=20 used 60%-80% ammonium, sodium or potassium chlorate or perchlorate, with = some=20 fuel such as carbon, sulphur, aluminium or vegetable meals. Some = aromatic nitro=20 compounds improved flame propagation, and paraffin or castor oil was = added as a=20 desensitizer. In Germany, a little nitroglycerine or collodion cotton = was added=20 to increase the brisance. These explosives were used in mining and = quarrying,=20 not for military purposes, for which they released scarce = nitrates.

Pyrotechnics

In pyrotechnics we are principally concerned with solid state = oxidizer-fuel=20 mixtures in relatively small amounts. The heat produced by the reaction = is used=20 to drive other chemical reactions, to change the physical or chemical = state of=20 some other substances, or to create some desired physical effect. Small = amounts=20 of gas may be the object, but equally often the reactions may be = gasless, or=20 produce a slag. The mixtures are usually much more sensitive than those = used for=20 propellants or explosives, which is tolerable because of the smaller = amounts=20 involved. However, there may be great hazards in manufacturing and = storage when=20 large quantities are present, although each individual is small. A = firecracker=20 containing 1 g of explosive is a trivial hazard, but a million such = firecrackers=20 in a warehouse would make a bang that can be heard a ways off.

Propellants and explosives may be used in pyrotechnics, of course. = For=20 example, a rocket to lift a device to an altitude may use a black powder = motor=20 grain, and another black powder explosive charge to deploy the payload, = with a=20 pyrotechnic delay charge between them.

Some of the effects produced by pyrotechnic devices are: light, = sound, smoke,=20 delays, and motion. The light may be a brief, intense flash, a continued = illumination of an area, or a flare to be seen, a signal light, perhaps = colored,=20 a shower of sparks or stars, or an intermittent light. The sound can be = a bang,=20 a whistle or a crackle. The smoke can be for concealment or for = signalling, and=20 of various colors. The motion can be the inversion of a dimple or the = extension=20 of a bellows, or the expulsion of a projectile, or a rotation. A = pyrotechnic=20 reaction can close an electric circuit, or can open it. It can produce a = puff of=20 flame. It can produce gas for pressurizing a safety bag in a car, or for = some=20 other instantaneous use. The gas must usually be at low temperature. A=20 pyrotechnic device is for one-time use, of course. Its virtue is that it = holds=20 itself in quiet readiness until it is needed.

A squib was a little black powder twisted up in a piece of = tissue=20 paper. The paper was lighted on one end, and the squib was thrown. The = powder=20 burned, sending out a shower of sparks, and the gas emitted sent the = squib=20 darting here and there on the ground. This device was called a = serpenteau=20 in French, either from its darting motion, or from an old word for = tissue paper,=20 serpente. In Spanish, the firework was called a = buscapi=E9s, a=20 foot-chaser, for obvious reasons, a word now used for a firecracker. = Later, in=20 English, a squib became a written lampoon, to chase its target in print. = A tube=20 filled with powder for igniting artillery came to be called a squib, = like the=20 French p=E9tard or amorce. Today, a squib is an = electrically-ignited=20 device that ejects a puff of flame, used for igniting rocket propellant, = black=20 powder and similar materials sensitive to flame. A squib will generally = not=20 ignite shock-sensitive high explosives, such as dynamite or TNT, for = which a=20 detonator is necessary.

The oxidizers used in pyrotechnics are mainly the high-energy = nitrates,=20 chlorates, perchlorates, and the low-energy metal oxides. The fuels are = metals,=20 such as Zn, Al and Mg, carbon, phosphorus and sulphur, and various = organic=20 materials. The preferred potassium nitrate, chlorate, and perchlorate = are often=20 replaced by the cheaper sodium and ammonium salts where the hygroscopic = nature=20 of these salts is not detrimental. Antimony sulphide,=20 Sb₂S₃, calcium silicide, CaSi₂, and = other=20 easily-oxidized substances are often seen. These are very sensitive = substances,=20 and their mixing is not something that should be done in the home or = general=20 laboratory. Those who manufacture the devices are aware of the dangers, = and know=20 how to meet them.

Potassium chlorate, KClO₃, is a kind of wonder ingredient = in=20 pyrotechnics, a powerful oxidizer that give a low-temperature reaction, = so that=20 colors will be distinct. It was discovered by Berthollet in 1785, and is = the=20 most popular oxidizer in pyrotechnics. It is too unstable for use in = explosives.=20 All pyrotechnic colors tend to bleach to white in a hot reaction, as = emission=20 through the spectrum is enhanced relative to the specific colored = emission bands=20 and lines. It is only relatively recently that strongly-colored displays = have=20 been possible through the use of chlorate. However, chlorate is unstable = and a=20 very treacherous friend. With sufficient stimulus, it can explode all by = itself.=20 If ground dry with any organic material, it is sure to explode. It is = this=20 sensitivity, however, that makes it valuable, as well as its peculiarly = loose=20 crystal structure.

Potassium chlorate is the substance used for laboratory preparation = of=20 oxygen. The chlorate decomposes slowly on heating, giving off oxygen = (and a=20 little chlorine, making the oxygen unsuitable for breathing). The = reaction is=20 accelerated by a little MnO₂ as a catalyst. This is a safe = experiment=20 (chlorate will not explode on its own in small amounts), but the = chlorate must=20 be respected and never allowed to approach any oxidizable material, = especially=20 things like sulphur or phosphorus. Filter paper soaked in 40% chloric = acid=20 bursts into flame when it dries. The same might happen with potassium = chlorate=20 solution. Potassium chlorate and sugar is called partisan's = mixture from=20 it use by revolutionaries and such. It can be exploded by concentrated = sulphuric=20 acid that eats through a metal container in a few hours or days. Any=20 revolutionaries using chlorates usually blow themselves up sooner or = later.

A typical reaction is 2KClO₃ + 3S → 2KCl + = 3SO₂, often=20 used in smoke mixtures. Using tabulated heats of formation, this = reaction yields=20 223 kcal, or 653 kcal/kg. Potassium chlorate alone gives only 10 = kcal/mol or 82=20 kcal/kg, so the addition of a fuel makes considerable difference. One = might=20 think that the sulphur reaction began with 2KClO₃ → = 2KCl +=20 3O₂, and continued with S + O₂ → = SO₂, but this=20 is not the case. Before the temperature has increased enough for the = chlorate to=20 decompose, the sulphur has melted and S₃ molecules insinuate=20 themselves into the loose crystals of the chlorate to begin the reaction = at just=20 below 160=B0C. That is, the sulphur picks apart the chlorate, and the = reaction=20 begins at a low temperature.

Perchlorates are much less sensitive than chlorates, and should = replace them=20 wherever possible. Nitrates are still less sensitive, and can be used = with=20 confidence. Strontium nitrate gives the flame a red color, while barium = nitrate=20 gives a green color. Strontium or barium anywhere in a mix gives the = same color=20 effects as the nitrate. Yellow is made with sodium oxalate,=20 Na₂C₂O₄. Copper gives a blue flame, the = most=20 difficult color to produce. Paris Green, a copper arsenate, makes a fine = blue,=20 but the arsenic is toxic. A golden color can be made with Fe sparks. = These are=20 all emission colors, so they can be mixed additively. The reaction = temperature=20 must be kept low, or the colors will be bleached by thermal = radiation.

Other oxidizers sometimes used are potassium permanganate, = KMnO₄,=20 barium peroxide, BaO₂, barium chromate, BaCrO₄, = potassium=20 dichromate KCr₂O₇, as well as many metallic = oxides, such=20 as those of iron, lead and manganese. The insoluble chromates, as = desired for=20 pyrotechnic mixes, are easily available in pure form as yellow pigments. = Oxidizers are generally of intermediate cost, while fuels can be cheap = (C, S,=20 asphalt, sugars, rosin) or very costly (B, Ti, Zr). Mg and Al powders = are of=20 moderate cost. Boron is used as the dark-brown powder with particles of = about=20 1μm in size that is 84%-90% B, the rest O. Silicon is ground into a = dark-grey=20 powder of size 10μm and larger. Metallic powders can be very = hazardous=20 substances, since they react readily, and may even be pyrophoric. In = general,=20 pyrotechnic materials are insoluble, non-hygroscopic, not hydrated, and = have=20 high melting points. They are held by a binder, or as a compressed = powder.

Pyrotechnics are very useful for the production of smoke. = Smoke is a=20 colloidal suspension of solid particles in air, an aerosol, that = can be=20 created in various ways. A fog is a suspension of liquid droplets = in air,=20 and we'll make no distinction between smokes and fogs here, calling them = both=20 smoke. Classified by use, smokes can be screening or = signalling. A=20 screening smoke is intended to be dense and obscuring. A signalling = smoke is=20 intended to be easily seen, and perhaps colored. Evaporating a liquid = that then=20 condenses in small droplets is one way to make a smoke (water, for = example), or=20 partial burning of carbonaceous matter that leaves carbon particles is = another.=20 They are combined in the smoke produced by a fire in damp wood. Crude = oil was=20 atomized into the funnels of destroyers, partially burning to make a = dense black=20 screening smoke. Burning white phosphorus makes a dense cloud of white=20 phosphorus pentoxide, which combines with the moisture in air to make a = fog of=20 phosphoric acid. Silicon tetrachloride and ammonia can be atomized into = aircraft=20 exhaust to form, with the moisture, a smoke of silicic acid and ammonium = chloride that is used in skywriting. Smokes are also made from = chlorsulfonic=20 acid-sulphur trioxide solutions (agent FS) and titanium tetrachloride = (agent=20 FM). With the moisture of the air, FM makes titanic acid and hydrogen = chloride,=20 and is intensified by being used with ammonia, which forms = NH₄Cl.=20 These are chemical, not pyrotechnic, smokes.

Military smokes have been dominated by a zinc chloride aerosol = produced by a=20 pyrotechnic reaction between zinc dust and hexachloroethane, 3Zn +=20 C₂Cl₆, that was discovered in 1920 by Capt. Henri = Berger=20 in France. These are called, in general, HC smokes. The reaction is 3Zn = +=20 C₂Cl₆ → 3ZnCl₂ + 2C, which also = produces a good=20 amount of heat. The carbon makes the smoke dark gray, and the rapid = reaction=20 made so much heat that convection carried the smoke up into the air. = Adding some=20 potassium perchlorate to oxidize the carbon, and some chalk and ammonium = chloride to slow down the reaction and consume some of its heat made a = useful=20 smoke by 1941. The chalk neutralized any acid that might make the mix = extra=20 sensitive (it is found in many pyrotechnic mixes for this purpose).=20 Modifications during World War II substituted ZnO as an oxidizer instead = of=20 chlorate, and CCl₄ instead of the hexachloroethane. = TiO₂=20 was found not to be as satisfactory as the zinc. Manganese was also = added in=20 place of the Al tried in earlier experiments. The heat of reaction was = brought=20 down from 645 kcal/kg to 365 kcal/kg, reducing the lifting of the smoke = as well=20 as the revealing light by night. ZnCl₂ smokes cause headaches = when=20 personnel are exposed to them for longer than about 20 minutes.

Colored smokes are produced with the potassium chlorate-sulphur = reaction=20 discussed above, which vaporizes organic dyes that are about 40% of the = mix. The=20 low temperature of the reaction prevents decomposition of the dyes and = bleaching=20 of the light. Sodium bicarbonate is added as an alkalizer, flame = suppressor and=20 coolant. The mix is about 30% chlorate, 10% sulphur, 20% bicarbonate and = 40%=20 dye. Green, red, blue, yellow, violet and orange are the usual colors. = In place=20 of the sulphur, sucrose or lactose are used as fuels in standard smoke = mixes to=20 avoid the effects of the SO₂ on personnel. Smokes colored by = dyes may=20 be largely governed by subtractive color mixing, so combinations of dyes = must be=20 carefully chosen or the result will only be a muddy darkness if the = smokes are=20 well mixed. Smokes that are not mixed may well give an additive color at = a=20 distance. Red smoke was colored with Para Red (paranitraniline red), or=20 1-methylaminoanthraquinone. Blue and yellow dyes will give green only if = neither=20 absorbs strongly in the green. The usual blue dye is Indigo, the yellow = dye=20 Auramine, and they do give green when mixed. Indigo will sublime at = 300=B0C, but=20 decomposes at 392=B0C, so the smoke must be kept cool (this is true of = all the=20 dyes). Red and blue dyes give violet, while yellow and red produce an = orange=20 color.

Among the most curious pyrotechnic devices are those which will act = as=20 electrical switches, opening or closing a circuit when they are ignited. = For=20 example, the reaction 2PbO + Si → SiO₂ + 2Pb begins = with=20 nonconducting reactants, but ends with conducting Pb that will join = wires=20 embedded in the mixture. The reaction Fe + 3BaO₂ →=20 Fe₂O₃ + 3BaO begins with conducting iron filings, = and ends=20 with an insulating slag. Boron with metallic oxides, in a thermite-like=20 reaction, behaves the same way, going from nonconducting to conducting. = Lead or=20 tin, with selenium or tellurium and a barium peroxide oxidizer, also = becomes=20 conducting after the reaction.

None of the reactions in the preceding paragraph emits a lot of gas, = so the=20 slag is not blown away and the device is not disrupted. Such = gasless=20 reactions are very useful in delay mixtures. In fact, the lead=20 oxide-silicon reaction can be made to burn at 1.5 - 2 cm/s. The similar = Si +=20 PbO₂ → SiO₂ + Pb reaction burns at 5-6 cm/s. = Silicon=20 reacting with red lead, PbO=B7PbO₂, burns at an intermediate = rate.=20 KMnSO₄-Sb mixes burn at a very low rate, and can give delays = on the=20 order of seconds. It is very important for a delay mix to burn at a = predictable=20 rate. Gasless reactions are insensitive to pressure variations, and this = is one=20 reason for preferring them. Early delays with black powder were often no = more=20 than a train of powder in the open. In 1831, Bickford discovered the = slowing=20 effect of wrapping the powder with fabric or paper, which conducted away = the=20 heat that would otherwise accelerate the reaction. The powder may be = slowed by=20 additions of chalk or sodium bicarbonate. Bickford fuses burn at about 1 = ft/min=20 (0.5 cm/s), and a puff of flame shoots out of the end of the fuse when = it is=20 consumed, ideal for initiating black powder. Incidentally, the spelling = "fuse"=20 is commonly used for a chemical initiating device, and "fuze" for a = hardware=20 device that may use any principle of operation, including electronics or = clockwork. Most present-day fuzes are electronic.

Some pyrotechnic devices provide moderate heat in circumstances where = building a fire would be inconvenient. The self-heating food can = developed=20 during World War II used a 50:50 iron oxide-calcium silicide mix,=20 Fe₃O₄ and CaSi₂. The first fire for = this device=20 replaced the iron oxide with Pb₃O₄ and added a = little=20 china clay to make a more sensitive mix that was easily set off by the = lighting=20 match (included with the can). Calcium silicide is a very active = reducing agent,=20 so you will not see such cans in the supermarket. The M1 fire starter = was=20 essentially a match head of KClO₃ and = Sb₂S₃=20 glued together with dextrin, which, when scratched, ignited a small = reservoir of=20 napalm-thickened kerosene. A "Heat Block" used granulated iron as a = fuel, and=20 potassium perchlorate and barium chromate as oxidixers, with some other=20 ingredients, to make 210-230 kcal/kg.

Napalm is a soap of aluminium NAphthenate and aluminium PALMitate = that can be=20 used to thicken hydrocarbon liquids. It was patented by Prof. L. Fieser = in 1952.=20 It forms a gel with the hydrocarbon that suffers from syneresis = (evolution of=20 fluid as the gel ages), and is not an explosive or incendiary in itself. = The gel=20 is thixotropic, which means that it can be thrown as a liquid, and then = gels=20 again. Napalm is used for all sorts of nasty purposes, but was mainly = intended=20 for incendiary bombs.

The Goldschmidt process, patented in 1895, has been widely used since = then.=20 It is better known as thermite or Thermit, a trade name. A mix mainly of = aluminium powder and iron oxide is packed into a crucible and ignited by = a=20 magnesium ribbon and a starter mass, or some other means. It is not easy = to=20 light. The vigorous reaction results in a slag of aluminium oxide on = very hot=20 molten iron, up to 3000=B0C. The iron flows into a mold that is part of = the=20 crucible, usually to perform a weld. The iron oxide is the oxidizer, and = the=20 aluminum metal the fuel, in the gasless reaction. The same idea can be = used for=20 other metals, such as chromium, manganese and ferrotitanium, and also = for a=20 nefarious incendiary device.

Railways have used pyrotechnics as detonators or = torpedoes, and=20 as fusees. Detonators are placed on the rail, held by soft metal = wings=20 (Pb or Al) that are shaped around the rail head, and explode when run = over as an=20 audible signal of danger. They were invented by E. A. Cowper in England = in 1842,=20 when they included match chemicals (a crushable bottle of sulphuric = acid) and=20 gunpowder. The potassium chlorate-sulphur reaction is now more generally = used,=20 mixed with sand and chalk. Potassium perchlorate-antimony = sulphide-sulphur mixes=20 are also found. They are rather inert, and difficult to set off except = in the=20 intended way. Fusees are cylindrical, with a spike on one end to hold = them erect=20 after they are thrown or dropped (they come down spike lowermost). They = are=20 especially effective in falling snow, fog, and other difficult = situations. The=20 cap contains a scratcher with red phosphorus, and the top of the fusee = when the=20 cap is removed contains chlorates and perchlorates. As in a safety = match, the=20 reaction is begun by scratching the top of the fusee. The flare mix is = mainly=20 Sr(NO₃)₂ and perchlorate oxidizer, with sulphur, = sawdust=20 and miscellaneous ingredient as fuel. The strontium gives the red color. = Stearic=20 acid is added in small amounts to adjust the burning rate. A standard = fusee=20 burns for 10 minutes. In the past, green fusees were also used (when = green was=20 the signal for caution), as well as 5-minute fusees. The green was = obtained by=20 substituting barium nitrate for the strontium nitrate.

Some mixes, when pressed into a metal tube about an inch in diameter = and=20 ingnited, produce a very loud whistle, descending quickly from a high = pitch to a=20 lower as the mix burns. The rate of burning of the mix is strongly = affected by=20 pressure. The open part of the tube acts as a resonator, with a velocity = node at=20 the surface of the mix, and a pressure node at the open end, so the = length is a=20 quarter wavelength at the sound speed in the gas produced. The pressure = is=20 maximum at the velocity node, and varies at the resonant frequency of = the tube,=20 radiating strongly at the open end. Typical frequencies are from 5 kHz = to 1 kHz.=20 25:75 gallic acid (its structure is mentioned above in connection with = aromatic=20 explosives) and KClO₃ was an early whistling mix. 70:30 = Potassium=20 perchlorate and potassium benzoate, 60:40 potassium picrate and = potassium=20 nitrate, and 72.5:27.5 potassium perchlorate and sodium salicylate = (aspirin) are=20 others. The last is the standard U.S. whistling mix. Whistling mixes = have the=20 unfortunate tendency to explode without warning.

The German Pfeifpatrone of World War II was a handgun-launched = parachute flare that gave an intense flash of light, a cloud of smoke, = and a=20 shrill whistle in rapid succession. The U.S. had a Day-Nite = signal, a=20 metal can that belched red fire from one end, and orange smoke from the = other.=20 These, apparently, are still manufactured for use as an emergency = signal. As we=20 have seen in the past few paragraphs, pyrotechnics is an excellent means = for=20 attracting attention. The Very pistol was patented in 1878, and adopted = by the=20 British services in 1888. It had a 1" bore, and fired a cartridge like a = shotgun=20 shell, which contained a pyrotechnic star (of various colours) instead = of shot.=20

Pyrotechnics uses one-shot devices that cannot be tested before use.=20 Nevertheless, their reliability must be satisfactory, especially for = military=20 and safety use. This can be assured through the use of statistics = and=20 rigorous destructive testing. Methods are presented in any text on = practical=20 statistics. Redundancy also decreases the probability of failure. = If a=20 device fails only once in 1000 times, then two redundant devices will = fail only=20 once in 1,000,000 times (on the average).

Fireworks

Fires and illuminations have always been an entertainment, a = celebration, or=20 a display of religious superstition. The summer son et lumi=E8re = displays=20 are examples. Pyrotechnics is a perfect medium for these festivities, = combining=20 light, sound and motion in a colorful and impressive way. The displays=20 celebrating the millennium a year early on New Year's 2000 were = memorable. A=20 good pyrotechnic display is majestic and stirring when the observer is = close to=20 it, so that it happens not only in all directions, but also overhead, = and can be=20 smelled and occasionally felt. Although this can be managed with safety, = most=20 displays are now seen from a distance, like a picture in a museum, = because of=20 the obsession with zero-risk. Residential neighborhoods are not a = suitable venue=20 for fireworks, it is clear. Loutish excesses have led to severe legal=20 prohibitions, which are necessary but unfortunate. Until the 19th = century, there=20 was little color in pyrotechnic displays, but the brilliance was still=20 impressive. The Chinese were the first to brighten their celebrations by = pyrotechnic fires. Incidentally, the original Chinese firecracker was = not an=20 explosive, but a green bamboo joint that made a loud crack when thrown = on a=20 fire. Many fireworks still come from China and Japan.

Until the 19th century, European fireworks displays were based on an=20 elaborate "machine" or "temple," an architectural display which could = involve=20 transparencies or cutouts illuminated from the back, as well as = fountains of=20 fire and other effects, which could be very bright and impressive, but = not=20 coloured. A common type of machine was in the shape of a pointed = obelisk, which=20 erupted in fire. These displays were generally close to the ground. = After the=20 introduction of chlorate oxidizers and colour effects, displays were = mounted on=20 "lancework" that merely supported the pyrotechnics and presented images = outlined=20 in fire. The use of rockets and mortars with air bursts became the = principal=20 part of a display, as it is at the present time, so the display was = mainly in=20 the air.

The common sparkler is a steel wire coated with a KClO₄-Al = mixture=20 that makes white sparks. Added Ba(NO₃)₂ makes the = sparks=20 green, and SrCO₃ makes red sparks. Fe filings and the barium = nitrate=20 make golden sparks. The binder used is dextrine, a gum made from starch. = The=20 sparks can be received on the skin with equanimity. Wowsers have = attempted to=20 ban sparklers as dangerous, though a more innocuous firework that gives = so much=20 pleasure can hardly be imagined. Any child waving a wire can probably = cause=20 injury sometime, but sparklers are probably less dangerous than rubber = ducks.=20 They can certainly be allowed in residential areas, and are a safe = outlet for=20 festive feelings.

The M-80 firecracker is a flash-and-sound device for army training. = It is=20 about 0.5 inch in diameter and 1.25 inch long, with a fuse coming out of = the=20 middle. The body is spiral-wound chipboard covered with Kraft paper. The = charge=20 is 2.5 g of 4:1:1 KClO₄-Al-Sb₂S₃ mix, = or a 1:1=20 Sb₂S₃-S mix. The Al is the dark colloidal powdered = aluminium, which is mixed with the antimony sulphide first. Then the = perchlorate=20 is put on top of it, and the tube sealed. Then the tube is "rumbled" in = a barrel=20 of sawdust to mix things well. The mixture is quite hazardous if mixed = by hand.=20 A fuse is then put in a hole punched in the side. Firecrackers that can = be sold=20 legally today are very feeble things, with a maximum load of 50mg. You = can hold=20 them in your hand when they explode. More people probably injure = themselves=20 trying to make proper firecrackers at home than would be injured by more = realistic 1-gram firecrackers. Firecrackers generally contain a = flash-and-sound=20 mix, not black powder. The crusade against fireworks is strange in a = country=20 where 40,000 die annually in car crashes and over a million are = seriously=20 injured. It's probably the usual envy of people having too much fun. I,=20 personally, would not use fireworks, am annoyed by hearing them go off, = and=20 consider them vulgar, but I am not a great fan of compulsion or = interfering with=20 the course of evolution. It is impossible to buy a firecracker with more = than=20 50mg of charge, but ammunition is easy to obtain everywhere. = Insanity.

To illustrate the dangers of fireworks, the explosion in a Paris toy = store on=20 14 May 1878 can be adduced. It happened in a storage room where six to = eight=20 million amorces, paper caps each containing only 10 mg of = explosive, were=20 stored. Individually, these were innocuous devices. Somehow, a few = caught fire=20 (probably someone was smoking) and although they were soon extinguished, = the=20 fire caught on a storage box, and all the millions of caps, containing = about 64=20 kg of explosive, went up in an explosion that killed 14 people. Such = high-order=20 accidents are the reason why manufacturing and handling fireworks is a = job for=20 experts only.

The Roman candle illustrates a lot about pyrotechnics. When lighted, = it=20 expels variously-colored stars at intervals, usually six stars, as it is = held in=20 the hand. There are successive modules consisting of delay mix, star and = expelling charge. The gas-producing delay mix makes a colored flame = shoot from=20 the end of the tube as it burns. The flame burns around the outside of a = star to=20 reach the black powder expelling charge behind it, which ignites and = propels the=20 star out of the tube, to burn in its trajectory. Then the next module of = mix=20 burns, and so on. The tube is convolute-wound (like a cylinder, = overlapping,=20 parallel to the grain) so it is strong and will not burst. The end of = the=20 chipboard is feathered so the inside is smooth and round. Roman candles = are not=20 safe in the hands of idiots. Roman candles are named after the use of = candles in=20 Mardi Gras festivities in Rome, where people tried to extinguish each = other's=20 candles and relighted them, and were first used in Britain.

A=20 stick rocket, shown in the diagram at the left, has a stout paper body,=20 convolute wound. A stick about three times as long as the rocket body is = firmly=20 attached to stabilize the flight. The propellant grain, ignited by a = squib or=20 fuse inserted in the nozzle, produces gas that exits from the ceramic = nozzle at=20 high velocity, propelling the rocket. The design of the grain is = critical, so=20 that as much of it burns evenly without burning through to the body. The = grain=20 ignites the delay mixture, which allows the rocket to coast to the top = of its=20 trajectory before it ignites the bursting charge, which expels the = payload in=20 the nose cone, which may be a sound-and-flash device or a parachute = star.

An aerial shell is fired from a short mortar using coarse black = powder. The=20 black powder is ignited by a fuse, or, better, by an electrical squib. = Shells=20 are usually 2" to 8" in diameter, but can be much larger, up to 24". A = delay=20 fuse is lighted when the black powder burns, which detonates a bursting = charge=20 of black powder at the height of the trajectory. This disperses the = stars and=20 salutes (another name for firecrackers) radially. Concentric shells can = give=20 multiple bursts timed by delay fuses. A good spherical Japanese or = Chinese shell=20 gives a beautiful crysanthemum effect to the burst. American shells are=20 cylindrical, because cylinders are easier to make. Shells are another = example of=20 how black powder is still used for many purposes in pyrotechnics. It is = much=20 safer than the other pyrotechnic mixes.

The British "cracker" or American "party popper" contains a string = that when=20 pulled at both ends makes a small report and liberates a party favor = from the=20 burst paper container. To make a cracker, the string is laid out, a loop = is=20 formed in it, and a drop of 68% KClO₃, 12% red P, 9% S, and = 11% chalk=20 is dropped on it (there are other mixes). The mix is unstable, but only = 16 mg is=20 used, so it can do no harm. When it dries, the device is wrapped as = required.=20 When the string is pulled, the crumbling of the drop detonates it, and = it makes=20 a small report. "Snap n'Pops" consist of a cigarette paper wrapped = around a=20 little sand coated with 0.8 mg of silver fulminate. When this is thrown = down, it=20 makes a little bang. This is a very safe firework, also suitable for = residential=20 areas. "Caps" used to be available in rolls for use in "cap pistols." = They were=20 on red paper, with little dots where the mix was placed between the = layers. When=20 struck, it would make a bang and a little smoke. The mix was Armstrong's = Mixture, 67% KClO₃, 27% red phosphorus, 3% sulphur, 3%=20 CaCO₃ dissolved in water with some gum arabic or similar = binder. A=20 small dab, containing a few mg of mix, was put on a paper backing and = allowed to=20 dry. Such devices are individually wholly innocuous, but in millions may = pose a=20 hazard, as in the explosion in Paris mentioned above. I would = confidently bet=20 that they have now disappeared from toy stores. Years ago, boys made a = device=20 from two bolts and a nut that held match heads, scraped off = strike-anywhere=20 matches, between the two bolts. When thrown rather firmly, a bang was=20 produced.

Novelty fireworks include the cigarette load. This is a small wooden = peg=20 coated with a little lead azide (Pb(N₃)₂). When = the fire=20 reaches it, it makes a little bang and blows tobacco about. Candles with = wicks=20 soaked in perchlorate ignite again when blown out. Snakes are = small=20 pellets that expand greatly when lighted. They were originally made with = mercuric compounds, but these have been banned, probably in an excess of = regulative mania, since they contained very little mercury and were very = seldom=20 used. Substitutes are available, however, so those with a passion for = them are=20 not frustrated. The American Hotfoot uses a match head inserted between = sole and=20 upper of a shoe. The match is surreptitiously lighted, and when the fire = reaches=20 the match head, the result is a source of merriment.

Safety

Safety with all of the materials mentioned in this article results = from=20 appreciation of the lessons learned from two centuries of practical = experience.=20 The greatest hazards, by far, are in the manufacturing processes where = materials=20 are combined and the finished devices produced. The ingredients are = individually=20 rather harmless. In some cases, they present a poisoning hazard, and in = others a=20 fire hazard, but only under exceptional conditions an explosion hazard. = The=20 finished devices are also safe, and can often be mistreated in shocking = ways=20 without danger. It is in the mixing that all the danger arises, and the=20 manufacturers have been made well-acquainted with the hazards by hard=20 experience. Special measures are taken to ensure as much safety as = possible in=20 the manufacturing procedures, and to limit the damage in case of = surprise.

Storage and transport of explosives and pyrotechnics is carried out = safely by=20 observing the lessons of past experience, codified in manuals of = procedure.=20 Pyrotechnic devices stored in large quantities may be as dangerous as=20 explosives, and in some cases even more dangerous because of the more = sensitive=20 compounds that may be used, in small quantities in the individual = device, but in=20 dangerous amounts in mass. The lesson of the caps in Paris is = instructive.

The user is exposed to the least hazard of all. If good procedures = are used,=20 then there is little danger to be anticipated. Dynamite will not explode = until=20 the detonator is placed. Detonators will not explode if shorted and not = exposed=20 to extreme conditions (but they are more dangerous to workers than the=20 dynamite).

A person may want to experiment with these mixtures to find out more = about=20 them, to obtain entertainment and to gain practical experience. With=20 electronics, or chemistry in general, this is to be commended. With = pyrotechnic=20 mixtures, it is folly. This is not the usual case of wowsers trying to = spoil=20 people's fun (as is so very frequent today), but something completely = different.=20 You may hear warnings about "only experts should do this" in connection = with=20 dangerous things, but here any expert would unequivocally reject casual=20 experimentation. The problem is that the situations that you would = establish, in=20 things like purity of materials, grain sizes, order of procedure steps, = and so=20 forth, would lead to unpredictable outcomes. This is only = exacerbated by=20 amateurism, and even professionalism would be in danger. For example, if = you try=20 to make gunpowder with "flowers of sulphur" you will be in danger of = premature=20 explosion, since flowers of sulphur contain a little acid, and is much = more=20 easily oxidized than the "flour of sulphur" that you really want. = Pyrotechnics=20 contains many such lovely traps as this. Of course, you will not succeed = in=20 making black powder, but very possibly will succeed in blowing off parts = of your=20 body. Some student's last experiments have been finding out what happens = when=20 potassium chlorate is substituted for potassium nitrate. Explosives = research=20 laboratories and manufacturers have procedures for detecting and = eliminating=20 dangers of this type that the individual cannot emulate. Experiments = that you=20 may think about doing they would do by remote control behind sturdy = blast=20 shields.

One way to learn a lot more about pyrotechnics might be to volunteer = to help=20 people who present public fireworks displays. You would get valuable = safety=20 training, and could work with these devices yourself. I do not know how=20 practical this suggestion may be, but would like to know if it is = possible. The=20 hobby of model rocketry may still be available, but I have not heard = much about=20 it lately. Anything involving black powder will probably be relatively = safe, and=20 will teach a lot about explosives.

References

J. H. McLain, Pyrotechnics (Philadelphia: Franklin Institute = Press,=20 1980). This is an absolutely excellent and extremely informative book = that=20 suffers only from an inadequate index, though references to the = literature are=20 extensive.

H. Ellern, Modern Pyrotechnics (New York: Chemical Publishing = Co.,=20 1961). Another excellent book, with much interesting practical = information.=20 Particularly good on spontaneous combustion.

H. Brunswig (C. E. Munroe and A. L. Kibler, transl.), = Explosives (New=20 York: John Wiley & Sons, 1912).

A. St. H. Brock, A History of Fireworks (London: Harrap, = 1947).=20 History of pleasure fireworks by a member of the prominent British = fireworks=20 family.

J. Bebie, Manual of Explosives, Military Pyrotechnics and Chemical = Warfare=20 Agents (Boulder, CO: Paladin Press, 1942). An excellent dictionary = of all=20 the terms, trade names, code names and other lore pertaining to = explosives.

J. Akhavan, The Chemistry of Explosives, (London: The Royal = Society of=20 Chemistry, 1998).

Return to Physics=20 Index

Composed by J. B. Calvert
Created 21 December = 2002
Last=20 revised 18 March 2004

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