A Primer on Explosives

It has been repeated so often that it is now accepted as fact that the Chinese invented black powder around the 9^th century AD.   This may be the truth or it may be a myth perpetuated after Marco Polo wrote memoirs of his 13^th century experience in the East.   There is actually some evidence to suggest that gunpowder may have originated in Greece or India.  It is not disputed that the Chinese were apparently the first to show significant progress in the refinement of black powder and the development of fireworks, the first to propel rockets and the first to attempt to intimidate enemies with explosives.

Introduced in 1961 the RPG-7 imaged above shares several admirable qualities with other Russian weapon designs; namely it is cheap, uncomplicated, efficient and rugged.   This rocket grenade launcher is encountered around the world and has been involved in most every armed conflict of any consequence for the last 40 or 50 years.  In Somalia or Iraq such a weapon might be acquired for about $200 and additional rockets might cost about $20 to $30 apiece.   In black markets of other parts of the world an old and worn RPG-7 might command about $1,000.  A standard HEAT (High Explosive Anti Tank) rocket as shown in this image would be obsolete and ineffective against modern tank armor but still devastating against softer targets.  Fragmentation grenades and small thermobaric warheads are also available for this launcher.   In the image the solid booster charge, solid rocket propellant and the unlabeled conically shaped HEAT warhead are all “explosive” or are potentially detonate-able although the first two chemical compounds merely deflagrate in a controlled manner.

Deflagration vs. Detonation

For a chemical to be considered explosive it must rapidly form gases and heat and exhibit the ability to be initiated by shock or by heat.  Chemical explosions are simply accelerated rates of chemical decomposition.   A campfire is an example of chemical decomposition as is gunpowder burning in a shotgun barrel.  Gunpowder, fireworks and air-fuel mixtures in internal combustion engines are “low explosives” and examples of “deflagration”.    Deflagration (from Latin – “to burn down”) is a rapid but subsonic (342 m/sec or less, 1,126 ft /sec or less) decomposition where a combustible and an oxidant are reacting at the molecular surface area, called a “flame front”.   The fake looking explosions in most Hollywood movies are examples of deflagration; where pyrotechnic stunt coordinators make big yellow-orange fireballs by setting off containers of gasoline.   In contrast, “high explosives” exhibit detonation.   Detonation is a supersonic reaction where a “shock front” rather than a flame front, travels through the explosive.   Detonations propagate through shock compression and can reach speeds of up to 9,000 meters/sec (29,527 ft. /sec).

* For comparison the fastest bullets or shells from rifles or cannons have a top speed of about 4,000 feet/sec (1,219 m/s).  Although the unconfined propellants do not deflagrate that fast, the buildup of pressure confined, accelerates the projectiles to that speed. 

* If detonated at the same instant that a fast bullet was fired from a rifle, a six mile length of primer cord filled with an explosive like RDX (which has a high detonation velocity),  would have vanished in its entirety before the bullet had gone one mile.  

* Without the confinement of a barrel and closed breach, a loose rifle or pistol cartridge when tossed into a fire – would pop like a child’s firecracker and be about as dangerous.  The brass case might split, some coals will be thrown around and the bullet may or may not bounce a few feet.

Shockwave

The caption or information included in the following video explains that it displays the destruction or disposal of a solid fuel rocket motor from a retired ballistic missile.  The caption claims that 38,000 lbs of HD 1.1 propellant are detonated.   HD 1.1 (Hazard Division 1.1) implies a nitramine-based explosive like RMX or HMX (both are commonly used for solid rocket fuels as well as components in the lion’s share of modern military grade plastic explosives used by the US and GB).

Huge Explosion and Shockwave

The problem with this video is that at this level of camera zoom, the supersonic shockwave or blast wave passes too quickly to be seen.  What can be seen and what is erroneously referred to as a shockwave is actually the transonic sound wave.  The camera was filming from about a mile away, which can be verified by the fact that the shadow and sound (which travels about 1,125 fps) took about 5 seconds to reach the camera.

This following ho-hum video, appears to be a clip from a TV show.   Some impression of shockwave is captured by high speed cameras.

Huge Shockwave Captured at High-Speed | Military.com 

More shockwave information: http://physics.info/shock/

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Brisance, Stability & Sensitivity

    Brisance is a French word meaning “to break”.   The brisance or the shattering effect of an explosive is proportional to its speed of decomposition but is not a total measure of the compound’s work capacity.  Relative brisance between explosives might be determined by field testing or sand crush test.   Knowledge of relative brisance is useful to bomb and fragmenting shell design engineers or to Army engineers preparing to destroy bridges, bunkers or other structures.

Stability refers to the ability of an explosive to be stored without deterioration.  There is a large variation in the stability of various explosive chemical compounds.   Fulminate of mercury as used in the priming compounds of rifle or cannon cartridges might be considered to have a good stability.   A high percentage of mercury fulminate primed rifle cartridges from the WWI era might be expected to perform (if they were stored well) even though they are now approaching 100 years in age.   On the other hand, long stored nitroglycerin has a reputation for separating from the other constituents in dynamite.  Such old dynamite becomes “unstable” because it deteriorates; the nitroglycerin collects as a liquid and becomes dangerously “sensitive” again.   TNP (Trinitriphenol or picric acid) which was once used as a primary explosive in naval and land artillery shells prior to WWI, eventually tended to corrode the metal shell casing it was housed in and created new and dangerously sensitive chemicals.

Heat, friction and shock test are preformed on explosive compounds to determine their sensitivity rating.  Actually the scale used to gauge the relative sensitivity of different explosive compounds is called the “Figure of Insensitivity”.   For many years this scale was based upon the sensitivity of TNT which is very insensitive – with a rating of 100.  After the creation of faster explosives in WWII, TNT was supplanted as the most common comparison gauge by the more sensitive RDX (with a rating of 80).

* Invented way back in 1863, TNT (or trinitrotoluene) was so insensitive that it was a challenge to make it detonate.  It was used as a yellow paint pigment for 40 years before it finally saw use in munitions.  It was notorously toxic for factory workers to handle and still being manufactured today. 

It is useful to understand that the blasting agents which provide the horsepower of an explosion are seldom if ever used alone and that a chain (or train) of reactions is usually necessary to produce the desired blast.   Common explosives like dynamite, TNT, and C4 are so insensitive and safe to handle that they require a substantial physical shock to set them off.   The first stage in an explosive train is a small and sensitive “primary explosive”.   Once stimulated the less sensitive “secondary explosive” might be the end of a train or the stimulant to set off an even more insensitive third stage.  This final or tertiary stage provides the real horsepower in such a chain, and might consist of a very insensitive blasting agent like a compound of horse manure and diesel fuel or ANFO (Ammonium Nitrate and Fuel Oil).   In essence a lowly firecracker can be the initial stage of an explosive chain leading up to the eventual detonation of the mightiest thermonuclear device.

Igniters, Blasting Caps & Detonators

Commercial and military detonators might comprise a little explosive train in their own right, containing perhaps three separate compounds.  The most common primary explosives used in today’s blasting caps & detonators are DDNP (Diazodinitrophenol), mercury fulminate, lead azide, lead styphnate and tetryl (until recently).   Potassium chlorate and potassium perchlorate were once used in explosive primers and detonators.  Today’s blasting caps might house small – less sensitive secondary charges of PETN or RDX perhaps.

* Compounds like nitroglycerin and silver fulminate are simply too sensitive to safely handle.   Other compounds can be used as primary explosives but are seldom used in modern detonators for various reasons.  These would include:  acetone peroxide, ammonium permanganate, azo-clathrates, copper acetylide, HMTD, lead picrate, nitrogen trichloride, nitrogen triiodide, silver azide, silver acetylide, sodium azide, tetacene, tetrazoles and triperoxide.      

* Apparently the first blasting cap was realized in 1745 and consisted of black powder which was set off by a spark from a Leyden jar.   Later in 1750 Benjamin Franklin created a better blasting cap by stuffing two parallel wires and black powder into a rolled paper tube.  The first glowing “hot wire” detonator appeared in 1822.  The first pyrotechnic fuse blasting cap was created in 1863 by Alfred Nobel.  It used a burning fuse and mercury fulminate to detonate his relatively insensitive and patented, nitroglycerine based mining explosives.   Five years later someone else invented the first electrical blasting cap capable of detonating dynamite (this combined mercury fulminate and a spark gap igniter).   

* Today pyrotechnic fuse and fuse caps (more modern metal tube equivalents of Nobel’s original wood tube) are becoming rare because older explosives like dynamite and gelignite (blasting gelatin) have largely been displaced by more modern and usually cheaper alternatives that might also leave less noxious fumes behind after a blast.  Even electrical blasting caps are becoming less common for certain types of blasting; the wires are being replaced with little plastic air tubes.  Avoiding radio waves and static electricity, these tubes transfer shock to the primary explosive using air pressure. 

Military detonators are usually a bit stronger than their commercial counterparts.   Today #6 and #8 caps are the only commercial blasting caps mentioned.  A # 8 cap is traditionally about twice as powerful as # 6 and would be used for initiating less sensitive explosives.  There is no such thing as a commercial # 4 blasting cap, but had it existed it would have reflected the power of a cap containing .4 grams of mercury fulminate.  Cap  numbers less than 6 are apparently not made and cap numbers stronger than 8 don’t exist or have no convention standards.  There may have been a time when mercury fulminate was the only substance used in blasting caps but before long potassium chlorate began being mixed with it.  Potassium chlorate was cheaper than and enhanced the power of the mercury fulminate.  A traditional # 6 blasting cap of yesteryear contained .2 grams of 80/20 primary (fulminate/chlorate mix) and .4 grams of a booster like PETN.  A traditional No. 8 cap would have differed by housing twice the PETN.   Modern # 8 caps have increased substantially in power depending on the source and have become the common types, but there is a lot of variance in what is termed a # 8 detonator.  A “US Army (Engineer’s) Special Blasting Cap” or “Military No. 8” for example might contain as much as 13.5 grams of PETN or RDX.

A matchhead or firehead electrical blasting cap uses an electrical match that sits in a pyrotechnic ignition mix in the butt of the cap.  An electric match is nothing more than a thin wire with a dollop of substance like dried red phosphorus attached to it.  It is a wire that gets hot and basically ignites the equivalent of a ‘kitchen match’ and starts the explosives train.   Only 3 volts from 2 flashlight batteries is required to set off the simple electric matches or ‘engine starters’ of solid propellant toy rockets.   Multiple exploding bridgewire detonators are generally used to uniformly initiate the primary stage of a nuclear weapon.   In effect a high capacity electrical discharge vaporizes each very thin bridgewire which in turn initiates a shockwave that simulates the chemical reaction.  The Germans were the first to use this type of detonator with their demolition charges in WWII.

*Explosions that detonate or deflagrate produce a lot of gas.  A mole of a substance is its mass in grams, as determined by the atomic numbers of its constituents and at standard temperature and atmospheric pressure a mole of any kind of gas occupies the same 22.4 liters of volume.   Using nitroglycerine as an example: one mole of nitroglycerine (@ standard atmospheric pressure) produces about 7.25 moles or 162.4 liters of gas.  One gram of nitroglycerine (about the size of a pencil eraser) would expand enough to fill a 43 gallon container with gas after decomposition.  The products in this example will be about 3 parts carbon dioxide, 2.5 parts water vapor, 1.5 parts nitrogen and 0.25 parts oxygen.

* Oxygen is essential for combustion and without it explosive chemical decompositions would go nowhere.   Oxygen is not a fuel, but an oxidant.   Concentrated sources of oxygen support rapid combustion.   Peroxides, nitrates, chlorates, perchlorates and dichromates all donate oxygen to combustion.

* Liquefied oxygen is about 840 times as dense as gaseous oxygen.  If spilled, liquid oxygen might soak into porous asphalt, charcoal, wood or petroleum products which then might become dangerously explosive for a short while.

Chemical jargon

Chlorates, perchlorates, nitrates, fulminates, permanganates and peroxides can be thought of as oxidizers with weak bonds and negative charges.

– Chlorates are the salts of chloric acid (HClO3).  Combustibles like charcoal, certain metals, organic solvents, sawdust and sugar will easily deflagrate when mixed with a chlorate salt.  Many fireworks in the past were composed with mixtures of chlorates, with various combustibles providing different colors and effects.  In modern fireworks, more stable perchlorates have replaced chlorates as the oxidizer of choice.

– Perchlorates are the salts derived from perchloric acid (HClO4), and they are often produced by the electrolysis of chloride salts.  Aside from pyrotechnics, perchlorates are also used in automotive airbags and as solid rocket fuel.  The Space Shuttle launches were powered by two solid rocket boosters that each held 350 metric tons of ammonium perchlorate.

–  The nitrate ion (NO-3) is formed when nitric acid and an alcohol are joined.  A nitrite (NO-2) is the salt of nitrous acid and not the same thing.  The molecular formula for nitrous acid is HNO2, while the formula for the much more significant nitric acid is HNO3.

–  Fulminates contain the relatively unstable fulminate ion (CNO-).

–  A permanganate is an oxidizing compound containing the manganate ion (MnO4-).

–  Peroxides are oxidizing compounds with the oxide anion (O2)-2 or single bonded oxygen to oxygen atom.   Hydrogen peroxide (H2O2) should be called hydrogen dioxide.   Barium peroxide is used in military “tracer” ammunition and in other pyrotechnics while triacetone triperoxide (or TATP) is a hard to detect.  TATP (or acetone peroxide) is easily produced, sensitive and fairly unstable but popular with terrorist like the would-be “shoe bomber” in 2001.  The shoe bomb had a dry TATP detonator along with PETN as the secondary explosive.   Perspiration made the fuse damp and the odor of several matches gave the imbecile’s intentions away.   The ‘hypergolic bipropellant’ of the remarkable WWII rocket-powered fighter aircraft (the Messerschmitt Me 163 Komet) was mixed from “C-Stoff” (a methane-hydrazine fuel) and a high test peroxide oxidizer called “T-Stoff”.   “Hypergolic” means that the propellant components spontaneously ignited when combined.  In field test and during the short carrier of the Me 163 during the war, several aircraft and many people were lost in explosions when these volatile components were handled.   When dealing with this rocket-plane, pilots & ground crew wore special nylon or rubberized clothing just to keep their flesh from being dissolved by these fuels.

Public Domain
The Shoe Bomber’s Shoe

– Nitramines are generally refer to newer and more chemically sophisticated compounds having bigger molecules, higher density, more of the hard to split covalent bonds and more energy packed into a given space.  While some of the best modern military grade explosives are nitramines, the benchmark for explosive speed and efficiency is still nitroglycerine – invented some 1.7 centuries ago.

– A binary explosive simply implies that two components are required to be combined but individually neither component is explosive.  The insensitive and very common blasting agent ANFO could almost be considered a binary explosive.   HELIX™ High Energy Liquid Explosive Binary Energetic is also an example of a binary.

Reactive elements

In pure form the “alkali metals” (Li, Na, K, Rb, Cs & Fr) are reactive or can be potentially dangerous in their own right.   Lithium hydroxide (as stored) when converted to lithium deuteride is used as a fuel in hydrogen bombs.  Pure potassium (K) explodes when it touches water, while rubidium, caesium and francium get even more violent than that.   The alkali metals are rare, and are never found in a pure form in nature – because they are so reactive.   Sodium (Na) and potassium (K) chlorates, perchlorates, nitrates and permanganates are very prevalent in explosive compounds.   Another group of elements called the “alkali earth metals” (Be, Mg, Ca, Sr, Ba and Ra) are reactive, but not as much so as the alkali group.   Of these, manganese (Mg) is commonly mixed in explosives, calcium (Ca) in calcium carbide makes explosive acetylene gas and barium (Ba) has already been mentioned for its use in pyrotechnics and tracers.   Aluminum (Al) belongs to a separate group in the Periodic Table but it is also fairly reactive (which explains why it is never found as a pure form in nature).   Powdered aluminum is a common catalyst or additive, usually mixed into ammonium nitrate or other explosives to enhance the blast power.

*  Hydrogen (H) which is the lightest and most abundant element in the universe, does burn, and can be considered explosive (if exposed to a flame) in concentrations between 4% and 74% with the surrounding air.   Hydrogen can react spontaneously with fluorine and chlorine.  The main engine of a Space Shuttle burns oxygen and hydrogen.   At full power the flame from one of these engines, is invisible – or nearly so, because it emits light in the ultraviolet spectrum.  Checking a pipe for a hydrogen leak involves looking for an ultraviolet flame – as hydrogen combines with oxygen in the air.   Hydrogen fires are seldom as dangerous as gasoline fires.  In 1937 the famous Hindenburg blimp, which was full of hydrogen burst into flames as it approached its landing.  The visible flames of that disaster were the burning outside skin of the blimp.  The hydrogen gas however rose quickly, carrying the invisible ultraviolet flames with it.  The deaths that did occur were the result of falls or by burns from diesel fuel. 

Fluorine (element #9) is so extremely reactive and electronegative that almost all other elements including some of the noble metals and noble gases will form compounds with it.   Hydrofluoric acid was so dangerous that laboratory explosions killed “fluorine martyrs” (chemist) like flies in the 19th century and it took 74 years of concentrated effort just to isolate elemental fluorine.

Phosphorus (P) is a nonmetallic, very reactive element which has been used in many explosive munitions.  It is an additive to start fires and to create smoke.  It is or was used in kitchen matches, fireworks and in weaponized nerve agents.  This elusive element was discovered about four centuries ago when an alchemist dehydrated his own urine, and then noticed that it glowed in the dark.   In WWI, phosphorus was employed on incendiary ammunition against blimps and aircraft.   In WWII phosphorus bombs were dropped on London to start fires while other phosphorus munitions were used to ignite aircraft, tanks and other vehicles, for smokescreens and for anti-personnel mortar rounds.

* A modern incendiary bomb is likely to employ a thermite mixture of aluminum and ferric oxide. 

Black Powder

Throughout its history and up until recent times there was no “black powder”, but simply “gunpowder”.    Modern nitrocellulose based “smokeless powders” have become regular “gunpowder” in today’s parlance while the term “black powder” now invokes the old charcoal, sulfur and saltpeter propellant.   In the previous century the term “flash powder” might have denoted the finely ground and very volatile FFFFFg gunpowder used by photographers to generate more light exposure for pictures.   While originally, firecrackers were made with black powder; today’s firecrackers are usually constructed using a faster burning compound called ‘flash powder’ (with magnesium or aluminum powder and a potassium chlorate or perchlorate oxidizer).

True black powder is messy because only about ½ of the material is burned.  It leaves behind a lot of unburned residue that quickly fouls a weapon.   Most new, so-called “primitive firearms” that are used today are never loaded with true black powder – but instead with a cleaner substitute having equivalent deflagration properties.  The main deficiency of black powder (or similar replacement) lies in its inability to drive a projectile past a certain speed.  Black powder has a relative detonation velocity of 1,312 fps.  Confined in a firearm, its pressure peaks almost instantly and starts to drop even before the projectile leaves the barrel.   In contrast the even slower burning smokeless propellants that replaced it in modern firearms, achieve higher projectile speeds because they produce more gas and continue to push and accelerate the bullet as it travels down the barrel.  Cramming more black powder into a black powder muzzle might speed a bullet up to 1,750 or 1,800 fps, unless the increased pressure ruptures the firearm’s breech beforehand.

In 1275 A.D. the standard recipe for black powder was 1 part charcoal and 1 part sulfur to 4 parts saltpeter (potassium nitrate or niter).   More ideal proportions have been determined to be around 13.5% charcoal, 11.5% sulfur and 75% saltpeter.  Charcoal is the fuel; sulfur the binding agent and saltpeter the oxidizer.  After firing black powder’s fouling or residue is corrosive and purportedly contains carbon and sulfur, potassium carbonate, potassium sulfate, potassium sulfide, potassium thiosulphate and potassium thiocynate.

Many a powder company has disappeared in a cloud of smoke, flame and noise during the crushing stage of black powder production.  Static electricity can easily ignite black powder.  In the manufacture of black powder, metal implements are avoided because they might cause sparks or build up static energy.  Before the process called “corning” was adopted, early black powders tended to separate or sift apart; making them inconsistent and possibly unreliable.  Corning was a process where water was added to the gunpowder and was then pressed out.  While still moist the propellant could either be pressed through a screen to make granules or left to dry as cake that was later crushed into small powder.  Using the corning process the final product became more homogonous and the separate ingredients were bonded in crystalline form.

Crushing the powder into smaller grains created more surface area particulate, more flame front and therefore faster burning. In the old days, black powder was graded in regards to its granule size, from 1F to 5F.   “Fg” powder would be used in something big, like a cannon.   “FFg” was intended for large bore shotguns and rifles.   “FFFg” was the common powder for most rifles.   “FFFFg” was fast powder for pistols and for flintlocks with flash-pans.   “FFFFFg” was rare and very fast and as mentioned, was occasionally used by early photographers to produce light in dark settings.

Although black powder is smoky and pokey, it is still getting the job done after a good 700 years of field testing, on land and at sea.  Every manner of big game including African elephant, rhino and Cape buffalo has been felled by it.  At range, in the right hands, black powder rifles have preformed respectably, even by today’s standards.   In front of witnesses a frontiersman once knocked an Indian brave from his horse from 1,538 yards away (later measured, 0.87 mi), using a 50-90 caliber Sharps.

Another testament of black powder accuracy involves a British made sniper rifle used by the Confederacy during the Civil War.  The Whitworth rifle as designed by Sir Joseph Whitworth in 1856 employed a hexagon bore and a hexagon bullet of .45 caliber.  The Whitworth rifle was apparently accurate to well over 1,500 yards, but very expensive and the Confederacy only purchased a handful to issue to a few chosen snipers or skirmishers.   It seems that Yankee General John Sedgwick was unhorsed by a Whitworth during battle.  Sedgwick was rousting his troops who were hiding from sniper fire.  His famous last words were “They couldn’t hit an elephant at this distance” just before a fatal bullet fired from about 800 yards away, hit him square in the face.

A black powder firearm is a pretty crude weapon by today’s standards but in its simplicity lays a certain elegance and reliability.  Modern weapons require modern ammunition; a reserve stockpile or continuous source of new ammunition is important.  If sources of modern ammunition run dry then the weapon becomes useless for either hunting or self defense.   Individuals that know how to reload ammunition might fare better for a while – until their source of nitrocellulose powders, primers and bullets runs out.  A survivalist with a black powder firearm however, with the know-how can mold his own bullets, make his own propellant and probably even make his own percussion caps if needed.   Making black powder propellant is hardly rocket science and sources of information on this subject abound.  The differences in proportions are not too critical. The best homemade gunpowder will probably be inferior to commercial powder but should work.  It should not be considered an act of anarchy or the abetting of terrorism to provide the interested reader with a recipe here.

* The following recipe is a combination of ideas from too separate books (“Special Forces – Guerrilla Warfare Manual” by Scott Wimberley and “The Poor Man’s James Bond” by Kurt Saxon).  The proportions of 6 parts potassium nitrate to 4 parts charcoal and 1 part sulfur would comprise a fine mixture.  If the units of measure were cups then this mix would produce about 4 pounds of propellant (perhaps ½ the measure would be more appropriate).  The different components should be ground separately into small powders before beginning.  Here the methods from the two books deviate.  In one example the three components are mixed with a little water (1 part water – or preferably a better solvent like whiskey or rubbing alcohol) and allowed to set up into a malleable paste. The other method involves using more water (3 parts), heating the mix slowly until small bubbles rise, then dumping the mix into a large quantity of alcohol (10 parts), stirring and allowing it to gestate for 5 minutes.   In the second method the water is not allowed to boil, any mixture attempting to dry along the sides of the container is carefully stirred back in to keep it moist and after the 5 minute dip in alcohol the excess fluids are strained and squeezed out through a cheese cloth.  In both methods the somewhat firm paste or cake is pressed through a screen to produce uniform granules.  The granules can be set on a tray or flat surface to dry out in the sun. The quicker it dries the more effective it will be (a role in which alcohol plays a part). One author advocated sprinkling in a little graphite (if available) at this stage; the granules swirled in a lidded plastic bowl like Tupperware® become rounded and uniform. 

The advance of chemical Explosives

There are some 800 uniquely identifiable explosive compounds but admittedly some of these are simply compounds of more basic explosives already on the list.  This post will end here but a follow-up or second part will explore some of the more noteworthy explosive compounds, including: nitrostarch, nitric acid, mercury fulminate, sliver fulminate, guncotton, nitrocellulose, nitroglycerin, dynamite, gelignite, ballistite, lead azide, cordite, picric acid (trinitriphenol or TNP), lyddite, TNT, significant mixtures of TNT with other agents, tetryl, PETN, semtex, DDNP, RDX or hexogen, HMX or octogen and CL-20.