Explosives: Definition, Types and Classification

An explosive is a single chemical compound or a mixture that, when initiated by a small input of heat, shock, friction or electric energy, undergoes a very rapid self-sustaining exothermic decomposition. The reaction releases a large volume of hot gas, generates a steep pressure rise, and emits a shock wave that does mechanical work on the surrounding medium. Three features have to be present: rapid reaction rate, large gas volume per unit mass, and a high heat of reaction. Anything short of all three is not an explosive in the forensic sense.

The most important distinction in explosives classification is deflagration versus detonation. Both are exothermic decompositions, but the physics is different.

Deflagration is subsonic burning. The reaction front travels through the material by thermal conduction (and convective gas flow), advancing at millimetres per second to a few metres per second. Pressure builds gradually. Black powder, smokeless powder and pyrotechnic compositions deflagrate, which is why they make good propellants: they push a bullet or a rocket without shattering the barrel or the casing.

Detonation is supersonic, shock-driven decomposition. A shock wave compresses and heats the next layer of explosive faster than thermal conduction could, so the wave is self-sustaining. Reaction-front velocities are 1,500 m/s for the slowest secondaries and 9,000 m/s for HMX. Pressure spikes are in the tens of gigapascals. RDX, PETN, TNT and HMX detonate, which is why they shatter steel and concrete rather than push them. A material can deflagrate under one set of conditions and detonate under another (confinement, particle size, density, initiator strength), and this is called the deflagration-to-detonation transition, and it is what makes a stored stack of nitrate-fuel mix dangerous in a fire.

In summary: low explosives deflagrate at mm/s to a few m/s, are used as propellants and pyrotechnics, and are characterised by burn rate. High explosives detonate at 1,500 to 9,000 m/s, are used as main charges, and are characterised by velocity of detonation (VoD), brisance and oxygen balance.

The properties used to characterise high explosives are velocity of detonation, brisance, sensitivity, oxygen balance, and energy density.

Velocity of Detonation (VoD). The detonation-wave speed. Measured experimentally by ionisation probes or fibre-optic timers placed along a charge. Approximate values to memorise: ANFO 3,200 m/s, TNT 6,900 m/s, RDX 8,750 m/s, PETN 8,300 m/s, HMX 9,100 m/s, nitroglycerin 7,700 m/s. The bigger the VoD, the more powerful the secondary explosive at the same density.

Brisance. The shattering effect, related to the rate of pressure rise at the detonation front. Measured by the sand-crush test, the lead-block (Trauzl) test or the plate-dent test. RDX, HMX and PETN are high-brisance; ANFO and slurries are low-brisance heaving explosives, which is why ANFO is preferred for mining (heave rock) while RDX is preferred for cutting steel.

Sensitivity. How easily the explosive is initiated. Standardised by drop-hammer impact tests, friction tests, electric-spark tests and minimum initiating charge for shock. Primary explosives are highly sensitive (a static-electric discharge is enough). Secondaries are insensitive to drop and friction; they need a detonator. Tertiaries (ANFO, slurries, emulsions) are so insensitive they need a secondary booster on top of the detonator.

Oxygen balance. Negative oxygen balance means the explosive carries less oxygen than it needs to oxidise all its own carbon and hydrogen, so detonation gives sooty CO-rich gases. TNT (oxygen balance about −74%) is famously oxygen-deficient, which is why TNT smoke is black. ANFO is mixed to about zero oxygen balance (about 94% ammonium nitrate plus 6% fuel oil) for maximum energy release. Mining-grade explosives are engineered to oxygen balance to control fume colour and gallery air quality.

Energy density and crystal density. Energy per unit mass (kJ/g) and packed crystal density (g/cm³) determine how much work the same charge volume can do. HMX is the densest common military explosive (1.91 g/cm³), which is why it goes into compact warheads.

Low explosives (deflagrating). VoD typically below 1,000 m/s. Burn rather than detonate. The three named families examiners test are:

1. Black powder. The original gunpowder, a mechanical mixture of potassium nitrate (about 75%), charcoal (about 15%) and sulphur (about 10%). Used in fuse trains, signal flares, antique muzzle-loaders and pyrotechnics. Hygroscopic and dirty-burning.
2. Smokeless powder. Nitrocellulose-based propellants, classified by base count.Single baseis nitrocellulose (NC) alone.Double baseis nitrocellulose plus nitroglycerin (NC + NG).Triple baseadds nitroguanidine (NC + NG + NQ) and is used in large-calibre artillery for cool-burning, low-flash performance. Smokeless powder is the propellant in all modern small-arms ammunition; primer chemistry sits next to it in the firearm-injuries and topics.
3. Pyrotechnics. Compositions designed for light, heat, smoke or sound rather than mechanical work. Includes signal flares, illumination compositions, smoke generators, firecrackers and incendiary mixtures.

High explosives, primary (initiating). Highly sensitive to heat, shock, friction and spark. Used in milligram quantities inside detonators and primers to convert a small mechanical or electrical input into a shock wave strong enough to set off a secondary. The canonical Indian-syllabus four are lead azide (Pb(N3)2), lead styphnate (lead 2,4,6-trinitroresorcinate), mercury fulminate (Hg(ONC)2) and DDNP (diazodinitrophenol). Lead styphnate plus barium nitrate plus antimony sulphide is also the classical small-arms primer mix that leaves the Pb-Ba-Sb three-component GSR particle profile.

High explosives, secondary (main charge). Insensitive to drop, friction and small flame. Need a detonator. VoD 4,000 to 9,000 m/s. The names every examiners aspirant must be able to identify by acronym and chemistry:

• TNT (2,4,6-trinitrotoluene). Nitroaromatic. Melt-cast pourable, oxygen balance −74%, VoD 6,900 m/s. Long the military standard.
• RDX (Royal Demolition eXplosive, cyclotrimethylenetrinitramine, cyclonite, hexogen). Nitroamine. VoD 8,750 m/s. The Indian military and terrorist-use canonical explosive. Always in Indian MCQs.
• HMX (octogen, cyclotetramethylenetetranitramine). Nitroamine. VoD about 9,100 m/s, density 1.91 g/cm³. The most energetic conventional explosive in common military use.
• PETN (pentaerythritol tetranitrate). Nitrate ester. VoD 8,300 m/s. Used in detonating cord and as a component of Semtex.
• Nitroglycerin (NG). Liquid nitrate ester, VoD 7,700 m/s. Extremely shock-sensitive in pure form, hence Nobel's invention of dynamite by sorbing it on kieselguhr or wood pulp.
• Tetryl (2,4,6-trinitrophenylmethylnitramine). Booster explosive, now phased out but still in the syllabus.
• Picric acid (2,4,6-trinitrophenol). Yellow crystalline nitroaromatic. Historical First World War filling. Forms shock-sensitive metal picrates with iron, lead and copper, which is why ageing picric stocks are a known fire-service hazard.

High explosives, tertiary (blasting agents). Insensitive enough that they need a secondary booster, not just a detonator. Used in mining and civil blasting where bulk and cost matter more than brisance.

• ANFO (ammonium nitrate + fuel oil).94% AN + 6% FO by mass. VoD 3,200 m/s. The single most widely used commercial blasting agent on earth. Also the explosive in the Oklahoma City bombing (1995) and several IED attacks worldwide.
• Slurries and emulsions. Ammonium-nitrate-based water-gel formulations sensitised by aluminium powder, micro-balloons or nitrate ester. Indian mining houses use brand names like NCDM, Indogel and similar.

Plastic explosives. A subset of secondaries where a high-VoD nitramine or nitrate ester is bound into a polymer matrix to give a mouldable putty.

• C-4.About 91% RDX with a plasticiser-binder (commonly polyisobutylene + di(2-ethylhexyl) sebacate + motor oil). VoD about 8,040 m/s.
• Semtex. Czech-origin mix of RDX and PETN with a styrene-butadiene binder. VoD about 7,400 m/s. Notorious for low vapour pressure, which made it hard to detect by sniffer dogs before taggants were mandated.
• PE-3A and PE-4.British and Commonwealth plastic explosives, RDX-based.
• PEK1.The Indian-Army issue plastic explosive, RDX-based with a domestic binder; produced under Ordnance Factories Board / now Munitions India Limited.

The numbers vary by a few hundred m/s across sources because VoD depends on packing density, particle size and confinement. The order from slowest to fastest detonation velocity is: black powder and smokeless powder (deflagrating, not detonating) < ANFO < TNT < NG < PETN < RDX < HMX. Oxygen balance approximations: RDX −21.6%, HMX −21.6%, PETN −10%, TNT −74%, NG +3.5%, ANFO close to 0%.

The same substances can be classified along three additional axes.

By use. Military(RDX, HMX, TNT, C-4, Semtex, PEK1, PE-3A) versuscommercial(ANFO, slurries, emulsions, dynamite, detonating cord) versusimprovised(homemade nitrate-mix, urea nitrate, TATP, HMTD, fertiliser bombs). Indian IED casework crosses the commercial-improvised line: a stolen mining-grade slurry stick triggered by a commercial detonator and packed into a tiffin box is the prototypical Maoist or terrorist device, and the related topic on country-made bombs and IEDs picks this up in detail.

By chemistry. Three broad chemical families and a fourth catch-all.Nitroaromaticsare toluene or phenol rings with nitro groups (TNT, picric acid, tetryl).Nitramineshave N–NO2 bonds (RDX, HMX, tetryl, nitroguanidine).Nitrate estershave O–NO2 bonds (PETN, NG, NC, ethylene glycol dinitrate).Inorganic and primary explosivesare the rest (lead azide, lead styphnate, mercury fulminate, ammonium nitrate, peroxides like TATP and HMTD).

By UN transport hazard class. The ICAO/IATA Dangerous Goods regulations and the UN Yellow Book classify explosives under Class 1, with six divisions. Class 1.1 explosives have a mass-explosion hazard (most military secondaries); Class 1.2 have a projection hazard but not mass explosion; Class 1.3 have a fire hazard plus minor blast (most propellants); Class 1.4 have minor hazard (commercial fireworks); Class 1.5 are very insensitive substances with a mass-explosion hazard (ANFO and blasting agents); Class 1.6 are extremely insensitive detonating articles. ANFO is Class 1.5 (very insensitive, mass-explosion hazard); RDX-based main charges are Class 1.1 (mass-explosion hazard).

Indian law treats explosives under two parallel statutes. The Explosives Act 1884 (with the Explosives Rules 2008) regulates manufacture, possession, sale, transport, import and use of commercial and industrial explosives. The Explosive Substances Act 1908 is a criminal statute aimed at unlawful use, attempted use and conspiracy with explosives.

Explosives Act 1884 + Explosives Rules 2008.The Rules classify explosives into seven classes for the purpose of licensing and storage.

• Class 1gunpowder and similar deflagrating mixtures.
• Class 2nitrate mixtures (ANFO and similar).
• Class 3nitro-compounds (TNT, RDX, picric acid, PETN, NG and other nitrate esters and nitramines).
• Class 4chlorate mixtures.
• Class 5fulminates (lead azide, mercury fulminate, primary initiators).
• Class 6ammunition (loaded cartridges, shells).
• Class 7fireworks and similar pyrotechnics.

The licensing authority is the Petroleum and Explosives Safety Organisation (PESO)at Nagpur, under the Department for Promotion of Industry and Internal Trade (DPIIT), Ministry of Commerce and Industry. PESO issues manufacture, import, possession, sale and transport licences and inspects explosives factories and magazines.

Explosive Substances Act 1908.Section 3 punishes causing an explosion likely to endanger life or property with up to imprisonment for life. Section 4 covers attempts. Section 5 covers possession of explosive substances under suspicious circumstances. Section 6 covers abetment. Trials under this Act in India have prosecuted RDX-based bombings including the 1993 Mumbai serial blasts, the 2006 Mumbai suburban train bombings, the 26/11 charge sheet and the 2019 Pulwama attack (which used a vehicle-borne IED; NIA investigations established approximately 35 kg of military-grade RDX was smuggled from Pakistan and supplemented with locally procured ammonium nitrate and gelatin sticks). Ammonium-nitrate-based attacks including the 2007 Hyderabad twin bombings and the 2010 German Bakery, Pune bombing were also tried under this Act.

These cases are routinely cited in MCQs as anchor examples for "which explosive was used in..", though examiners more often phrases the question generically. Both the forensic-evidence-in-court rules under BSA 2023and the analyst's lab report sit on top of these statutes when an explosives case reaches trial. Shrapnel-injury and firearm-versus-blast wound interpretation overlaps with the entry-exit and range determinationanalysis covered in the firearms book chapter.