Explosives Classification: Low vs High, Primary vs Secondary

An explosive is a material that, when triggered, undergoes rapid exothermic decomposition releasing large volumes of gas. The critical variable that distinguishes the two major classes is the speed at which the reaction front moves through the material.

In deflagration, the reaction propagates by thermal conduction and convection from one layer of material to the next. Propagation velocities are subsonic relative to the unreacted material: typically 1 to 400 metres per second for black powder, 400 to 1,000 metres per second for smokeless propellants under confinement. The reaction is fundamentally a rapid combustion. Gas pressure builds within the confining vessel and drives the mechanical work (propelling a projectile, rupturing a pipe). Remove the confinement and deflagration is harmless burning.

In detonation, the reaction is driven by a supersonic shock wave that compresses and initiates each successive layer of unreacted material. The Chapman-Jouguet velocity, the minimum stable detonation velocity for a given explosive, ranges from about 6,000 metres per second for ANFO (ammonium nitrate fuel oil) to 9,100 metres per second for HMX (high-melting explosive). The shock wave itself does the chemical work, not heat transfer. This is why detonating explosives require no confinement to function and why they shatter materials instead of simply rupturing them.

The forensic consequence is direct. A pipe-bomb using black powder kills primarily through fragmentation of the container driven by deflagration gas pressure. A similar-sized device using RDX (Research Department Explosive) kills through the combination of brisance (shattering power of the detonation shock wave), the blast overpressure (the rapid pressure rise ahead of the shock front), and thermal effects. The damage patterns, fragment morphology, and chemical residue signatures are chemically distinct.

Oxygen balance quantifies how far a given explosive composition deviates from stoichiometric combustion. A composition with zero oxygen balance produces only CO2 and H2O from its carbon and hydrogen, with all nitrogen going to N2, a theoretically ideal detonation product mixture. Positive oxygen balance means excess oxidiser; negative oxygen balance means incomplete oxidation and the production of CO, soot, and partially oxidised residues. TNT has an oxygen balance of minus 73.9 per cent, meaning it is severely oxygen-deficient and produces significant CO and soot in post-blast debris. PETN (pentaerythritol tetranitrate) has an oxygen balance of minus 10.1 per cent, closer to zero. ANFO formulated at the 94:6 ammonium nitrate to fuel oil ratio has an oxygen balance near zero, making it chemically efficient. These differences in oxygen balance directly affect which post-blast products are detectable and in what ratios.

Black powder, also called gunpowder, is a mechanical mixture of potassium nitrate (the oxidiser, approximately 75 per cent by mass), charcoal (the fuel, approximately 15 per cent), and sulphur (the burn-rate accelerant and binder, approximately 10 per cent). The standard formulation traces to Chinese pyrotechnic texts from the 9th century CE, with modern ratios stabilised in the 16th century. The reaction when ignited is roughly:

2 KNO3 + 3C + S produces K2S + N2 + 3CO2

In practice, the post-blast residue includes potassium carbonate, potassium sulphate, potassium sulphite, and thiocyanate alongside partially unreacted sulphur and charcoal. Ion chromatography for potassium, nitrate, sulphate, and carbonate anions is the primary confirmatory method. The residue is relatively stable and detectable on surfaces for days to weeks.

Smokeless propellants replaced black powder in firearm ammunition starting in the 1880s (the French Poudre B in 1884, developed by Paul Vieille). They are classified by base composition. Single-base propellants contain nitrocellulose (NC) as the sole energetic ingredient, plasticised with diphenylamine (the stabiliser that captures the nitrous acid that accelerates NC degradation). Double-base propellants add nitroglycerin (NG) to NC, increasing energy density and producing measurable NG residue after discharge. Triple-base propellants (used in large artillery and naval ordnance) add nitroguanidine to the NC and NG matrix, reducing barrel erosion at the cost of increased sensitivity.

The forensic significance of smokeless propellant residue extends beyond firearm discharge. Improvised devices that use commercially available smokeless propellant from rifle cartridges or sporting powder stocks produce characteristic residue profiles that can be compared against manufacturing databases. The FBI Laboratory's explosives reference collection and the UK DSTL database include hundreds of commercial smokeless propellant formulations for this purpose.

Primary explosives are high explosives that are sensitive enough to detonate in response to relatively modest stimuli: friction, percussion, electrostatic discharge, spark, flame, shock, or even pressure. They detonate without confinement and without a priming charge, which is what makes them useful as initiators in a detonator and dangerous to handle.

Mercury fulminate, Hg(ONC)2, was the first practical primary explosive, used by Alfred Nobel in his early blasting cap designs and in the 19th-century revolver percussion cap industry. It is extraordinarily sensitive to percussion, which is why it was the primary ingredient in firearm percussion caps from the 1820s until it began to be phased out in favour of lead styphnate in the mid-20th century. Post-blast detection relies on mercury analysis by ICP-MS or cold-vapour atomic fluorescence spectrometry. Mercury fulminate is now rarely used in modern commercial or military applications.

Lead azide, Pb(N3)2, is the workhorse primary explosive in modern commercial and military detonators across the US, UK, India, and most of Europe. Its lead content contributes to the GSR particle signature, and the gunshot residue three-component particle chemistry module explains how detonator-derived lead particles are distinguished from conventional primer particles by SEM-EDX morphology. It is sensitive to flame, spark, and shock, and is incompatible with copper (it forms copper azide, which is even more sensitive). Commercial detonators from Orica, Dyno Nobel, and Ideal Electric typically use lead azide in the primary charge. Post-blast detection uses lead analysis by ICP-MS combined with azide anion detection by ion chromatography. The combination of both analytes in trace evidence strongly indicates a detonator initiation train.

Diazodinitrophenol (DDNP, also called dinol) is a primary explosive used in detonators where lead-free formulations are required, particularly in European markets with restrictions on heavy metals in pyrotechnics under the EU Directive 2011/65/EU on the restriction of hazardous substances. DDNP is detectable by GC-MS and LC-MS/MS.

Lead styphnate (lead 2,4,6-trinitroresorcinate), the primary explosive in most modern centerfire primer mixtures alongside lead azide and barium nitrate, is also a primary explosive that appears in gunshot residue alongside the classic Pb-Sb-Ba three-component signature. Its presence in a post-blast sample alongside other initiator chemistry indicates a detonator-based initiation train rather than simple ignition.

Secondary explosives are high explosives that are insensitive to normal friction, spark, and percussion stimuli. They require either a primary explosive shock or a blasting cap to detonate. This insensitivity is a design feature: it makes them safe to manufacture, transport, and store in bulk quantities. It also means they are the main explosive mass in military munitions, commercial blasting, and most improvised explosive devices (IEDs) large enough to cause mass-casualty events.

TNT (2,4,6-trinitrotoluene) is the reference secondary explosive against which explosive energy is measured in TNT equivalent. Detonation velocity: approximately 6,900 metres per second. Density: 1.654 g/cm3. Melting point: 80.35°C. The low melting point was the 20th century's key manufacturing advantage: TNT can be melted and cast into any shape, and it can be mixed with other explosives in the molten state to form composite formulations. Composition B (Comp B) is 60 per cent RDX and 40 per cent TNT by mass, cast from the melt and used in artillery shells, mortar rounds, and anti-tank mines. Torpex is 42 per cent RDX, 40 per cent TNT, 18 per cent powdered aluminium, used in naval munitions. Post-blast TNT residue is characterised by the dinitrotoluene (DNT) isomers (2,4-DNT and 2,6-DNT) that appear as decomposition products and manufacturing impurities, detectable by GC-ECD.

RDX (cyclotrimethylenetrinitramine, hexogen, or T4) has a detonation velocity of approximately 8,750 metres per second and a density of 1.816 g/cm3. It is the highest-energy component in most military plastic explosives: C-4 (the US military standard plastic explosive) is 91 per cent RDX with polyisobutylene and di(2-ethylhexyl)sebacate as the plasticiser binder. Semtex H, manufactured by Explosia in Pardubice, Czech Republic (and before the Velvet Revolution by the Czechoslovak state enterprise), is a mixture of RDX and PETN in a styrene-butadiene rubber binder. Semtex is characterised by its red-orange colour and characteristic IR fingerprint.

PETN (pentaerythritol tetranitrate) has a detonation velocity of approximately 8,400 metres per second and is used in detonating cord (det cord), flexible sheet explosives (Detasheet, Primasheet), and detonator base charges. Its low critical diameter makes it useful in thin configurations. PETN decomposes to produce pentaerythritol, erythritol tetranitrate, and pentaerythritol dinitrate as residues, detectable by LC-MS/MS in negative-ion ESI mode.

HMX (cyclotetramethylenetetranitramine, octogen) is the most powerful of the four conventional secondary explosives with a detonation velocity of approximately 9,100 metres per second. It is used in military applications requiring maximum brisance in a compact geometry, including shaped charges and missile warheads. HMX co-occurs with RDX as a manufacturing impurity in Composition B and some grades of military RDX.

Oxygen balance (OB) is expressed as a percentage and calculated from the molecular formula CaHbNcOd:

OB% = 1600 x (d - 2a - b/2) / M

where M is the molecular weight. A positive OB means the molecule has more oxygen than needed to completely oxidise carbon to CO2 and hydrogen to H2O. A negative OB means oxygen-deficient combustion. Most organic explosives have negative OB because the nitro groups, while donating oxygen to the reaction, cannot fully compensate for the fuel elements.

Brisance is the shattering effect of a detonation, distinct from heave (the lifting and moving of overburden). It is measured by the lead cylinder compression test (the Trauzl block test, developed by the Austrian chemist Isidor Trauzl in 1885) or by the sand-crush test. A high-brisance explosive produces a sharp, high-amplitude pressure wave that shatters rock, steel, and concrete close to the charge. TNT has a Trauzl block expansion value of approximately 300 cm3; RDX approximately 480 cm3; PETN approximately 520 cm3.

At blast scenes, brisance is readable in the damage pattern: cratering depth in soil, spalling pattern in concrete walls, the type of fragmentation observed in nearby structural steel. US Army Corps of Engineers blast-effect guidelines, UK Ministry of Defence explosive safety standards (JSP 482), and India's Explosive Rules 2008 (under the Explosives Act 1884, amended) all reference detonation velocity and brisance class as the primary physical parameters for explosion classification. Australia's AUSEG explosives safety guide and Canada's NRCAN Mine Safety program use equivalent frameworks.

The United Nations Model Regulations on the Transport of Dangerous Goods (the UN Orange Book, published biennially by the UN Committee of Experts on the Transport of Dangerous Goods) place all explosive articles and substances in Class 1. The six divisions of Class 1 are directly relevant to forensic classification:

Division 1.1: substances and articles which have a mass explosion hazard (primary explosives, secondary explosives in bulk). Division 1.2: articles which have a projection hazard but not a mass explosion hazard. Division 1.3: fire hazard with minor blast or projection hazard (some propellants). Division 1.4: no significant hazard (consumer fireworks in many jurisdictions). Division 1.5: very insensitive blasting agents (ANFO, emulsion explosives). Division 1.6: extremely insensitive detonating articles.

The US Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) publishes the Annual List of Explosive Materials under the authority of 18 USC 841(d). The 2024 list includes primary explosives (lead azide, mercury fulminate, DDNP), secondary explosives (TNT, RDX, PETN, HMX, ANFO), improvised formulations (TATP, HMTD), and propellant powders classified separately under 18 USC 845. Manufacturers, importers, dealers, and users of listed materials require ATF federal explosives licences (FEL) or permits. Post-blast forensic evidence from an ATF-listed material is introduced through this regulatory framework at trial.

In India, the Explosives Act 1884 (as amended) and the Explosive Rules 2008 promulgated under it by the Petroleum and Explosives Safety Organisation (PESO, formerly the Chief Inspector of Explosives) govern manufacture, possession, use, transport, and import of explosives. The Rules classify explosives into six categories (A through F) mapped approximately to UN Class 1 divisions. Category A includes blasting explosives (primary and secondary high explosives). Category B includes propellant powders. Possession of any explosive without a valid PESO licence is cognisable and non-bailable under the Explosive Substances Act 1908, with punishment up to 20 years imprisonment.

The UK's Explosives Regulations 2014 (SI 2014/1638), the EU Directive 2014/28/EU on civil explosives, and Australia's national model Work Health and Safety (Explosives) Regulations 2017 use equivalent licensing frameworks. In all jurisdictions, the forensic chemist's identification of an explosive by class creates the nexus between the chemical evidence and the statutory offence.