Explosives Classification: Low, High, Primary, Secondary, Tertiary

Deflagration is a combustion process in which the reaction front moves through the unreacted material at a velocity below the speed of sound in that material, typically anywhere from a few millimetres per second (a slow-burning safety fuse) to several hundred metres per second for a fast-burning propellant grain. The mechanism of propagation is thermal: heat released by the burning layer heats the adjacent unreacted layer to ignition temperature. The pressure wave produced by a deflagrating material travels ahead of the reaction front but the two are not coupled.

Detonation is fundamentally different. The reaction front is a supersonic shock wave that compresses the unreacted explosive ahead of it. The compressed material is simultaneously raised in temperature and pressure to the point where it reacts instantaneously. The chemical energy release feeds the shock wave, sustaining it. Detonation velocities for practical high explosives range from approximately 4,500 metres per second (low-grade ANFO) to 9,100 metres per second for HMX in a pressed formulation. This is why detonating a high explosive in the open air, without confinement, still produces devastating brisance (shattering effect).

The transition between deflagration and detonation is called deflagration-to-detonation transition (DDT) and is not instantaneous. Confinement accelerates DDT: a burning propellant that deflagrates harmlessly in the open can detonate if ignited inside a closed pipe because rising pressure accelerates the burn front. This is the mechanism behind many improvised pipe bomb incidents and behind the industrial accidents documented in the US Chemical Safety Board's investigation database. The DDT risk underpins why low explosives are stored and transported under strict confinement controls in most jurisdictions.

Low explosives are materials that, under normal conditions of use, deflagrate rather than detonate. The class includes black powder (gunpowder), smokeless powder (single-base, double-base, triple-base propellants), and pyrotechnic compositions (stars, flares, signals, and incendiaries). All three are oxygen-balanced systems: they carry their own oxidiser in the formulation and do not require atmospheric oxygen to combust.

Black powder is the oldest manufactured explosive in systematic use, with origins traced to Tang dynasty China (seventh century CE) and European military adoption by the thirteenth century. Its composition is approximately 75 per cent potassium nitrate (the oxidiser), 15 per cent charcoal (the fuel), and 10 per cent sulfur (a fuel that also lowers ignition temperature). The potassium nitrate furnishes oxygen within the molecule; combustion produces carbon dioxide, carbon monoxide, sulfur dioxide, potassium carbonate, potassium sulfate, and potassium sulfide as residues. These inorganic ionic residues are detectible by ion chromatography (IC) and capillary electrophoresis in post-blast debris. The ATF (Bureau of Alcohol, Tobacco, Firearms and Explosives) in the United States lists nitrate, carbonate, and sulfide as targeted residue ions in the Standard Operating Procedures for post-blast analysis of black powder scenes. UK Forensic Explosive Laboratory (FEL) at Fort Halstead and the German Bundeskriminalamt (BKA) explosives section use compatible ion chromatographic methods.

Smokeless powder is the propellant used in modern small arms cartridges, shotgun shells, and artillery rounds. Single-base powders contain nitrocellulose as the sole energetic component; double-base powders add nitroglycerin to increase the energy density; triple-base powders add nitroguanidine to reduce muzzle flash and barrel erosion. The residues of smokeless powder combustion include diphenylamine (and its nitrated derivatives: 2-nitrodiphenylamine, 4-nitrodiphenylamine) which are stabilisers, as well as nitrocellulose fragments and nitroglycerin in unburnt particles. Detection of these organic residues by GC-MS or LC-MS/MS is the primary analytical approach for smokeless powder identification in improvised device cases.

Pyrotechnic compositions are physical mixtures of oxidiser and fuel formulated to produce heat, light, sound, smoke, or a combination. Metal fuels (aluminium, magnesium, titanium) paired with oxidisers such as potassium perchlorate, barium nitrate, or strontium nitrate are standard. Forensic identification relies on SEM-EDX (scanning electron microscopy with energy-dispersive X-ray spectroscopy) for elemental mapping, and IC for ionic species. In India, the Explosives Act 1884 and the Petroleum, Explosives and Safety Organisation (PESO) licensing framework explicitly classify fireworks compositions as "low explosives" subject to manufacturing, storage, and transport rules distinct from those governing high explosives.

Primary explosives are defined by their high sensitivity to mechanical shock, friction, electrostatic discharge, and heat. A small input of energy, often as little as a spark from an electrostatic discharge or a sharp blow from a firing pin, is sufficient to trigger detonation. This sensitivity is the property that makes them useful as initiating materials: they convert a small, controllable stimulus into a shock capable of initiating a less-sensitive secondary explosive.

Mercury fulminate (Hg(CNO)2) was the first widely used primary explosive in percussion caps, patented by Joshua Shaw in the US in 1814. It is now rarely used in new designs because mercury is an environmental hazard and lead-free alternatives perform comparably, but it appears in historical devices and in forensic case archives. Lead azide (Pb(N3)2) is the standard primary explosive in modern detonator caps and blasting caps across most of the world. It detonates readily on impact or when exposed to copper salts, which is why lead azide must not be allowed to contact copper or brass hardware (the resulting copper azide is even more sensitive). Lead styphnate (lead 2,4,6-trinitroresorcinate) is used as a sensitising agent in percussion primers for firearms cartridges; its function is to provide the initiating spark for the main primer charge of lead azide or similar compounds. Diazodinitrophenol (DDNP) is a secondary initiating explosive used in some detonators as an alternative to lead azide, particularly in jurisdictions moving toward lead-free compositions for environmental reasons.

The handling and regulatory treatment of primary explosives is the most restrictive tier in every explosives regime. In the United States, ATF classification places primary explosives under Class A requirements with additional licensing controls. In India, Schedule I of the Explosives Rules 2008 lists initiating explosives separately from propellants and high explosives. The UK's Explosives Regulations 2014 (implementing EU Directive 2014/28/EU on the harmonisation of laws relating to the making available on the market and supervision of explosives for civil uses) require that primary explosives manufactured, imported, or stored carry CE marking and batch traceability documentation.

Secondary explosives are relatively insensitive to routine mechanical stimuli. They will not detonate from a spark, a hammer blow, or a rifle shot under normal conditions. They require an initiating shock delivered by a primary explosive (typically through a detonator or blasting cap) to set off the sustained detonation wave. This insensitivity is a practical safety feature that makes storage, transportation, and handling manageable at industrial and military scale.

The most important secondary explosives in forensic casework are:

TNT (2,4,6-trinitrotoluene) was the dominant military explosive of both world wars and remains a reference standard for blast yield. Its detonation velocity is approximately 6,900 m/s and its TNT equivalent (the energy basis for blast effect comparison) is, by definition, 1.0. It is used as a melt-cast carrier for RDX in formulations such as Composition B (60% RDX, 40% TNT). TNT residues are detectable in post-blast debris by GC-MS, HPLC-UV, and ion mobility spectrometry.

RDX (1,3,5-trinitroperhydro-1,3,5-triazine; cyclonite) has a detonation velocity of approximately 8,750 m/s and is the primary energetic component of military C-4 (91% RDX in a plasticiser matrix) and Semtex-H (a Czech-manufactured plastic explosive combining RDX and PETN). The United Nations Monitoring Group documented the widespread use of Semtex in sub-Saharan Africa and the Middle East in improvised devices throughout the 2000s and 2010s.

PETN (pentaerythritol tetranitrate) detonates at approximately 8,400 m/s and is used as the explosive core of detonating cord and as a component of Semtex-A. The 2009 Detroit "underwear bomber" (Umar Farouk Abdulmutallab) carried a PETN device; the failed initiation was due to the device design, not the explosive itself.

HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazocane; octogen) achieves the highest detonation velocity of the practical nitramine class at approximately 9,100 m/s. It is used in polymer-bonded explosives (PBX) for precision munitions and in nuclear weapon triggers where the extremely brief, uniform shock front is critical. Commercial availability is tightly controlled; its presence in a device strongly suggests military supply chain diversion.

In the commercial sector, dynamite (nitroglycerin adsorbed onto kieselguhr or other carriers) and emulsion explosives (ammonium nitrate-based) are the dominant secondary explosive types in mining, quarrying, and civil engineering. ANFO (ammonium nitrate fuel oil) is discussed separately in Section 5 because of its large-scale civil and improvised use.

Tertiary explosives (also called blasting agents in regulatory terminology) are the least sensitive tier of the high explosive classification. They require not just a primary explosive initiator but also an intermediate booster charge of a more sensitive secondary explosive to achieve reliable detonation. Ammonium nitrate (AN) in bulk is the archetypal tertiary explosive and blasting agent.

Pure ammonium nitrate is an oxidiser, not an explosive by itself at ambient temperature. Its classification shifts when it is sensitised. ANFO (ammonium nitrate fuel oil), a mixture of 94 per cent AN prills and 6 per cent diesel fuel oil, is the most widely used commercial blasting agent in the world. Mining operations in Australia, South Africa, Canada, and the United States consume millions of tonnes of ANFO annually. Its detonation velocity ranges from approximately 4,500 m/s in unconfined conditions to 4,800 m/s in boreholes. ANFO requires a booster (typically a cast-pentolite or PETN-based booster) to detonate reliably; a blasting cap alone is insufficient.

Emulsion explosives replace the fuel oil with a water-in-oil emulsion carrying ammonium nitrate as the oxidiser phase. This configuration is more water-resistant than ANFO and detonates more reliably in wet boreholes. Heavy ANFO (a blend of ANFO and emulsion) is the standard fill for large-diameter mining blast holes globally.

The forensic significance of ammonium nitrate in improvised devices is well-documented. The 1995 Oklahoma City bombing used a vehicle-borne device with approximately 2,200 kg of ANFO, plus liquid nitromethane as a sensitiser, initiated by a blasting cap assembly from a stolen commercial shipment. The 2011 Oslo attacks (Anders Breivik) used a vehicle bomb with ANFO manufactured from fertiliser-grade ammonium nitrate. In India, the 2006 Malegaon bombings and several IED recoveries in Jammu and Kashmir have involved ammonium nitrate-based compositions, leading PESO to tighten licensing conditions for fertiliser-grade AN procurement and to require tracking documentation for purchases above a set threshold.

Regulatory responses to the misuse of ammonium nitrate have been multi-jurisdictional and consistent in direction. The US ATF and DHS issued the Ammonium Nitrate Security Program rule under 6 CFR Part 31. The EU passed Regulation (EU) 2019/1148 on the marketing and use of explosives precursors, which restricts the sale of ammonium nitrate above 16 per cent nitrogen content to professional users only and requires suspicious transaction reporting by retailers. The UK retained equivalent controls post-Brexit under the Explosives Precursors and Poisons Act 2022.

The classification framework is not academic in a post-blast scene. It directly shapes the investigation sequence: what analytical methods to deploy, what residue targets to prioritise, what physical damage patterns to document, and what supply chain to trace.

For low explosive scenes, the investigator expects deflagration signatures: blackening, scorching, and sooting without the deep crater or radially ejected fragmentation pattern characteristic of high explosive detonations. Black powder residues (nitrate, carbonate, sulfide ions) are collected by swabbing directly or by rinsing with deionised water and submitting the rinsate for ion chromatography. Smokeless powder residues (nitroglycerin, nitrocellulose fragments, stabiliser compounds) are extracted with acetonitrile from debris and analysed by GC-MS or LC-MS/MS.

For high explosive scenes, the primary target analytes are intact parent explosives and their degradation products. Secondary explosives (TNT, RDX, PETN, HMX) are recovered from soil, water, and debris by solid-phase extraction or liquid-liquid extraction and analysed by HPLC, GC-MS, GC-ECD, or LC-MS/MS. The UK Defence Science and Technology Laboratory (Dstl) and the US Army Criminal Investigation Laboratory (USACIL) both publish validated methods for these matrices. Ion mobility spectrometry (IMS) is used for field screening.

For ANFO and AN-based devices, nitrate ion at elevated concentrations is the primary indicator (routine background nitrate in soil must be characterised), supplemented by carbon residues and, where sensitiser was present, other organic markers. Intact AN prills with distinctive prill morphology may survive in debris and can be compared with reference prills from suspected source batches.

The physical damage patterns differ systematically. A high explosive detonation produces a defined seat of explosion with crushed, spalled, or cratered material at the burst point, radially ejected fragments, and a characteristic pressure-time profile (brief, extremely high peak overpressure). A low explosive deflagration in a confined space (a pipe bomb) produces fragmentation driven by the container wall failure, with burn residues distributed across the interior surface of the fragments and a longer, lower pressure pulse. These physical signatures, documented at scene and correlated with analytical residue findings, form the forensic case structure that supports a charge of possession or use of explosives under the relevant national statute.