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An examination of the precursor chemicals used in the manufacture of explosive compositions found within Improvised Explosive Devices (IEDs)

Chapter 3:Precursor use in IED Manufacture

In order to combat the spread of precursor chemical materials, it is important to understand first the weakness of an IED; the outcome required by the perpetrator; and to observe what achieves success. Predictive threat analysis is important to understand how the terrorist or armed actor will respond to the application of a constraint.

In terms of weakness, the IED tells us everything we need to know if exploited. It relies on an explosive chain of events to deliver an outcome. Break that chain (e.g. the passage of stimulus from detonator to main charge) and the IED fails. Three areas of potential focus are:

  1. restricting access to the means of initiation itself (such as military and commercial detonators);
  2. restricting access to the precursors of detonator and main charge compositions; and/or
  3. ‘dumbing down’ the viability of an explosive through substitution (less reactive/energetic chemicals) and standardization (availability below a certain potency).

From the perpetrator’s perspective, precursors are used throughout the explosive chain if military ordnance is not available, and a number of factors influence choice: the fuels and oxidizers available in their local supply chain; the desired effects and potency of the HME; the HME’s efficiency in terms of cost and destructive effect; the HME’s stability and safety during production and transportation; the ability to replicate the HME created and its effects; and the ability of the security services to trace the purchase and use of those precursor chemicals.

Apply influence to any of these factors and the wind changes in our favour. The starting point is to define what is actually being used and the reasons why, as this influences the response.


The most up-to-date analysis of major global attacks outside war zones using HME was conducted by the National Academy of Sciences (NAS) in 2018 [4]. It was not considered necessary to include war zones since the control of the State in most cases was absent and does not, therefore, reflect any legislative approach. Their research identified a number of large, high profile IED attacks between 1970 and 2016. An update to their research by the author can be found at Annex A with details of fuel, oxidizer, booster and initiator annotated from exploitation (where disclosed). Northern Ireland is excluded in assessment given that there were over 500 recorded incidents of large-scale HME use between 1970 and 2007.

However, what is certain from Operation BANNER – the operational name for the British Armed Forces’ operation in Northern Ireland from 1969 to 2007- is that the preferred HME remained AN, and that improvised detonators were used in <1% of all events [5].

An Afghan and coalition security force uncovered a Taliban weapons cache during an operation in Helmand province, Afghanistan, 2012, containing materials for constructing IEDs, including ammonium nitrate, HME, and detonation triggers. Department of Defence (CC BY-NC 2.0).

Annex A presents a clear trend, very much driven by supply chain availability and shared knowledge amongst user groups. For example, the oxidising agents chosen are oxygen-rich ionic solids that decompose at moderate temperatures and liberate oxygen gas. These materials have been chosen because they have been readily available in reasonably pure form, in the proper particle size and at reasonable cost. They are stable over a wide temperature range, yet readily decompose to give up their oxygen to the explosion reaction. We therefore have a start point in terms of favoured oxidizers. As for fuels, these are extremely common organic compounds such as coffee, spices, sugars or metal powders, thereby making regulation difficult.

Annex A also demonstrates that the most effective and common HME is based on AN, peroxide, or chlorate compositions. The reason for this has been supply chain availability, ease of modification, and power output. Combinations of improvised detonators, military/commercial detonators, boosters and main charges have all been tried (again supply chain dependent) but events in the last decade have seen efficient use of HME compositions incorporating TATP, lead azide, mercury fulminate, DDNP, ETN and potassium chlorate. For example, peroxide-based explosives have been used in 6 of 14 (43%) attacks involving explosives by Jihadist groups in the West between 2014 and 2018 [6].

Recommendations: Focus should be applied on the precursors, fuels and oxidizers actually being used. Predictive threat analysis should be focused on supply chain availability and other potential precursors of interest within a region.

Precursor chemicals for detonators (primary HME)

Initiating substances (primaries) are chemical compounds or mixtures used in improvised igniters

or detonators to bring about burning or detonation. As such, primary compositions are the key to success in the explosion process whereby external impulse is transformed to rapid burning (thrust) or a detonation wave. These substances need to be readily initiated by flame and friction. Only a moderate proportion of the attacks in Annex A have positively identified an improvised detonator. Where they are identified then OPE such as HMTD, TATP or DDNP have been used. AOAV’s consultancy base enjoys access to over 160,000 IED technical reports7 from Northern Ireland, the Balkans, Iraq, Afghanistan, East Africa and Yemen. Expert judgement suggests that 5% of IEDs contain improvised detonators globally, and that use is specific to organisational ‘know how’ and the local supply chain. This figure of 5% should be considered ‘qualitative’ since a fundamental weakness in our knowledge is the limited exploitation that takes place within UN agencies and the absence of a global repository.

The primary explosives of specific interest in HME manufacture are listed in Annex B. Each chemical can provide sufficient impulse to sustain an explosive reaction. The precursors necessary for their manufacture are also listed. Each of these chemicals has a specific role in successful detonation, which is not discussed here. Concentrated acids are common in synthesis.

Recommendations: A global repository is required to determine the physical / chemical make-up of IEDs, which will influence the upstream capacity building measures necessary to prevent their use. Strengthening vulnerable institutions in stockpile management reduces the incidence of IED attacks involving military and commercial explosives. HMA may wish to review its stance on the exploitation of IEDs in its areas of operation.

Sodium azide is widely available for use in emergency inflation systems. It takes the form of coarse pellets, which are usually ignited by an electrical charge, to produce a rapid expansion of gas. If these pellets are dissolved in water and treated with the presence of lead nitrate, dextrin and sodium hydroxide, lead azide crystals are formed. Lead azide is one of the most efficient detonators in the region of 3880 m/s, overly sufficient to detonate the HME most commonly used. There is no specific limitation of use for lead nitrate or sodium azide, although viable alternatives for many applications exist.

Lead Azide (CC BY-SA 3.0)

Examples of the most common HME main charges are shown in Annex C, along with the precursor chemicals used to manufacture them. Velocities of detonation and TNT equivalence are listed for the most prevalent, from which blast radius, fragmentation throw, or effects on structures and people can be estimated. Again, AN, peroxides, chlorates, perchlorates and metal powders are common but there is also divergence towards other nitrate/nitrite compositions (such as potassium, sodium and barium), and potassium permanganate. Again, the catalysts are concentrated acids and experimentation with fuels is considerable.

PRECURSORS FOR PROPELLING AND INCENDIARY CHARGES (PROPELLANT HME) Examples of propellant explosives of interest in HME manufacture are listed in Annex D. There is nothing remarkable here that is not already established.

The chemical precursors identified in Annexes A – D are consolidated in Table 1. We would expect from their prolific use this past 50 years that control measures have been applied, including the oxidizers and more energetic fuels (such as the metal powders which, when used at certain percentage and particle size in HME compositions, are known to enhance explosive performance). As stated earlier, it is impractical to ban, monitor or substitute every fuel or oxidizer that may be considered in IED manufacture, but where viable explosive compositions have been used successfully, every effort should be applied.

Comment: Table 1 provides visibility of other chemical precursors that terrorists may drift towards as the supply chain diminishes.

Chapter 4:The Regulation

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