Petrol Bombs and Incendiary Devices

Petrol Bombs

Prior to training to become a forensic fire investigator I thought that all bottles containing a flammable liquid with a bit of ignited fabric attached to them was a petrol bomb. It turns out that I was wrong in that assumption and also that many others make a similar mistake. There are quite careful definitions of a petrol bomb as an explosive substance and as not all incendiary devices can be categorised as petrol bombs they are not considered explosive substances.

A petrol bomb is an ‘explosive substance’ under the Section 3 (b) of the 1883 Explosive Substances Act. There is a Court of Appeal stated case of Regina v Bouch [Judgement delivered on the 15th of July 1982] which states that a breakable container holding petrol and with a wick of absorbent material inserted into its neck, which when lit and thrown causes a pyrotechnic effect constitutes an ‘explosive substance’ in Section 9 of the Explosives Substances Act of 1883.

In essence because a petrol bomb produces an instantaneous fireball when triggered by being thrown there is a pyrotechnic effect (similar to a flare) and so it can be defined as an explosive substance.

The reason for the pyrotechnic effect is that the breaking and dispersal of the liquid petrol within the container occurs simultaneously with the ignition of those dispersed petrol droplets. This sudden conversion of the liquid from a confined volume into an aerosol of much larger volume and the associated expansion caused by the heat produced due to the ignition of the petrol droplets is attended by a pressure force which can be violent depending on the constitution and circumstances.

As we can see the criteria for a petrol bomb and thus any used or unused incendiary devices to be considered an explosive substance is very clearly defined. However there are several ways in which an incendiary device can be made, they may not produce the same explosive effects as a true petrol bomb but they can be equally as effective in cause panic and fire damage.

Sometimes additions are made to petrol bombs; such as dissolving polystyrene or swarfega into the liquid petrol to cause the liquid to stick to the target, nails or other heavy objects for ballast to improve flight characteristics or sugar and soap flakes to try to enhance the burning characteristics. Some of these alterations to the petrol bomb cause it to no longer be classified as such.

Incendiary Devices

If any part of a petrol bomb as defined above were replaced with a material or substance that did not meet with the explicit criteria then it would no longer be an explosive substance for the purposes of Section 3 (b) of the 1883 Explosive Services Act. For example though white spirit is a commonly encountered ignitable liquid it will not produce a pyrotechnic effect when put into a breakable container with an ignited wick though the neck and thrown. However the white spirit will ignite when the container breaks and continue to burn this would therefore constitute a viable incendiary device and could still cause a reasonable amount of fire damage to the person or building at which it was thrown.

There are other variations which would mean that the device would not meet the requirements to be described as an explosive substance under the 1883 Act but which would still have similar effects. For example if the absorbent wick was not inserted through the neck but was instead attached to the outside of the breakable container then this would not fulfil the criteria. However if an alkaline substance, such as potassium nitrate, were dissolved within the liquid petrol inside the bottle and an acidic substance, such as concentrated sulphuric acid, were absorbed into the wick adhering to the outside of the bottle these two substances would react when the bottle breaks and the heat produced would ignite the aerosolised petrol. Though this would not be considered and explosive substance under the 1883 Act it would still produce a fireball and be a very effective incendiary device. This type of device is safer to handle in use and would not enable the target to see the thrower as there would be no visible flame prior to the container breaking on contact.

If a container were used that did not conform to the ‘readily breakable’ requirement, such as a plastic bottle, then this would not comprise an explosive substance but could still be an effective incendiary device. As the vessel will not break on contact there will be no fireball effect however as the container lands the ignitable liquid will spill and be ignited and so a pool fire would result. Again not an explosive result but certainly an effective incendiary device.

Examination of Petrol Bombs and Incendiary Devices

If devices are found unused and intact then they are usually easily identified as comprising either a petrol bomb or an incendiary device. Each item would be examined visually to ensure that the criteria for a petrol bomb is filled and if it is not what the differences are. If a device has been thrown and so is no longer intact a visual examination will still take place however there may be a degree of reconstruction to enable the scientist to determine whether or not the wick was inserted through the neck of the container.

The liquid within the container or the residues of the ignitable liquid on the remains of the container would be analysed to determine whether or not it was petrol. The identity of the liquid would enable the scientist to determine whether or not the device was or could have been a petrol bomb as defined under the 1883 Explosive Substances Act or if it was an effective incendiary device.

The composition of the wick and the type of container used can be used to provide intelligence information about the potential manufacturer of the device and links to other devices recently found. Depending on the type of container used there is also potential for DNA or fingerprint evidence to be identified on it.

If a specific set up of an incendiary device has been created, for example a device is designed to be triggered by a mechanism other than throwing, it is possible to set up a suitable reconstruction to determine whether or not the specific arrangement in the case could have resulted in the activation of the device and whether or not the device, once triggered could have had an incendiary effect.

In summary a ‘petrol bomb’ isn’t always a petrol bomb but that doesn’t mean that it isn’t an effective incendiary device. All devices have the potential to cause incendiary effects and these can be tested.