Explosives and Explosions

I. Characteristics of Explosives and Explosions

• An explosion is a very rapid chemical reaction that produces heat and gaseous products

• Explosions produce a large amount of heat (an exothermic reaction) in a very short time period

• Molecular fragmentation converts the solid explosive material into an enormous number of gas molecules which will occupy a much greater volume, further enhanced by the very high temperature of the explosion

I. Characteristics of Explosives and Explosions

• The rapidly expanding gases compress the air creating a physical force known as a shock wave

• It is the shock wave that is responsible for much of the damage associated with an explosion

II. The Three Major Classes of Explosives

1. Low Explosives:• Low explosives are materials that burn rapidly and will only

explode if contained• Examples include smokeless powder and fireworks

(pyrothechnics)• Smokeless powders consist of nitrocellulose and black powder

II. The Three Major Classes of Explosives

2. Primary High Explosives:• Are used as primers or detonators• Are sensitive to shock, heat, and electrical spark• Examples include nitroglycerine, and mercury fulminate &

lead styphnate which are used as primers in cartridges and blasting caps

II. The Three Major Classes of Explosives

3. Secondary High Explosives:• High explosives do not have to be contained to explode• Are relatively stable and safe to handle• Initiation requires an electrical spark, fuse, intense heat, or

sharp blow• Examples include dynamite,

TNT, PETN, RDX, & ammonium nitrate

III. The Explosive Train or Device

• The explosive train is the combination of components needed for a successful explosion (i.e. a bomb)

• An explosive device or train has three primary components:– The igniter starts the event (e.g. a spark)– The primer or detonator contains a primary

high explosive (e.g. blasting cap)– The main charge is a low explosive or

secondary high explosive (e.g. dynamite)

IV. The Role of the Scene Investigator

• The investigator must sort through all of the debris to find any portion of the explosive device and residue

• The debris is examined and any large pieces that do not appear to have been near the center of the explosion are removed

• The remaining debris is sifted through screens and examined for portions of the explosive device or items that show evidence of being close to the seat of the explosion

• Promising pieces are forwarded to the lab

V. Laboratory Analysis of Explosives and Explosive Residues

1. Examination of an Unexploded Device:• The device must first be rendered safe• A detailed description is then made of each component

and how they are connected to form the explosive train• The chemical nature of the explosive ingredients is then

determined, as an investigative lead and possible identification of the manufacturer