C4 is made up of explosives, plastic binder, plasticizer and, usually, marker or odorizing taggant chemicals such as 2,3-dimethyl-2,3-dinitrobutane to help detect the explosive and identify its source.
The explosive in C4 is (cyclonite or cyclotrimethylene trinitramine), which makes up around 91% of C4 by mass. The plasticizer is diethylhexyl (5.3%) or dioctyl sebacate and the binder is usually polyisobutylene (2.1%).
C4 is manufactured by combining the noted ingredients with binder dissolved in a solvent. The solvent is then evaporated and the mixture dried and filtered. The final material is an off-white solid with a texture similar to modelling clay.
A major advantage of C4 is that it can easily be molded into any desired shape. C4 can be pressed into gaps, cracks, holes and voids in buildings, bridges, equipment or machinery. Similarly, it can easily be inserted into empty shaped charge cases of the type used by military engineers.
C4 is very stable and insensitive to most physical shocks. C4 cannot be detonated by a gunshot or by dropping it onto a hard surface. It does not explode when set on fire or exposed to microwave radiation. Detonation can only be initiated by a combination of extreme heat and a shockwave, as when a detonator inserted into it is fired.