Flammable solids, spontaneously combustible and Dangerous when wet
Class 4.1 – Flammable solids
These will burn easily, more so than ordinary combustible materials such as wood and paper. The burning may be fierce and rapid, creating great heat. Some 4.1 are desensitised explosives, e.g. wetted trinitrotoluene (TNT) which would otherwise be in Class 1. Some are self-reactive, and are liable to break down chemically, if they get above certain temperature, or are subjected to shock etc.
They may then decompose explosively, or burn vigorously, or produce toxic gases or vapours.
Class 4.2 – Spontaneously combustible
Division 4.2 goods are either solids or liquids. They will ignite spontaneously in contact with oxygen. They must be kept in airtight packages or as liquids under an inert gas or liquid blanket. Pyrophoric materials will ignite within five minutes of coming into contact with air and are always assigned to packing group I. Other materials will ignite only when in large amounts and after longer periods of time. These are in Packing Group II or III, depending on classification tests
Class 4.3 – Dangerous when wet
Division 4.3 goods react with water, either as liquid or as vapour, and generate flammable gas. This can be ignited by the heat of the reaction. They must be kept in watertight containers, hermetically sealed to avoid the entry of moisture or water vapour