Class 4.1: Flammable Solids
4.1 Flammable Solids: Solid substances that are easily ignited and readily combustible (nitrocellulose, magnesium, safety or strike-anywhere matches).
Class 4.2: Spontaneously Combustible Solids
4.2 Spontaneously Combustible: Solid substances that ignite spontaneously (aluminium alkyls, white phosphorus).
Class 4.3: Dangerous when Wet
4.3 Dangerous when Wet: Solid substances that emit a flammable gas when wet or react violently with water (sodium, calcium, potassium, calcium carbide).