HAZMAT Class 4 Flammable solids

Flammable solids are any materials in the solid phase of matter that can readily undergo combustion in the presence of a source of ignition under standard circumstances, i.e. without:

Division 4.1: Flammable Solid

Flammable solids are any of the following three types of materials:

Division 4.2: Spontaneously Combustible

Spontaneously combustible material is:

Division 4.3: Dangerous When Wet

Dangerous when wet material is material that, by contact with water, is liable to become spontaneously flammable or to give off flammable or toxic gas at a rate greater than 1 liter per kilogram of the material, per hour, when tested in accordance with the UN Manual of Tests and Criteria. Pure alkali metals are known examples of this.

...
Wikipedia