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 four 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.
Source: United States Code of Federal Regulations, Title 49 CFR §177.848 - Segregation of hazardous materials.
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