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Flammable Solid

DOT flammable solid placard

Be sure you're in compliance with DOT placards and labels from Safety Emporium.


A flammable solid is defined by the U.S. Department of Transportation (DOT) quite extensively (see 49 CFR 173.124). Three broad classes are:

  1. Desensitized explosives such as those wetted with sufficient water, alcohol, or plasticizer to suppress explosive properties.
  2. Self-reactive materials that are thermally unstable and that can undergo a strongly exothermic (heat-evolving) decomposition even without the participation of oxygen (air). Certain exclusions apply.
  3. Readily combustible solids. Examples include:
    1. Solids which may cause a fire through friction, such as matches.
    2. Pyrophoric (literally, "fire-loving") materials, those that can ignite with no external ignition source within five minutes after coming in contact with air.
    3. Self-heating materials, those that exhibit spontaneous ignition or heat themselves to a temperature of 200 deg.C (392 deg.F) during a 24-hour test period. (This behavior is called spontaneous combustion)
    4. Dangerous when wet materials, those that react with water to become spontaneously flammable or to give off flammable gas or toxic gas at a rate greater than 1 liter per kilogram of the material, per hour.

Paragraph B.7 of Appendix B of the 2012 version of the OSHA Hazard Communication Standard (HCS 2012), 29 CFR 1910.1200 defines a flammable solid as one that is "a readily combustible solid, or which may cause or contribute to fire through friction."

Additional Info

Paragraph B.7 of HCS 2012 outlines several principles for the hazard classification of a flammable solid and then assigns materials to one of two Categories using the following criteria:

Category Criteria
1 Burning rate test:
  Chemicals other than metal powders:
    (a) wetted zone does not stop fire; and
    (b) burning time < 45 s or burning rate > 2.2 mm/s
  Metal powders: burning time ≤ 5 min
2 Burning rate test:
  Chemicals other than metal powders:
    (a) wetted zone stops the fire for at least 4 min; and
    (b) burning time < 45 s or burning rate > 2.2 mm/s
  Metal powders: burning time > 5 min and ≤ 10 min
GHS flame pictogram

Flame pictograms and other GHS-compliant labels and signs are available from Safety Emporium.

The labeling provisions of HCS 2012 require that Category 1 or 2 flammable solid containers carry the flame pictogram (shown on the right) to provide a quick visual warning of the flammability hazard. The GHS does not require these symbols to appear on the SDS itself, however most responsible manufacturers will do so to preserve the correspondence between a substance's SDS and label.

Because of the special hazards posed by flammable solids (and other dangerous goods), shipment of these materials can be difficult. For example, the US Postal Service will not accept shipments of flammable solids by air, and will only ship these by ground subject to certain restrictions. Likewise, United Parcel Service requires the shipper to be HazMat-certified and has limitations on quantities. All carriers, including FedEx, have special handling fees for such shipments.

DOT has very hefty fines for violation of dangerous goods shipping regulations. For example, in 2014, Amazon was fined $91,000 for shipping an improperly packaged and labeled one gallon container of flammable liquid that sprung a leak during transport. Later that year, they were assessed a $350,000 fine for a leaky package that injured several UPS workers.

SDS Relevance

Always use caution around flammable (or potentially flammable solids). For example, most people do not normally think of aluminum metal as a flammable solid, but when finally divided into dust (or heated to high temperature) it can readily ignite with a fierce flame that can not be extinguished with a typical fire extinguisher. Uranium is another metal that some may not recognize as flammable.

Badger Class D extinguisher

Get your Class D and other specialty extinguishers from Safety Emporium.

If you are using flammable metals (lithium, sodium, potassium etc.) or flammable metal compounds (butyllithium, diethylzinc etc.) recognize that using water or carbon dioxide fire extinguishers can fuel fires of these sorts. You must have a class D extinguisher on hand if you are using these materials. See this reference for details.

Industrial flammable metal fires are some of the most terrifying incidents one can imagine. Some examples include:

Further Reading

See also: combustible, flammable, flash point, water reactive.

Additional definitions from Google and OneLook.

Entry last updated: Sunday, February 2, 2020. This page is copyright 2000-2020 by ILPI. Unauthorized duplication or posting on other web sites is expressly prohibited. Send suggestions, comments, and new entry desires (include the URL if applicable) to us by email.

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