Pyrophoric materials (solid or liquid) that, even in small quantities and without an external ignition source, can ignite within five minutes after coming in contact with air; or self-heating materials that, when in contact with air and without an energy supply, are liable to self-heat.

Note: Labels used to identify a subsidiary risk must have the class number obliterated.

Also see:

What are the Federal Regulations for Hazardous Materials Labeling?