Rags that are contaminated with vegetable or animal oil are susceptible. Coal , especially soft coal in dust form, is liable to ignite spontaneously. The principal conditions believed to affect susceptibility are fineness of particles, absorptivity for oxygen, moisture, presence of pyrites, height of pile and temperature of the environment. Charcoal made from hardwood appears to be susceptible. Spontaneous heating occurs very readily in fresh charcoal, and the more finely divided it is, the greater the hazard.
Carbon black is formed by incomplete combustion of natural gas or liquid hydrocarbons.
Lamp black is formed by burning low-grade heavy oils with insufficient air. Both are fine carbon particles susceptible to spontaneous heating, especially if they are freshly prepared. Wood subjected to prolonged periods of heating and cooling cycles may ignite spontaneously. The alternate heating and cooling processes may chemically decompose the wood to produce pyrophoric carbon, which oxidizes rapidly. Many incidents of spontaneous ignition have occurred with wood in contact with hot surfaces such as hot water and steam pipes.