Old newspapers tend to turn yellow and brittle with age because of a relatively slow oxidation rate. Since paper is a carbohydrate, the yellowing is a result of the oxidation or slow burning of carbon and hydrogen. Oil is another carbon compound that oxidizes in contact with air. When oily rags are left to accumulate around the house, for example, the heat of oxidation may not be able to escape and after enough heat accumulates, the oily rags may be set aflame. This process is known as spontaneous combustion.
Ignition is the process of initiating self-sustained combustion.
In a chemical context, all elements that are not in their highest oxidation state can undergo oxidation. Some metals oxidize so slowly that the heat generated during oxidation is dissipated so that the ignition temperature is never reached. Certain metals, such as zirconium and uranium, are described as combustible metals because of their potential for rapid oxidation under appropriate conditions. The use of magnesium as a flashlight powder for photography is a common example of a combustible metal. This temperature may be defined as the lowest temperature which will cause a flame or glow, to start and spread through the substance or material.