How can spontaneous combustion begin in stored hay or oily rags?

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Spontaneous combustion occurs when combustible materials, such as hay or oily rags, undergo a slow oxidation process while stored in bulk. This process—which may involve bacterial fermentation or atmospheric oxidation—generates heat that cannot easily escape from the center of the mass.1

As this heat accumulates, the internal temperature of the material gradually rises. If the temperature reaches a critical threshold, it triggers the outbreak of fire without the need for an external heat source. To prevent this, farmers often dry crops before storage or use forced air circulation to inhibit fermentation, while soft coal is sometimes wetted to suppress aerial oxidation.1