Linseed oil is a natural product that a lot of people think is harmless. It is typically used to refinish or preserve wood. However, if you soak a rag with linseed oil, it can ignite on its own with no spark required.
It's called spontaneous combustion. It happens when linseed oil is exposed to oxygen, and causes a chemical reaction hot enough to burn a rag.
"It is certainly among only a few items that gives off heat when it comes in contact with other natural products like cotton and given the right atmosphere, the right amount of ventilation, it does self-ignite so that makes it extremely dangerous," says Washington D.C. Fire Chief Dennis Rubin.
Linseed oil is responsible for plenty of fires including one in Philadelphia which was one of the biggest high-rise fires in U.S. history.