Linseed oil is a yellowish oil obtained from the dried, ripened seeds of the flax plant (Linum usitatissimum, Linaceae). Linseed oil is a drying oil, meaning it can solidify into a solid form. Due to this property, linseed oil is used on its own or blended with other oils, resins, and solvents as an impregnator and varnish in wood finishing, as a pigment binder in oil paints, as a plasticiser and hardener in putty, and in the manufacture of linoleum.
Linseed oil is still widely used for the finishing and refinishing of furniture and timber products. These oils don’t dry like paint (through the evaporation of a solvent or water). Instead, they dry through the same chemical process that generates fire – oxidation. This process generates heat and in some cases this heat may be sufficient to ignite the material it is on and then catch fire to anything nearby.
Linseed oil can spontaneously combust through an exothermic reaction and rags soaked in linseed oil can catch fire without an external ignition source, caused by the chemical reaction between the oil and oxygen, generating heat. If the heat isn’t dissipated quickly enough, the temperature can rise to the point of ignition – only about 120 degrees.
You would never see a piece of furniture spontaneously combust because the oil oxidizes in open air so the surface never even gets warm to the touch. But a pile of oil-soaked rags can. In every case of spontaneous combustion of drying oils that we have found, the cause has been a pile of oil soaked rags. It seems that, as the oil oxidizes, the rags act as an insulator, allowing the oxidizing oil to become hot enough to cause the cloth to smoke and eventually ignite. The bigger the pile, the greater the possible heat and the greater the risk. Temperature is also a factor. The warmer it is, the quicker the rags can reach ignition temperature.
Industry advice for the safe disposal of linseed oil soaked rags, endorsed by Fire & Rescue Services include:
- Safe Disposal: Do not discard oily rags in a closed container or in a pile where they can't dry out.
- Proper Drying: Spread out oily rags in a well-ventilated area to dry before disposal.
- Soaking in Water: Soak oily rags in water for several days before disposal.
- Use Metal Containers: If using a container, ensure it's made of metal with a tight-fitting lid and fill it with water.
Heads Up!
We have a case study about the spontaneous combustion of linseed oil on our Heads Up website.
For further information please contact Prevention@esfrs.org or call 0303 999 1000