Spontaneous Combustion and Wood Finishing

Charred rags that combusted spontaneously and damaged a
Lake Forest Park home being renovated in January 2016
Photo courtesy Northshore Fire

By Jeff LaFlam, 

Fire Marshal

Northshore Fire Department

Getting ready to begin your wood finishing project - and you wonder if you should be concerned about the stories you’ve heard about spontaneous combustion involving oily rags.

You should be. More than 3,000 structure fires are caused by spontaneous combustion each year.

The Northshore Fire Department has responded to three structure fires over that last several years that were caused by spontaneous combustion. Each fire caused significant damage.

Spontaneous combustion occurs when a material in contact with air can heat up sufficiently (without an outside heat source) to burn.

The oils in some wastes and rubbish can slowly react with oxygen in the air. This reaction creates heat that can build up over time if the wastes are left undisturbed.

Oils from grains and plants are more prone to spontaneous combustion than petroleum oils. When the heat level in a "self-heating material" is high enough (i.e., when the temperature reaches the auto-ignition temperature), a fire may start. 

Residents were evacuated after spontaneous combustion of rags
caused a large fire in Lake Forest Park in August 2017
Photo by Jacob Kimerer

What causes it?

With linseed oil and other oils used to finish wood, including some exterior deck sealers and wood stains, heat is generated during the drying process. This is because these oils do not dry like paint (through the evaporation of a solvent or water). Instead, they dry through the same process that generates fire... oxidation.

For example, rags soaked with vegetable oil in the bottom of a pail could heat up enough to cause spontaneous combustion of the rag.

However, the same oil-soaked rag would not be expected to heat up on a clothes line because there would sufficient contact with moving air that would prevent heat from building up.

It is simple to prevent spontaneous combustion.

1. Materials (rags, etc.) that are prone to spontaneous combustion should always be stored in metal containers with covers in place. These containers should either be filled with water or the container lid must fit tightly to prevent air from entering the container.

2. Soaked rags can be spread out on an exterior, non-combustible surface and allowed to dry completely. The rags can then be properly disposed of.

Always read and follow warning labels carefully! Work safely and your home and family will thank you.