Attention woodworkers: Don’t wad up rags soaked with oil-based wood finishes! Don’t toss them into a pile! Don’t throw them into a trash can!
Oily rags can spontaneously combust!
Shown above is a bottle of Klean Strip boiled linseed oil, but the same advice applies to many different types of oil-based wood stains and finishes.
When certain oils and oil-based wood finishes dry and cure, the reaction is exothermic, which means they give off heat. The combination of heat that builds up and cannot dissipate fast enough, flammable materials, such as rags, and oxygen, can lead to the real and dangerous risk of spontaneous combustion.
Basically, under certain conditions, oily rags can self-ignite. And once that happens, they will catch fire, burn, and the fire can then spread.
Oily rags have started fires.
TL;DR: Common woodworking oil-based stains and finishes create heat as they cure. Wadded up oily towels and rags can trap this heat, causing temperatures to increase faster than the buildup can dissipate. Once the temperature gets hot enough, everything can ignite and start a fire.
According to the NFPA, there are an average of 1,700 home fires per year caused by spontaneous combustion, and an average of 900 home fires per year due to oily rags.
Spontaneous combustion is very real, and it can happen to you.
READ THE CONTAINER. Follow manufacturers’ recommendations!! There will usually be warnings, as well as disposal guidance.
Some oils don’t cure and dry, and will not self-heat. If you’re unsure, treat every oil-based wood stain or finish as if they could potentially spontaneously combust.
I am NOT an authority on this. It is your responsibility to conduct your own research and determine how you could or should minimize the risks of spontaneous combustion.
Here’s a reference page from the National Fire Protection Association (PDF). Here’s what they say to do and not do with rags soaked with paint or stain:
Never leave cleaning rags in a pile. At the end of the day, take the rags outside to dry.
Hang the rags outside or spread them on the ground. Weigh them down. Do this so they do not blow away. Make sure they are not in a pile. Keep them away from buildings.
Put dried rags in a metal container. Make sure the cover is tight. Fill the container with a water and detergent solution. This will break down the oils.
Keep containers of oily rags in a cool place. Keep them out of direct sunlight. Keep them away from other heat sources. Check with your town for information on disposing of them.
The UL also has a reference page on this, and they also advocate that you Dry, Dunk (in water), and Dispose of oily rags.
Manufacturers’ advice can vary. For instance, Klean Strip’s boiled linseed oil container advises that users should hand-wash rags with water and detergent immediately after use and prior to storage or disposal.
Tried and True says this:
Place all oily rags in a jar or bucket of water. Then store in a plastic bag/bucket, seal and dispose of them in the garbage. DO NOT LEAVE OILY SOAKED RAGS OUT WITHOUT DISPOSING OF THEM PROPERLY. If you leave a rag soaked with linseed oil out, the rag will begin to oxidize and heat up, leading to a risk of spontaneous combustion if left for a long time with an adequate heat source. Avoid this by having a water-filled jar or bucket ready with you while you are working for disposal.
While recommendations can vary, I generally follow the “dry, dunk, dispose” method. Some brands recommend a “dunk, dispose” method, others advise “dry, dispose.”
Disposal methods can vary, depending on location, and possibly volume; most oily rag fire prevention advice that I’ve read seems to be aimed at hobbyists and homeowners, and not higher volume waste producers such as professional, commercial, or industrial woodworking shops.
If you don’t have an old can, fill a resealable bag with water, submerge the rag inside and seal the bag.
Glass jars are usually advised against, I’m guessing because the contents can heat up in sunlight.
You can also buy self-closing metal cans (Justrite 6-gal via Amazon), but they’re generally meant for temporary storage and have “empty every night” labeling.
In my opinion, the best way to minimize the risk of oily rag spontaneous combustion is to understand what can happen and why, and by forming and adhering to a plan based on fire prevention guidance.
TL;DR: Fire prevention authorities and oil finish product manufacturers offer guidance on how to minimize the chances of spontaneous combustion. Most recommendations seem to involve i) preventing the buildup of heat by creating conditions that maximize heat dissipation during drying or curing, ii) preventing oxidation and combustion through the use of sealed containers, or a combination of methods.
It’s my understanding that DRYING helps to limit heat build-up, DUNKING in water helps to seal out oxygen (presumably to prevent residual curing or limit combustion), and DISPOSAL is self-explanatory.
Have a plan for each step of the way.
Personally, the rare times I use an oil finish, I dry my rags flat on the driveway, weighed down by a rock. Once dry, I dispose of the rags in a sealed water-filled freezer bag. The last time I checked, the dry rags are considered household waste. If I ever have a big project, I’ll likely store them in a sealed water-filled metal can (with a few drops of [dishwashing] detergent as NFPA-recommended) and take them to a collection center.
If in doubt or you want to learn more, contact a fire prevention authority or the manufacturer of the product you intend to use for guidance.
Why did I decide to post about this today? Another unfortunate woodworker learned about oily rags and spontaneous combustion the hard way: