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.
Lowe’s (and other retailers) also sell empty 1-quart and 1-gallon, in case you don’t have your own sealable metal containers.
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.
NFPA Guidance: Safety with Oily Rags
UL: Dry, Dunk, Dispose to Safely Treat Oil-Soaked Rags
Auto-Ignition Demonstration, by the Wilmette, IL Fire Department
Why did I decide to post about this today? Another unfortunate woodworker learned about oily rags and spontaneous combustion the hard way:
I have been dealing with this issue for 30 years.
Newbies don’t know.
But what’s worse is professionals don’t know.
Seen too many fires due to this.
This is why I don’t use any oil based products in my shop any more. Water and alcohol based finishes do the job for me.
This is why they make those fireproof bins with shutting sealing lid for dirty flamable rags
Good info for anyone who doesn’t know, good reminder for those of us who do. I also usually go with the “lay outside under a rock” method. Same goes for disposable applicators, chip brushes, etc that might be soaked in oil products, or cups of extra resin that heat up when kicking off. After a couple days when they’re bone dry they go in a bag in the trash. At home, if the weather is crappy, I’ll spread them out flat on the swept-up cement floor away from anything else. If I’m using oil-based products at home I’m pretty much done working after application, so last thing is spread out the rags. Never had any problem with flat rags on cool concrete.
Koko The Talking Ape
Well, I think it needs to be made clear the distinction between materials that are very flammable (like rags soaked in turpentine, paraffin or mineral oil), and materials that will spontaneously self-ignite (like rags soaked in linseed oil.)
Linseed oil and a few other oils (like tung and I think walnut oil) are “drying oils,” meaning they, INCLUDING products containing them like varnishes, “oils”, oil paint and other wood finishing products, will combine with oxgen over time BY THEMSELVES, hardening into a film. The process releases heat, but spread out over a surface of a piece of furniture, the heat is negligible. But concentrated in for instance a wadded-up rag, it can create enough heat to ignite itself or other flammable materials.
So for those, saafe disposal means either oxidizing them completely so the reaction won’t continue, or depriving them of oxygen. The first you can do by just spreading the rag out on a concrete floor and letting it completely harden (which might take a while.) I like that better than depriving them of oxygen by doing things such as submerging them in water, because water can leak out or evaporate, leaving in effect a fire bomb on a very slow fuse. The rag with hardened linseed oil is harmless (though still mildly flammable) and can be thrown away like paint-stained clothing.
That’s different than rags soaked with a non-drying oil like mineral oil. These won’t generate heat on their own, and won’t spontaneously flash into flame unless something heats them past their ignition temperature. Flames or even sparks can do that, and measures should be taken to prevent that, but they will not ignite BY THEMSELVES.
So for those items, you can leave them out to let the volatile components evaporate, if any, then toss. If you like, you can cover them in sand or seal them in an airtight metal container, and that’s relatively safe because they won’t ignite on their own.
So it’s important to know what is the particular oil in question. If it’s some kind of wood finish like varnish or oil-based paint, it’s best to assume it’s contains a drying oil, and let it oxidize in a safe, isolated spot as Stuart says (or use a paint hardener or some other way to neutralize it.) For other flammable but non-self igniting liquids, it’s a little easier.
Am I wrong? Did I miss anything?
I did write in the post:
I don’t want to be any more specific because I feel it’s better for woodworkers to read product labels rather than to trust anything they read online.
When fact-checking something for this post, I read on an artist’s forum where someone told others they dump linseed oil rags into a box and wait for everything to dry before they toss into the main trash. That’s terrible advice and exactly what could cause a fire.
Mineral oil is a non-drying oil that is not prone to self-ignite. But the list of self-heating drying oils and finishes is much longer.
Most woodworking oil-based finishes will dry, cure, or polymerize, and are prone to self-heating and at risk for spontaneous combustion.
There are exceptions.
So, yes, the distinction between spontaneous combustible and non-self-heating oils is important, but it’s a distinction that I leave readers to make for themselves.
It’s good practice to know the nature of the finish you are using, rather than relying on someone else to tell you the difference. There are also so many different varieties out there that I didn’t want to risk someone getting the wrong impression and mistreating the 1-in-1000 exception for a particular blend of components.
First job was in a paint factory, this was written in large lettering on and around every single garbage and disposal area. It applied to all oil based products.
What about tack clothes used in wood working to lift off the last traces of sawdust? It doesn’t seem to dry out.
Lee Valley’s tack cloths are “resin-impregnated.” (https://www.leevalley.com/en-us/shop/tools/supplies/finishing/20060-tack-cloth?item=56Z2001)
I have never seen any cautionary labeling regarding off-the-shelf tack cloths.
Someone was reading Reddit Woodworking…..
Are you talking about tack cloths or the post topic inspiration? If the latter, I included a link at the end of the post to the Reddit thread that absolutely did prompt me to post this topic ASAP.
Oily waste cans were a fixture in our cabinet and metal fabrications shops.
Our procedures required that they be emptied every day. Some old woodworking books recommended that oily rags be hung out (as on a clothesline) separately to dry. We would soak them in water and bag them up. While we never had an OSHA citation on this – nor did we ever have a dumpster fire – I’m guessing that there may be other or even better procedures for dealing with oily rags.
That’s the same one I linked to towards the end of the post.
They’re often sold for temporary storage in commercial/industrial spaces, but then you still need a disposal procedure in place for where rags can go afterwards.
Individual users that generate small amounts of oily rag waste volume can still use them, but there’s not much guidance out there – likely due to liability concerns.
Labeling says “empty every night,” but this could be due to recommended safety practices or OSHA regulations that require said practice.
I was reluctant to talk too much about safety cans, as they’re not a “use this and you’ll never have to worry about oily rags” type of product. They’re a “put your oily rags here rather than the trash can until you can neutralize them at the end of the day” type of product.
I have also seen clothesline recommendations, but official advice seems to stick more to “place flat on cement or driveway with a weight” recommendations.
The problem with hanging on a clothesline, should the rag catch fire, it can easily fly off or drop on a combustible surface, such as a pile of leaves or mulch. A clothesline can also burn through and break, and now you have a swinging flame on a rope, or a fuse leading to the house. Of course, that’s an extreme case, but possible.
When I worked in safety long time ago, it amazed me how many professional shops I visited did not have any procedures dealing with oily rags. Thanks, for the reminder.
Fireball Tool did some experimentation on this – they indeed combust in a trashcan when soaked. https://www.youtube.com/watch?v=Cw5MqfrxM9g
one – fully agree that care has to be made. I would like to add – this isn’t just a woodwork issue.
Car and equipment MX – same issues. bunch of oily rage or solvent rags. While it might be trained a bit more in the auto and equipment MX world you still see people screw this up and have issues.
Regardless I recommend every garage have a bag of sand handy. bag, bucket, etc. and yes a fire extinguisher but sand is also useful
Yes, that is a very good point, it’s not just oil paint or linseed oil which has this risk. Every machine shop I have worked in had steel safety disposal cans for oily waste. I have seen them at many mechanic’s shops as well.
Sand has uses but I’d say it’s no substitute for an A-B-C rated fire extinguisher. That can be used on general materials, electrical fires, and flammable liquids like oil/grease/fuel, etc.
Yep, it a lot more than woodworking, I worked in a printing plant in the 80s and we had a safety issue/procedure with rags from both the rags with residual ink on them, and ones with solvent on them. We had several industrial safety cans that were about 5 gallons. The cans were lined with heavy plastic bags, and then had water in them. They also added a heavy detergent that smelled like Lestoil (sp). The place had a rag service that removed the bags every several days and processed them.
Not only oily rags will exhibit spontaneous combustion, so will fine wood chips and saw dust if they become somewhat damp, not wet. I witnessed this at a local non-chain lumber yard that would custom cut lumber for customers. Since the set up was not part of a shop, employees would just throw the sawdust just outside one of the large doors. Most of the time the saw dust would wind up being spread around the gravel lot, but at least once the mound got large enough and a trickle from a roof valley splattered enough water to get the breakdown going.
Any set of materials (reagents as a chemist might call them) that can interact to produce an exothermic reaction can result in fire. Finely divided materials (be they organic or even metals) just help promote chemical reactions. Add an oxidizing agent – or catalyst to speed the reaction – plus an energy source (could be a simple as sunlight, heat of a room or a static discharge) and the reaction will be set off. We all recognize wood as combustible – but aluminum dust or iron filings are also an issue. Storing pool chemicals (e.g. chlorinating agents) and fertilizer improperly can also be a recipe for disaster. Flour, rice or wheat all seem pretty benign sitting in a bag in the cupboard – but grain elevator fires and explosions can tell a different story,
Yep, wet grain can spontaneously combust (I have a relative who was in the grain moisture meter business).
Powdered aluminum makes great rocket fuel. It’s also hazardous to human lungs. Both of these issues are important to PBF (powder bed fusion) 3D metal printers – they use inert gases (mainly nitrogen) in the print chamber, and you have to take good care of the powder. (Resin for 3D resin printers is also very nasty, but not explosive).
Aren’t iron filings and sawdust are the primary ingredients in “Hot Hands”? I guess in the right ratios even that could get hot enough to combust.
I’ve had good luck spreading rags out, and weighing them down if necessary, to dry a day, and then burning them in a burn barrel or other suitable metal type burning apparatus. That way I don’t have to worry about them anymore. YMMV. I can see green sawn wood piled up combusting. Just like grass, just like hay.
Back when I started in business burn barrels or more formal incinerators were not uncommon. But in my locale and many others they now are forbidden for environmental reasons. We looked into what to do with waste wood at our cabinet shop. Some suggested we install a wood burning stove in the breakout room. Local ordinances and our insurance underwriters quashed that idea. We did give away hardwood scraps (like maple and oak offcuts) to local scout troops for camp fires.
Dry, I attach to My Fence.
Dunk. After Drying I put the Rags in an old Coffee Can and fill It with Water and seal the Lid.
I’ve followed this procedure since the mid 70’s. I watched a neighbor’s house burn down because of oily rags. Also, I worked across the street from a cabinet shop who just disposed of in the trash. When the trash truck picked up their trash the truck caught on fire.
I just throw them on the burn pile.
Thanks for the post. I’m guilty of using old glass jars with metal lids filled with water, (I might have a salsa problem) but after reading this, I’ll make 2 changes. Adding the soapy water cleaning step, and storing in a metal container instead of a glass jar.
If they’re full of water, that should provide at least two safeguards against ignition. I would have thought that was safe – but I am not a scientist, fire marshal or anything else that would qualify me to state facts rather than opinions here. 🙂
Slightly off topic, but I personally like the theory that aliens have never invaded Earth because we have an atmosphere with an objectively terrifying amount of oxygen. But seriously, don’t play around with BLO. I left a rag out on some concrete one time to see what happened. Lit itself on fire.
Years ago, knowing better but not thinking, I put a pile of linseed-oil soaked rags in the laundry tub of a compact stacked washer-dryer that was in a closet and left for an event two hours away. Halfway there I realized what I’d done, turned around and went straight home. The rags were very hot. I don’t know how hot because we didn’t have IR thermometers back then. I turned on the water (“dunk”) and put them through a rinse cycle then disposed of them – I don’t remember how, but I”m sure I was cautious about the method, having a pretty close call.
That was in 1985 and I have treated oil-soaked rags with utmost respect since that time. This post is a great reminder and a great lesson for anyone who isn’t aware of the risk.
Also, just don’t use linseed oil for anything. It ain’t 1776 anymore.
A lot of wood finishes and stains have similar warnings, not just linseed oil.
Stuart is correct, any kind of “drying oil” has this risk. And many wood finishes, paints, etc, contain not only drying oil, but also chemical additives that accelerate the curing reaction and therefore generate a higher risk of combustion.
While Linseed oil is far from perfect I think it has a lot of advantages that other finishes don’t have. If I finish something with linseed oil and the finish needs a touchup or the wood gets damaged and needs repair, the process is very simple: just apply more. The finish is soaked into the wood, the repair or touchup blends right in with minimal effort. On the other hand, something like polyurethane or lacquer or varnish is a hard layer on top of the wood. Touchups require sanding first and then re-applying the finish, and it’s often tricky to get repairs to look right afterward. Natural oils like linseed, tung, teak, etc, don’t have that problem. The presence of water can cause ghosting if it gets underneath a lacquer or varnish, that never happens with oils because the oils soak into the wood.
It also has a tactile advantage. The surface is a bit grippy for lack of a better word. I really prefer that over finishes for things like rifle or shotgun stocks or wooden tool handles. If your hands are wet due to weather or sweat you can still get a good hold on whatever it is you’re holding, while a varnished finish is a lot more slippery.
Most men my age (58) learned this in junior high wood shop.. Sadly I don’t think my local high school has wood shop or any industrial arts classes anymore 🙁