Spontaneous combustion – why? – what’s the reaction that starts it?
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I’m just curious after cleaning up an oil mess today. We know that rags soaked with oils like tung oil and linseed oil can spontaneously combust. But why rags? Why wouldn’t oil soaked wood, or oil soaked sawdust spontaneously combust? Or could it? Can motor oil react the same way? Does anyone know what happens chemically to cause the combustion?
Replies
Oil type critical
It's the oils that dry that cause spontanious combustion as far as I know. I always leave my finishing rags spread out and hanging over the back of my bench stool till they are crispy hard, 40+ years and no problems yet, knock on wood.
hanging them to dry is good
but if you are doing numerous coats in a day, one runs out of room to hang them. nitril goves have bev come quite popular in the lst decade and I go thru quite a few when I am finishing. before I take them off I always ball up the materials that I have been imediatley using and pull one glove over it which in effect creates anice tight ball around the rags and eliminates oxygen from getting to it( the ignition). never had a problem. have carefully followed this for years. always put thenm on a bare spot on the cement flloor just in case never had a surprise. it never hurts to be careful with this stuff. carelessness always costs. it doesn't take long for that stuff to ignite if you are sloppy.
ron
It does work!
I didn't have time to hang out some rags once for some reason or other and I tossed them in my fireplace and closed the glass doors just to be safe before I went to bed. This was a cold fireplace in the middle of summer. The next morning I smelled smoke and when I opened the glass fireplace doors the smoldering rags burst into flame. Oil that has a drying agent in it will catch fire. At one time I had a large popcorn tin inside a larger popcorn tin with a layer of sand in-between and underneath. I would soak the rags in water and seal them inside the inner tin and then seal the outer tin as well until I could get them outside to hang up and dry.
I once heard a story about rags that were saved just before a move. The moving van caught fire while on the road and the guy lost everything in the fire.
I don't use much oil anymore except for Tried and True brand which has no drying agent but I still treat oily rags with great respect. Can't help with the chemistry part of your question even though I was a chem. major many years ago (1957).
Tried and True may have no drying agents (in the form of metallic driers), but these are still drying oils and can still potentially create spontaneous combustion. They, like raw linseed oil, dry slowly, and run less risk than some other drying oils and varnishes, but that's just an artifact of being really slow curing--mostly a negative thing in my point of view. The point is that it is the basic curing process of the oils or varnishes that create the problem--"drying agents" only speed the process.
I absolutely agree with Howard. Spread them out and let the rags dry. Other solutions are just stop gaps. For example, there are air tight containers for such rags in industrial situations. BUT, that only works because in those contexts there is a next step. The rags are collected and then burned. If you have kept the air away then the potential still exists when air eventually does reach the oil.
T&T
SteveS
You are right about T&T, in fact I am so paranoid about oily rags I treat them all with great respect, even motor oil. The secret to using T&T is that it is almost impossible to put down too little and therefore the application rag has very very little on it and the wipe off rags and the burnish rags have almost none. I have two wires running the length of each side of my shop that 25 ft extension cords run on, sliding on caribiners. They make great "clothes lines" to hang rags on to dry.
As it says in the first line of T&T's description of their products, "Tried and True are based in a polymerized linseed oil system.
Polymerized means that the linseed oil is heated to a particular temperature and then cooled. The heating causes partial polymerization (curing) allowing the oil to further polymerize more rapidly when applied to the wood. Heating linseed oil is the old and very unsafe process used to cause linseed oil to cure much faster. It's the basis for the word "boiled" in relation to faster drying linseed oil.
The point is that anything--whether polymerization or adding chemical metallic compounds--added will cause the the more rapid build up of heat as the oil further polymerizes to the point of becoming a solid. Tried & True is just as dangerous boiled linseed oil made by the addition of metal driers.
The basis for the word "boiled"
"Linseed oil dries very slowly, so manufacturers add metallic driers to speed up the curing process. This faster mix is called boiled linseed oil, in spite of the fact that it is not actually boiled. Manufacturers thicken it by blowing air through it, making it apear like a pot of boiling liquid."
The Woodfinishing Book Michael Dresdner Taunton Press 1992
I agree about the danger. I was always careful with rags, even more so when I saw some burst into flames in my fireplace that morning mentioned above.
There are three things needed to start and sustain fire--heat, oxygen and fuel.
When a drying oil dries/cures it cures by taking on oxygen which reacts with the oil. This reaction creates heat. In an enclosed space (wadded up in a pail) the heat is concentrated and unable to be dissipated as rapidly as it is created. Therefore, the heat builds until it reaches the point where it ignites. The rag material burns at a relatively low temperature so it doesn't take much heat to cause combustion.
If the oil is spread out such as on a board, the heat is able to easily escape so it never builds up the point of combustion.
The drying oil most relevant to woodworkers is linseed oil--most particularly "boiled" linseed oil. The chemicals added to linseed oil to speed up the curing process causes the heat building reaction to occur more vigorously causing the heat to build much faster. Raw linseed oil cure much more slowly therefore it does not concentrate the heat faster than it can be dissipated. Real, pure tung oil, and plant oils like walnut and soya are much slower drying so are no real danger. Non-drying oil like mineral oil, motor oil, olive oil, etc. and not a combustion danger either.
So the question is how to deal with rags used to apply or wipe boiled linseed oil or BLO products like Watco, Minwax Tung Oil Finish, most "Danish Oils (all these products contain an accelerated drying linseed oil in spite of the brand name) or any oil/varnish mixture home brew. In my opinion, the best procedure is to open up any oil soaked rag and put it over a clothes line (preferably outdoors) or lay it opened it up on the lawn or driveway. This dissipates the heat. Once it the rag has dried and become stiff, you can dispose of it in your normal garbage. Some put the rags in a bucket of water but I don't recommend this. The rags will not burn in the water but when you take them out and they dry out, they still contain un-oxidized BLO. It can again start to heat up. In addition, you now have bucket of contaminated water you need to deal with.
Three things...
I agree completely with Howie on this one. Rags are probably the most dangerous because of several factors, they hold lots of oil, therefore more fuel, and have large surface area (where drying takes place) therefore more heat generation, sawdust also holds a lot of oil but because it packs tightly it tends to choke off the oxygen needed to start and maintain the combustion, and as Howie said, a board actually has very little oil and it's spread out over a broad surface and therefore allows heat to escape. Although I've never tested it, I suspect a mixture of wood chips and sawdust with oil might be just as dangerous as rags.
The big issue with drying oil is that the drying process can be accelerated by higher temperatures. So as rags dry, the oxidation of the oil gives off heat, if the heat can't escape, temperatue goes up, temperature goes up and the remaining oil dries even faster, faster drying, more heat generated, higher temperatures, faster drying, etc. So the process is quickly self-accelerating.
I don't always use oils, but when I do usually toss rags into my woodstove and burn 'em under controlled conditions.
The most interesting woodworker in the world [this is a pop culture ref, not intended to be a factual statement]
Steve
Oily Rags and Spontaneous combustion
I find this discussion really fascinating and I think I have been storing oily rags dangerously incorrect. I have done this method for years, using an empty paint can, gallon, quart, size doesn't matter as long as the container is metal and has a lid. I especially like the 8 oz size, after completing a project stuff the rags into the container, seal and dispose. I stuff the rags into the container and seal the lid tightly. After reading others discussion I suspect I have been doing a no no. I haven't as yet had rags burst into flames when I reopened the can, but it sounds as though this is possible. I thought with limited oxygen inside the sealed can, drying stops therefore heat cannot build up. It sounds as though I am wrong. I started this trick 40 years ago, when my uncle showed me how he disposed of oily rags. I have used this method even when I painted a project or my house. Comments please.
Best disposal is wet
Dad taught me to wring them out and then dip them into a solution of 1-gallon of water, 1-cup of borax, and about 6 tablepoons of dish soap.
The dishsoap, helps the water pentrate the oil, and emulsifies the oil. The Borax is a fire retardant.
A friend of mine had threw some linseed oil soaked rags that he tossed in the garbage can in his garage, they caught fire melted the garbage can and smoked the house up, lucky for him the garbage can was in the middle of the garage floor and the fire did not spread. He had just bought the house less than a year before. I always spread oil soaked rags out on the grill of my webber then if they did catch fire they would not cause a problem. The reaction that causes this is the oil curring this causes heat and a balled up rag can produce enough heat to start burning. Best to be careful with this.
A Little Experiment
Here is something you can carefully try. Get a kitchen food thermometer, a metal coffee can (or something else of similar size) and a rag wetted with boiled linseed oil or any oil/varnish mixture like Watco, Minwax Tung Oil Finish or home brew. Wad up the oil soaked rag and put the thermometer probe end into the center of the wadded up rag. Put the rag and thermometer in the can and put the can in the middle of your driveway. Don't cover the can. Now give it some time and monitor the thermometer. You will see it begin to register a heat rise. See how high the temperature gets (to the limit of your thermometer).
This experiment will give you a real world appreciation for the risks with drying oil "spontaneous combustion".
I have an old tupperware container with the tight fitting lid. I fill it half full of water, put all my rags, disposable brushes, etc. in the water and seal the container. After a day or so, I will take them outside and lay them on something non-combustible until they dry. I'm really cautious with this type of thing as I saw some combust in a high school shop many, many years ago. Not a pretty sight.
Spontaneous combustion or spontaneous ignition
Spontaneous combustion (or ignition) is a serious topic that goes under the radar far too often. Based on my research here are quick answers to your quesions; an oxidation process is the cause of spontaneous combustion; rags are susceptible because of the surface area for oxidation; sawdust is a potential material for spontaneous combustion; petroleum products don't oxidize and are not subject to spontaneous combustion.
Here's the most concise explanation and prevention description I have found:
"Spontaneous combustion can occur when towels, rags, booth filters, paint scrapings, steel wool, masking and other materials wetted or saturated with these products are not handled or disposed of properly. Saturated rags, filters and other materials can combust spontaneously when the oils or resins generate enough heat through auto-oxidation to exceed the ignition temperature of the rags or other materials. Spontaneous combustion can begin without any flame, spark, heat or other ignition source. Saturated rags, filters and other materials contaminated with the coating material should not be disposed of in regular waste containers. To dispose of these materials properly, thoroughly wet the contaminated materials with water and place them loosely inside a separate, sealed metal container."
http://www.hytechsales.com/spontaneous_combustion.html
Here's the most understandable technical description:
http://www.nrc-cnrc.gc.ca/eng/ibp/irc/cbd/building-digest-189.html
I have always been cautious about oily rags since a teacher in a chemistry class did a demonstration. He soaked a rag in oil and wadded it up and put it inside a glass container on his desk. The top of the container was not sealed, so air was able to get in. Then he continued his lecture and suddenly the rags burst into flame. No other ignition source. It's a lesson I have never forgot. I always lay my rags over the edge of a trash can so that there is no overlap of the fabric. After they are completely dry I throw them away.
Jim
Quantity of Oxygen
Curing does not need very much oxygen. Burning needs a lot more.
Consequently, if one puts oil-soaked rags in a sealed container there can be enough oxygen present to cure the oils and raise the temperature but not enough to support combustion. If the rags get hot enough then when you open the container........
An analogous situation is when there is a fire in a closed room. We are told not ot open the door because this will let in fresh oxygen and whoosh! Suddenly the problem is a LOT more serious.
spontanious combustion
Plant based oils such as tung, or linseed dry though polymerization, which means the formation of cross-bonds between the oil molecules. This consumes oxygen, and produces heat. Petroleum oils do not do this. If the heat can easily dissipate the there is no problem, but if the rags are piled up heat can reach combustion levels.
A good illustration of this is to make a pile of freshly cut grass, and leave it for a few days. Don't stick your hand into the pile or you might burn yourself. Farmers must be very careful in stacking bales of hay that is overly moist. Many barns have burned down from moist hay. In these situations there is not polymerization, but bacterial action , again using oxygen.
Tom Higby
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