Hi Chris,

if your rag was used to clean vegetable oil (with unsaturated fatty acids) it might undergo spotaneous combustion, for instance see here, or here. When organic conpunds oxidise heat is produced.

Thor

Hi Chris, Any combustible material if finely divided(has it's surface area greatly increased) will much more easily ignite. So if a combustible material is soaked up with rag then it offers a huge surface area to the air(oxygen) and the chemical process of oxidation(burning) will not only start much more easily but will also be much more rapid.

When a young and possibly foolish child, I found that in being taught basic chemistry & having an inquiring mind that certain chemicals readily available from the grocer & hardware store could be mixed to create an interesting explosive. Applying theory from class, a pestle and mortar could reduce the two powders to a very fine dust, & in this condition the two once mixed could be fired by merely hitting it with hammer due to the intimacy of the fuel and oxidizing agent.

On a lighter note a bottle of Co2 of any sort is the best form of fire extinguisher in a small workshop since it will make no mess or damage tools Etc. A 3' length of reinforced hose attached to the bottle will serve to administer the gas and can be passed through a small gap to fill a room or building. In using Co2 one needs to remember that YOU also need to be able to continue to breath. Once caught in a cellar when a Co2 bottle burst I only had seconds to get out. On liquid fuel fires the result is spectacular, an invisible hand snuffing the flame out

I hope nobody need to use this information.

Good Luck Noel

Not spontaneous combustion but following on from Noel posting many finely divided materials will burn fiercely or even explode if treated carelessly .

Common problem prone materials are coal dust , flour and mill puff .

Has some practical uses as well – finely divided coal dust is used in some power stations where it burns both very hot and more cleanly than lump coal . Also you don’t need a grate system – the coal is burned more like paraffin in a blowlamp .

MikeW

I believe there is more than one way that spontaneous combustion can take place. Damp hay will heat up due to bacterial action, and it can apparently get hot enough to initiate ordinary combustion. Damp coal will do a similar thing, and spraying water over it to cool it off is only of limited value since it helps keep the bacteria alive.

I think though that with the drying oils, the problem is that they can oxidise at room temperatures, this being how they dry anyway, and the process generates heat. With a varnished surface this is no problem, but in the confines of an oily rag problems can ensue. Epoxy resins also generate heat when they set, and if you use a fast hardener in a warm climate the stuff can catch fire in the pot while you are still busy trying to apply it to the job.

We tend to think of reactions like combustion taking place at or above a certain temperature, but if you talk to the quantum mechanics you will find it is more complex than that. To react, the two atoms or molecules have to overcome an energy barrier, and that corresponds to them having a certain energy, eg temperature. However, even if the temperature is lower, there is a probability that a reaction will take place. The hotter it is the more probable it is that any given pair will happen to react, but even at room temperature, some reaction can take place, depending on the material. So some materials will sit around at room temperature quietly oxidising. This is a bit like water sitting around evaporating even though it is not hot enough to boil.

John

I do not intend to query the chemistry. I just want to give some simple advice about how to deal with the potential outbreak of fire due to oxidation reactions.

There are 2 main recommendations in facilities where sprinkler systems are not available.

All rags, especially those contaminated with paint, varnish etc. should be placed in a bucket or other suitable receptacle and covered with water, thus excluding the air from contact with the contaminant.

Keep a foam extinguisher handy, messy when used but easier to clean-up than the residue from a fire. Do not use CO2 in a confined space.

DAHIK.

Jerry

Another OT post, on the subject of coal dust. Many years ago when I was designing electronics for active noise control we fitted a system to a coal mill at a cement works. The mill crushed coal to a powder and then blew it into a rotating furnace (150 feet long and 8 feet diameter) where it burnt along with crushed limestone and other ingredents. We were trying to quieten the fan (driven by a 2.5MW DC motor) that blew the coal dust into the furnace. We were told that there were two ways to build a coal mill to prevent damage caused by the coal dust exploding. One, run in an inert atmosphere or two, build it like a brick built outhouse so it would survive the inevitable explosion. The coal mill we worked on used the latter method. Unfortunately, when the coal dust did explode, the design calculations were proved to be wrong, as the explosion did rather a lot of damage. Ironically our loudspeakers with their paper cones survived the explosion.

Regards,

Andrew