You see it on every washer and dryer: “Do not wash or dry clothing in this machine that has ever had any oil on it, it can be a fire hazard” (paraphrased)
I was under the impression that a dryer never gets hotter than, say, 170 F inside, even on the hottest settings. Isn’t that far below ignition point for any flammable material?
The risk is not that the washer heats up the clothes to the ignition temperature of the oil. The risk is that the oil that’s left in the clothes continues to oxidise naturally: Oil, and also fat, consists of carbohydrates with more chemical energy than CO2, so there’s the possibility of an exothermic reaction with oxygen from air into CO2. If that heat is trapped in the clothes rather than conducted away, then the risk is that this brings the clothes up to their ignition point. It’s a rather remote risk, but things like that have been known to occur. The risk is greater with oil-soaked clothes than with, say, a bucket of oil because the oil in the clothes has a much larger total surface area that exposes it to oxygen, and the clothes tend to trap the heat.
For some oils the risk of ignition is not remote at all, for instance linseed oil and cottonseed oil can easily ignite spontaneously when they soak rags. No dryer needed.
I see, thanks. So this can only happen if the oil has been on the clothing for a long time, and then you wash it?
So, if, say, someone spilled a lot of olive oil (or oil-based paint, or gasoline, or whatever) over their shirt, then washed it right away and put it in the dryer, the oil hasn’t had time to oxidize naturally into anything else?
In addition to some oils being able to self ignite, even if the inside of your dryer drum isn’t above the ignition temperature of the oils, there’s still a heater in there. You might not want something flammable in a dryer evaporating that close to a gas flame or electric heater. I’m not sure if olive oil is a concern, but I’d be worried about oil paints or gasoline.
Note that the whole “oil-soaked rags igniting spontaneously” warning is somewhat overblown.
There are certain types of oils (tung, linseed) that are “drying” oils. They release heat when they polymerize. Most common oils (like motor oil) don’t have this property, and don’t present a spontaneous combustion threat.
Flash point of many oils is 130 degrees.
No the risk does not increase with a shorter time between oil stain and drying. In fact I suspect it would be inversely related ie the older the stain the more it would already have oxidized and be at less risk of oxidizing.
The risk (such as it is) of fire in a dryer through oxidizing oil stains is because if the fabric is already susceptible to exothermic oxidation then the heat of that, combined with the the heat of the dryer, and in a closed location where heat can’t easily escape, might be enough for ignition.
Having said all that, I suspect it is @Joey_P who hits the real nail on the head. The real risk may not be one of self-heating but of hydrocarbon stains giving off flammable fumes that may ignite if they circulate in the machinery of the dryer. The hot air blowing through the drum may be only 170F, but the actual heater element has to be hotter than that. And a single spark (such as from an operating relay) can cause ignition of flammable gas.
Motor oil can spontaneously combust if exposed to oxygen under pressure. TBF, not conditions found in your average dryer but more than one welder has had an exciting day not knowing that fact.
You can also get some impressive pyrotechnics if you pour it on granular pool chlorine (break fluid works better, though).
The two points above are going to be critical ones.
Reaching flash point and available ignition source.
The flash point is important. Some hydrocarbons have remarkably low flash points. Mineral spirits is down at 100F, turpentine as bad. Even special high flash point formations only get as high as 140F. Diesel is similarly low, if a little higher.
Motor oils are much higher.
A tumble dryer might get as hot as 150F. So very much a risk for someone cleaning up after painting. Not so much after changing the car’s oil, but if your fabric has diesel in it, bad.
I’ve known two people to burn down their houses by leaving a rag they used to wipe linseed oil with on a counter overnight.
I have had dirty linen bags start smoldering in dirty linen hutch out behind my kitchen a couple of times and several times in the bags themselves in the kitchen…. Dirty food and grease soaked rags do spontaneously catch fire… been there done that got the extinguisher recharge receipts or prove it.
What, no Tshirt?
The oxidation that leads to the build-up of heat is a slow reaction, but it is slow primarily compared to combustion, i.e., fire (which is also an oxidation, a very rapid one). It’s not something that goes on on the timescale of months. So I don’t think a long time since the spill reduces that risk.
Just the dirty sweat stained ones from cleaning up the powder gunk from the extinguishers….
I noticed the dampness of the rags and the amount of food gunk embedded in them affects wether they were able to get hot enough to combust.