Reading about oil painting, there is a lot of confusion and mythology about spontaneous combustion and linseed oil. Is it a thing? How can I avoid it? (spoilers: yes, and easily)
Let's start with Linseed Oil.
It's an oil from the
flax plant (
Linum usitatissimum). The same plant we use to make health food and
clothing. It's the base that (most) oil paints are made from and is a frequent ingredient to modify that paint and for cleaning brushes during the painting process. In painting, the most common linseed oil is artist quality refined linseed oil.
There are several versions of linseed oil out there, so here's the main categories we encounter.
Food linseed/flaxseed oil: Crushed seeds of the flax plant that has the solid bits filtered out and had some shelf stabilizers added to it so it doesn't go rancid.
Refined linseed oil: What painters use. Like food oil but goes through a refining process and doesn't have the additives. We want the oil to go rancid and in oil painting we call this "curing".
Industrial linseed and boiled linseed oil: Is often used in woodwork and industrial settings. It has solvents and other additives to help it dry (rather than cure) and these can be harmful to humans and the painting. If there is a health warning on the linseed oil, don't use it.
There are also different cultivars of flax, but they are pretty close like how cherry tomatoes are similar to roma or beefsteak.
I want to mention these because at it's heart, artist oil isn't very different than kitchen oils. When kept in a dark, cool place away from oxygen (like in a bottle), these oils are stable and have about as much risk as your bottle of olive oil of combusting spontaneously or otherwise.
To be clear: linseed oil in the jar is safe.
But what about the fire?
It's when the linseed (and just about any vegetable oil) comes in contact with fabric and is left in a pile, like a garbage can, the oil oxidizes. This produces heat and gases which are flamable when exposed to
enough heat. No spark needed.
I use the word "rags" but this also includes other flammable substitutes like paper towel, paper,
cardboard, etc. Rags are more common because they are more easily left in a pile or bin.
Is the risk real? Like come on? People would be burning down their house all the time, wouldn't they?
The risk is real.
In my life, I've known two artists that have lost their studio/home due to rag fires. They just kept the oil rags in a bin and took it out to the trash when they remembered. One gets in an argument with a fire marshal "but I always keep my rags like that, it's never been an issue before." It didn't change the cause of the fire.
The insurance does not cover what they call "stupidity".
More recently, we lost a good chunk of a
local high school due to "improperly disposed of vegetable oil combined with fabric in a poorly ventilated area" - aka oily rags.
It happens, but the conditions need to be right, and we don't always know when that's going to happen. But there are steps we can take to make sure the conditions are never right.
Okay, how do I not burn down my house/studio?
Dry flat: With very few exceptions, paintings don't normally burst into flames. They are basically oil sodden cloth, so it helps to know what stops them spontaneously combusting. The heat can't build up because the oil is dispersed across the surface and not bunched together like a rag pile.
Hanging rags to dry so they are in a single layer imitates this process. Laying flat on a non-absorbent surface or non-flammable surface also helps.
Note, hanging the rags on the edge of a cardboard box is not the safest way to do this because cardboard is basically "tree rag". It can absorb the oil and oxidize, produce heat, and fire. Better to lay flat on
concrete or hang flat on a line.
Water: The one I was taught is to put the rags in a plastic bag, add enough water to just cover the rag, then squeeze out all the remaining air and tie it tight. This works great for one or two rags and if one has a spare plastic bag around.
Avoid Air (temporary): Where I take my oil painting classes, they are a bit lax about how to dispose of oily rags. It frightens me. And if a fire happens, I don't want to always worry if it was my rag that tipped the balance. So I put my rags and paper towels in a ziplock (which I
reuse), squeeze as much air out as I can, and take them home to dispose of properly. This is up to an hour to get home, so I feel confident it won't combust. But I don't want to forget it in my bag where there are lots of flammable paints and oils.
A Rag Bin: Everywhere is going to have different standards, so feel free to look up your local HAZMAT or WorkSafe standards. Basically it's a metal bin, often filled part way with water, where the rags get put during the day. At the end of the day, this is emptied and taken to a tin bin away from the house/studio/mechanic shop/etc.
There are also simpler versions of this like using an old paint can like it showed in the
video.
When the bin is full, dispose of as one would cooking oil rags. Where I live, this is taken to the recycling center and they deal with it.
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