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Is there a risk of spontaneous combustion of cedarwood oil used with rattan diffuser reeds as in the photograph at the bottom?

An NFPA safety sheet talks about the risk of spontaneous combustion of "oily rags wet with flammable or combustible liquid," but does not specify which liquids are and are not a risk.

According to Popular Mechanics, it is the "drying oils" specifically that are a problem, e.g. linseed oil. Wikipedia describes drying oils as "consist[ing] of glycerol triesters of fatty acides...characterized by high levels of polyunsaturated fatty acids, especially alpha-linolenic acid."

This NIH study on the human health risks of cedarwood oil lists the typical composition of three different types of cedarwood oil. These have varying concentrations of thujopsene, cedrol, cedrenes, widdrol, methyl thujate, thujic acid, and thujaplicins. "The ISO standard specifies an alcohol content (expressed as cedrol) of 35-48% with a minimum cedrol content of 20% for Texas cedarwood oil" and a maximum cedrol content of 14% for Virginia oil.

The chemical names in the Popular Mechanics and NIH articles are different, but I lack the expertise to know whether that is meaningful either way. Can anyone with more expertise weigh in?

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The risk of spontaneous combustion is greatest when a drying oil (i.e., one readily oxidized) is exposed to air but is thermally insulated, so that heat builds up. The greater the surface area exposed to oxygen (e.g., when spread thinly over a rag), and the more effective the insulation (e.g., layers of surrounding cloth), the more likely it would ignite.

Though I could not find out how easily it is oxidized, it is certainly flammable. Since the diffuser is not insulated, heat buildup is less likely (though where the sticks cross inside the jar could be an issue). Keeping the diffuser in a large nonflammable container and away from any source of ignition seems like a good idea.

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  • $\begingroup$ The sticks in hat jar are not a risk. These things that stink up flats are sold everywhere. You need both the thermal insulation and the large surface of the rag before anything can happen. $\endgroup$
    – Karl
    Commented Aug 3, 2023 at 20:37
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Apparently thujopsene, cedrol, and cedrene are all sesquiterpenes. Sesquiterpenes are a class of terpenes, which according to Wikipedia are unsaturated hydrocarbons. According to the Handbook of Industrial Hydrocarbon Processes, unsaturated hydrocarbons can spontaneously combust. So the answer would seem to be yes, theoretically it could, and therefore it should be treated as a spontaneous combustion risk. I suspect that determining whether or not it is a significant risk would require empirical testing (e.g., here or here).

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