Apparently it's very uncommon and conditions have to be just right; linseed oil on loosely bundled rags in ventilated space on warm day etc. I put mine in a jam jar with a lid on.
Not that uncommon, unfortunately its a bit too common. Famous fire at Meridian plaza and lots of HSE cases in UK.
One Meridian Plaza - Wikipedia.
Its an inherent hazard with finishing oils.
Most finishing oils (Linseed, Tung, Teak, genuine turps) contain unsaturated chemical bonds, these react in air to polymerise, this is the drying process that gives a nice finish, its a type of oxidation and gives off heat.
On furniture the heat is converted away safely but in a confined space of a screwed-up rag the temperature rises - autoignition will set fire to the rag. If temperature rises above the flash point 93c for linseed oil, but only 40c if thinned with white spirit then spontaneous combustion occurs.
Also metal shavings oxidise giving off heat, so rags thrown in the bin can easily self ignite.
Precautions:
Storing the rag in a sealed jam jar, as
@Jacob does, keeps the oxygen down, so lessens the hazard.
A strong metal tin or fire proof tin is be recommended these days as glass can break, but same principle.
Other common sense approaches, keep the volume of rags to a minimum, don't slosh the stuff about, and don't' leave them soaking wet with oil. All minimise the risk of fire. I'm sure most woodworkers do this naturally as its enviromentally sensible and minimises waste/cost! But I'd always advice new comers to adopt this practice, as they may not be aware of the fire risk.
I hang them on a washing line outside to dry before putting back in a tin, or disposing of them. Best to put outside in dust bin once dry or burn them as
@Phil Pascoe does.
