There are a lot of resources online to learn about the hazards of spontaneous combustion. Better safe than sorry. Proper storage of rags is a good starting point.
I have had the opportunity to investigate fires professionally. In my professional experience I have witnessed the results of 3 spontaneous combustion fires. I have noticed a commonality between actual ignitions I have investigated and other case studies. In each case I have seen, the volume of rags is significant, and specifically, large enough to generate and TRAP heat. Two of the cases involved large uncovered containers full of rags soaked in a fence stain product at fencing companies with lackadaisical employees.
In the first case, employee statements and and physical evidence at the scene pointed to spontaneous combustion as the cause (no accidental ignition sources in area, area of origin was pile of rags in a trash can). In the second the employee statement was similar, but the can was left in the bed of a truck full of tools and fencing materials. The truck was parked in a secure yard, with video surveillance. A review of video showed approximately a 3 hour delay between parking and smoke visible from can. Another hour or so and it ignited, burning the truck completely in the process.
The last case surprised me thoroughly but I believe it was spontaneous combustion. A local BBQ restaurant owner's car burned in their driveway in the mid afternoon. The area of origin was outside the car, in a 5 gallon plastic bucket full of white cotton towels. The towels ignited and started plastic components of the vehicle, spreading across most of the vehicle. The towels were heavily charred. Statements were that the rags were used for cleaning in the restaurant. More rags were at the home but unburned. They were all soaked in animal fat, but devoid of moisture/water. The owner said many rags were damp with cleaning solution and many rags were completely soaked in animal fat. Before collecting rags into a bag or bucket they are dried on a rack to prevent mildew before laundering. On the week in question the rags were dried outside in 100 degree heat with very low humidity, leaving them very dry besides fats. They were then collected into buckets.
One bucket was left outside the car to be taken to the laundry (they still smell and the owner had a reasonably nice car). The temperature outside was 105. They were in the direct sun for approximately 5 hours before ignition.
In any case, don't leave piles of linseed rags laying about. They can self heat and spontaneously combust. I think it is unlikely if it's one rag, but I wouldn't bet on it.
Keith.
