Cotton rags and any kind of grease or oil may start to "selfheat".
Same proces as in coal or a haystack.
If you combine this with metal dust or turnings that are left after drilling in metal, you can get a tricky combination for a fire.
Cotton rags with lineseed can also be laid down flat for the lineseed to dry before disposal.
Risk is the sum of "chance" and "consequence".
The chance of these rags selfcombusting is probably not that high.
But the consequence if you have left them in the garbage can in the garage and return after a days work...
