Consider the following as representative of a torch applied to a pool of oil on a dungeon floor. Is it possible to get that pool ignited by applying a flame? No way! I emptied a whole book of matches on multiple attempts (photo shots) like this:
But if I take a little bit of string, dip the end in some oil, and light it like a wick, can I use it as a lamp? Quite easily!
I left this burning away happily for the better part of an hour. I can carry it around the house and even light up my always-dark bedroom with it reasonably well. (Actually, it's still going next to me as I write these words.) I've really got no idea how long it would take to burn away (can't see any noticeable difference in the wick or the pool). After an hour, for argument's sake, I tried to ignite the whole pool again -- Not gonna happen!