There has been a lot of discussion as to whether you can light the grease from a Grease spell on fire. The spell description is inconclusive. And a lot of ink has been spilled as to whether is is or isn’t flammable. The opposing arguments go generally like this:
Grease IS NOT Flammable: the spells doesn’t say it’s flammable, so it’s not flammable. If it was, it would say.
Grease IS Flammable: Well… normal grease is flammable, even lubricating greases can catch on fire.
So which is it? Well, grease certainly isn’t combustible, a word that gets bandied around with equal aplomb as flammable (and inflammable, which means the same thing, which is one of the many reasons why English is universally hated by non-English speakers). Combustible means that a substances can catch fire once the temperature reaches a certain point – most items are combustible, provided enough ambient temperature and other necessary conditions (eg oxygen). Flammable means that a substance will catch on fire IF A FLAME IS APPLIED TO IT. You can see the Venn diagram here between combustible and flammable. But for our purposes, I lean toward Grease being flammable. I am unconvinced that the spell description is not only inviolable but *comprehensive*. Therefore:
Grease spell IS flammable, doing damage equivalent to burning lamp oil (1d6 per round), albeit for every creature inside the confines of the spell. Catching on fire rules do, of course, apply. Here’s the catch: spell grease will not catch fire unless some other flame source – and it has to be FLAME, no lightning bolts or other chicanery – comes into contact with it.