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The whole idea of casting Grease is not only to knock your opponents prone but — for certain fire trigger happy wizards — to ignite it on fire (fireball as one option) causing potentially extra damage for the individual(s) in the grease.

What is unclear though is how much extra damage (if any) will the now ignited grease provide?

While the answers might be similar to this question/answers, that question deals with non-magical grease (flask of oil).

The grease spell has the components pork and butter, so I'm wondering if it would be different, or if it would just take the standard 5 points per round.

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Grease cannot be ignited.

Spells only do what they say they do, nothing more.

The full text of Grease is:

Slick grease covers the ground in a 10-foot square centered on a point within range and turns it into difficult terrain for the duration.

When the grease appears, each creature standing in its area must succeed on a Dexterity saving throw or fall prone. A creature that enters the area or ends its turn there must also succeed on a Dexterity saving throw or fall prone.

There are no references to the grease being ignitable, so it is not. It's important to note that not all kinds of grease are flammable in all situations. Pork rinds and butter, the material components for the spell, are not flammable in all situations, for example.

Moreover, Jeremy Crawford has tweeted support for this interpretation.

While it's a popular houserule to light it on fire, the resulting damage is purely up to the DM.

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I think there's only one reasonable interpretation of "spells only do what they say they do" in this case. The idea that this means that the grease can't be ignited puts a weird privilege on the property "flammable". In the real world, pretty much most things can be lit aflame, and grease is usually one of them.

Saying "the grease spell doesn't say the substance is flammable, so it is fireproof" is like saying "wall of stone doesn't say that the wall is visible, only that it's solid, so you can't see it". That's.... a route to madness. We use a common sense, English language interpretation of what "stone" is, and the same should apply to "grease".

Now, it may be that "flammable" is special, and there is a non-written rule that this is a property that nothing has unless otherwise stated. But, that seems to be exactly what Crawford is cautioning about in this tweet"There aren't secret rules." There definitely isn't a written rule about this.

So, what's the reasonable interpretation? The grease may indeed be flammable, but it's not so specially flammable as to cause significant extra fire damage — if it were, it would say so. I'd rule that it either burns in a flash that does no damage, or minimal damage like the non-magical damage from a lit torch as a weapon — save or take 1 point of fire damage.

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    \$\begingroup\$ There are plenty of real-world greases that are not readily ignited nor burn particularly well. There is no reason to suspect that the substance created by grease is not one of those. \$\endgroup\$ – KRyan Aug 29 '18 at 12:57
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    \$\begingroup\$ "Plenty", maybe, but it's distinctive enough that googling "non-flammable grease", gets many products that are labeled that way. On the other hand, if you search for "flammable grease", you get, well, this question (and other variants of it). In fact, in many pages of search results, I can't find any substance described as "flammable grease". Instead, that is the default assumption — in the real world, it seems that flame-resistance is the unique property that needs to be called out if it exists, not flammability, and there's no reason to suspect that this is different in D&D. \$\endgroup\$ – mattdm Aug 29 '18 at 14:02
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We have a wonderful cantrip available since Elemental Evil: create bonfire. It's great for an example on how to handle PCs lighting fires to hurt people.

Disclaimer: Applying physics to D&D is often a bad idea, the rules work a certain way because of game balancing! Also, this is obviously houserule.

That being said, most long chain saturated hydrocarbons that would be commonly called "grease" will burn well once heated up enough. If you want to experiment with this, grab a candle and try and light the side. Nothing happens. Take my word for the other half of the experiment, that once you heat up wax in a frying pan or in a tin can on a fire, it burns vigorously. The same thing with deep fryers. That oil burns well once it's heated, but when cold, it won't light. Based on this, my ruling has been that it takes a number of rounds to light up, and needs a wick (lit torch) to burn from unless a good fire based spell has been used to light it. Once lit, it does the same damage as a Create Bonfire cantrip, and if a creature passes through or begins their turn in the middle of the blaze, they will take damage (can't do a dexterity saving throw against something you're deliberately moving through). One whole D8 per round of damage. If they fail the save. Once it's had time to warm up and burn.

This interpretation is great for providing light, visual barriers, dangerous and damaging terrain, etc. However it limits its usage as a damaging spell. It also means it's not useful for putting out fires, unlike what was suggested on Twitter in response to Crawford's tweet.

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As a DM, I consider the lit grease from the Grease spell to have a similar property to the spell 'firebolt' as the source of the grease is magical as well. In and of itself it doesn't do an outrageous amount of damage, but there is a certain something to be said about a very temporary (a few rounds max) and mildly intimidating trail of fire, or to use grease to cover a Medium object/creature (acknowledged, this is my house rule) and then proceed with ignition. It causes no small amount of satisfaction with new players to have it described in detail just exactly how they lit the bandit's hair on fire.

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protected by Oblivious Sage Jun 2 at 14:53

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