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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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    \$\begingroup\$ The tricky thing with this ruling is it only explains why the grease doesn't catch fire on its own, or through a special action as defined by the spell. But by the same token, other spells such as Create Bonfire can ignite flammable materials, which a lot of things (including grease) are; the question up to the DM is how quickly it ignites and what happens when it does, e.g- Create Bonfire after d6 rounds could do it, spreading to fill the area for the Bonfire's remaining duration. The pieces are there in the rules, just not explicitly defined IMO. \$\endgroup\$
    – Haravikk
    Jul 12, 2020 at 10:43
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    \$\begingroup\$ Butter: it's flammable. Pork rinds: also, although apparently Funyuns are better. "In all situations", sure, there are exceptions — but ... in normal ones, those are things that burn easily. \$\endgroup\$
    – mattdm
    Mar 22, 2021 at 16:04
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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, 2018 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, 2018 at 14:02
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    \$\begingroup\$ Actually real world grease is not generally flammabe, even if it can be burned. You don't find "flammable grease" because that isn't a thing (try googling "fire paste" instead). I would treat grease covered wooden floor the same as non-greased wooden floor for purposes of flammability/damage from fire. If you disagree, pour eg. the grease from frying bacon on a rock/concrete and try to get it to ignite... \$\endgroup\$ Mar 22, 2021 at 7:54
  • \$\begingroup\$ @WakiNadiVellir I did a quick search for flaming bacon grease and I remain quite confident in this answer — bacon grease is absolutely flammable by the real-world definition of the term (Merriam-Webster: "capable of being easily ignited and of burning quickly"). \$\endgroup\$
    – mattdm
    Mar 22, 2021 at 15:31
  • \$\begingroup\$ And as for your "pour on concrete" example — remember, this is enough grease to make a person standing there suddenly fall down. And I'm hitting it with a mote of magical fire that can do enough damage to kill the average person. None of this adds up to the "grease cannot be ignited" statement of the accepted answer. \$\endgroup\$
    – mattdm
    Mar 22, 2021 at 15:31
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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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The Web spell clearly lays out what happens when the material conjured by the spell is ignited: "The webs are flammable. Any 5-foot cube of webs exposed to fire burns away in 1 round, dealing 2d4 fire damage to any creature that starts its turn in the fire."

Keep in mind, Web is a second level spell and therefore has this added effect, which is specifically mentioned. Compare that to the first level spell Grease. No explicit mention of flammability or fire damage to be dealt when ignited. I might reward a clever player an extra 1 point of damage for each creature in the target area for thinking to do this, but even that ruling would be generous.

Take a look at the item Alchemist's Fire. That specifically does fire damage from an ignitable source: 1d4 fire damage per round until a DC 10 Dex save is met, which costs an Action to roll. That is potentially devastating over ten rounds dealing an average of 25 points of damage if not dealt with (which is roughly the equivalent of Fireball), and if extinguished will cost the target its Action for that round. With one failure, it then costs at least TWO Actions to successfully extinguish (or potentially more). It's really the Action(s) that is costs that ends up being more expensive than the damage.

What I'm saying is, if you were to give Grease a similar effect as either of these other two examples, you'd be letting it punch significantly above its first level ranking. Careful not to grant it too much power, because if you do you are also likely allowing other things to be more powerful than they're intended to be as well. Beginner DMs beware: house ruling damage is a slippery slope, so only go this route if you keep good notes and have solid reasons for when you grant extra damage.

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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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