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I don't know if it's flammable or not or just supposed to be up to DM discretion. I don't know if it always produces a substance with the same properties, or could it be different from one cast to another. I also don't know if magical Grease is the same as non-magical grease.

Jeremy Crawford has stated that if it were flammable it would say so. I can't figure out if this means that it's not flammable, or that it's not necessarily flammable.

All we know is that it's slick and that it's called grease. We aren't specifically told if it's opaque, translucent, or transparent, but we can't assume it's not any of them just because it's not explicitly stated. I feel like we are supposed to assume it's not necessarily flammable, and that it's up to GM discretion, but I have no idea if that's the case, or if I'm missing something important.

So beyond the name of the substance, and knowing that the substance is slick, what, if any, other properties does grease (specifically the magical grease produced by the spell Grease) have?

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Anything besides what is written in the spell description is up to the DM

It is slippery

The only description we get of the substance from the spell deception is that it is "slick grease" and the rest describe its mechanical effects (difficult terrain, causes creatures to fall prone).

No other properties are defined

Besides the above, nothing else about the substance is defined rules-wise.

In 5e, anything unclear or vague in the rules automatically falls to the DM to decide. This case is no exception. There are no general descriptions of this magical grease anywhere outside of this spell, thus the DM at your table must decide what additional properties, if any, the grease has.

The grease is not flammable unless your DM says it is

As you alluded to in the question, Jeremy Crawford has indeed spoken about the flammability of grease here:

If the grease spell created a flammable substance, the spell would say so. It doesn't say so.

What this means is that nothing in the spell indicates that it is flammable, therefore it is not defined as flammable according to the rules. Something being flammable is something that is a significant property to have for a material and, as such, would be noted if it were intended to have it. This fits with the 5e ethos that spells do only what they say they do.

However, this does not mean that a DM couldn't say that grease, at their table, is flammable.


Even if grease is ruled as flammable, it still requires DM rules to make it work

Great, so, your grease is flammable.

  • How big of a flame is needed to ignite it and for how long does it need to be exposed?
  • Now how many rounds does the fire last once ignited?
  • How much fire damage does it do and do enemies get a save?

These are all questions that a DM needs to address even if grease is allowed to be flammable. And this is a big reason why the flammability of the grease is a DM decision in the first place. There are no rules for creating grease fires in 5e and nothing in the spell is written as or intended to allow it. Unlike color, or other properties this has significant mechanical effects.

Thus, it is the DM's perview to decide if they want to deal with the mechanic and, if so, how it should work.

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  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$ – SevenSidedDie Apr 23 '18 at 23:34
  • \$\begingroup\$ So Grease is neither flammable OR non-flammable until a DM adjudicates it one way or the other - correct? And either way is correct because WOtC didn't tell us if it's supposed to be flammable or not? \$\endgroup\$ – M C Apr 24 '18 at 15:18
  • \$\begingroup\$ @MC That is the gist of it. For better or worse, 5e is built to have the DM make rulings such as these. \$\endgroup\$ – Rubiksmoose Apr 24 '18 at 15:20
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It has no other properties. It just does what the spell says

JC's tweet is meant to reinforce the idea that spells do only what they say that they do. All of the information you need to successfully use a spell in any generic case is wrapped up in that spells description. Any special interactions have to be specifically noted somewhere.

So, to answer your question more up front, he's saying that the grease is not flammable. If it was definitely flammable, the spell description would have mentioned that.

What about its physical properties? (Color, Odor, flammability, etc.)

Since the spell does only what it says, we have to assume the intent is that the grease has no particular color and no particular odor. It also isn't flammable. We know this because the spell doesn't specify any of these properties. So, aside from being slick, making people fall prone, and turning an area into difficult terrain, the grease is nondescript.

If you want, you have the right as a DM to rule that the grease does in fact have a particular property. You could rule that the grease smells a certain way, or has a certain color, or that it is flammable. However, doing so is the equivalent of making a house rule and another DM would be equally justified with giving the grease different properties altogether.

TL;DR

JC's tweet describes to us that if a spell that was intended to have a given effect, then that effect would be printed in the spell description. As far as the grease is concerned, there is mention anywhere that the grease from the grease spell is flammable. Ergo, the designers of the game did not intend for the grease to be flammable.

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Unfortunately, the spell's description only states that which you have already pointed out; that it's slick and called grease. Beyond that it would be the DM's judgement as to whether it would be flammable or not.

However, given that this is a level 1 spell, it's likely that it's not intended to be used for mass damage as it would do if set alight; it's more comparable to something like Entangle.

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Another argument for Grease being non-flammable can be found in the description of Web:

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.

This suggests that if Grease was flammable, the spell would include that, as well as the rules for setting it on fire.

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The designer of the spell has said that if the spell created flammable grease, it would say so. It seems reasonable to conclude that the opposite - a layer of flame-proof fire retardant capable of extinguishing fires in its area of effect - is similarly covered. The spell doesn't say that it's a fire extinguisher, so it doesn't extinguish fires.

How can the grease created do both of these things at the same time? Easy. It's magical. The grease is conjured out of the essence of whatever it materializes on top of, adopting some of its properties.

In this way, a greased stone floor will not burn, because stone doesn't burn so the grease conjured doesn't either. A wooden floor conjures grease just as flammable as itself; its flammability is also unaffected. If the floor was already on fire when the spell is cast, the grease fuels the fire exactly the same way that the wooden floor it's on top of would. (If it matters, the grease does not even alter the rate at which the floor burns under it. It's magical, it can break natural law to some degree.) The grease spell does not end quicker because the grease is burning, either. If the area covered contains both stone and wooden flooring, then the grease has different properties depending on which spot of floor it covers.

Or the DM can rule that the grease is a magical substance. Air passes through it as if it wasn't even there. Therefore it will neither burn (because it can't react with air at all) nor be usable as a fire extinguisher (because objects so coated will still have an air supply).

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  • \$\begingroup\$ So the Grease has no net interaction with fire at all? It doesn't increase or decrease flammability? This answer would cause a lot of confusion without the (most excellent) explanation that it can behave this way due to it being magical. This seems like an amazing answer and takes care of the main problem I was having with JCs response about this spell. \$\endgroup\$ – M C May 1 '18 at 16:18
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The properties are the real-world properties of whatever the DM decides the natural interpretation of the term "slick grease" to be. The Wall of Stone spell says it creates a "nonmagical wall of solid stone". It never says that this wall is visible, but... it'd be silly to take the "spells do only what they say" to mean that this wall can't be seen.

There are a number of perfectly reasonable interpretations to this, but they're all in the "DM fleshes out the world" bucket, not "careful parsing of technical language and rule nuances using precedent set in the wording of other spells" territory.

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