Can control flames be used on a magically lit torch to burn a barn down?

Question

A lit torch is snuffed by Prestidigitation/Druidcraft. Because we've been overwhelmed by the enemy (in a barn), we decided to try setting the barn on fire with Control Flames.

Control Flames specifically says to choose non-magical fire.

You choose nonmagical flame that you can see within range and that fits within a 5-foot cube.

Is the torch, which was previously naturally lit, snuffed, then lit again using Prestidigitation/Druidcraft an eligible flame source for Control Flames?

Is it any different if the torch was not previously (naturally) lit?

Can we light the torch with Prestidigitation and then use Control Flames to set the barn on fire?

Answer 1

Control Flames will work on normal fires started by magic.

None of these spells produces a magical fire. They use magic to start a fire.

Druidcraft

You instantly light or snuff out a candle, a torch, or a small campfire.

Prestidigitation

You instantaneously light or snuff out a candle, a torch, or a small campfire.

Firebolt

A flammable object hit by this spell ignites if it isn't being worn or carried.

You'll notice that all of the things that might be lit by these cantrips have normal fuel (oil, wood, etc.). They are not fueled by magic (unlike Create Bonfire). Magic was only used to create the initial spark. The cantrips above take place instantaneously and are not sustained by the magic or caster.

Other Magic which starts normal fires through instantaneous ignition:

• Flaming Sphere (2nd level) - for the things it ignites
• Fireball (3rd level)
• Lightning Bolt (3rd level)
• Delayed Blast Fireball (7th level) - despite the concentration, the explosion is instantaneous
• Fire Storm (7th level)

Magic which sustains a magical fire for an amount of time:

• Create Bonfire (cantrip)
• Continual Flame (2nd level)
• Flaming Sphere (2nd level) - the sphere itself
• Heat Metal (2nd level) - within the metal itself
• Flame Blade (2nd level)
• Fire Shield (4th level)
• Wall of Fire (4th level)
• Incendiary Cloud (8th level)

There are also other fire spells which do not start fires, such as Chromatic Orb (fire) and Flame Strike. (All of these spells are in the PHB except Control Flames and Create Bonfire from XGtE.)

You can expand the fire into an adjacent 5' cube (containing fuel) each time you cast this cantrip.

If you spend 1 action to expand the fire into each ground level cube of the barn, the fire will do the rest. This might even be overkill, but you might as well keep expanding it since you're already standing there watching it (from within 60').
(Control Flames lets you sustain 3 non-instantaneous effects on the fire at once. You don't need any of those.)

Also note that you could just use the Firebolt cantrip (120') itself to start the barn on fire without getting so close.

Control Flames (60') could also be used, but would not be necessary. This would be safer than having to walk up to the barn with a torch in order to cause it to spread.

Answer 2

It is not magical

Both of those spells have a duration of instantaneous in the case of lighting fires. If the fire stayed magical, that would be an ongoing effect and it should have been included in the duration. As it was not, the magic fades instantly.

Contrast spells like flaming sphere that have a non-instantaneous duration.

Setting the barn on fire

Assuming there is enough flammable material (like hay) in the barn, you could even accomplish this without any magic. Though if time is of the essence, this may be too slow (ask your DM). Using control flames should drastically reduce the time needed to get the whole thing burning. This effect is also up to your DM, but I would rule it reduces the time needed to half.

Answer 3

Of course this is a DM's call, but realistically wood has to reach a certain temperature before it will burn (350 F). This is why campfires have to be "built" progressing from small highly flamable material and progressing to larger pieces of wood.

The way I see the mechanics of Control Flames is that it does not make things burn. Rather you control the flames themselves which allows you to control how a fire progresses, i.e: contain, expand, and direction.

Thus, to set the barn on fire there would need to be an initial source (such as hay bales) to apply heat to the structure. Then Control Flames can be used to direct the fire to key structural elements (ie concentrate the heat/flames) so that it starts burning faster.

Note: if the barn floor is covered with hay, you could use Control Flames to make a wall of fire between you and the enemy without setting the barn on fire. How long it lasts depends on how much hay you have.