Fire and explosions

Fire is smoke with fuel. The particles give off fuel and heat; where the gas is hot enough the fuel burns, making flame, more heat, smoke and a push outward. Everything in the Smoke guide holds here too.

Start fromCopy Anchor Link

• Fire: flames and smoke off a burner.
• Set on Fire: the same fire over the selected mesh's surface.
• Explosion: a burst of burning gas swelling into a fireball and a rising cloud.

The Fire starter, taken apartCopy Anchor Link

• 600 particles a frame from a flat burner, going up slowly (0.6 m/s, give or take 0.25), living a quarter of a second.
• Smoke Source: Radius 8 cm, Fuel 1, a little Smoke, Push 0.4. The fuel is what burns.
• The particles aren't shown: only the gas.

For a fire on an object, the Set on Fire starter births them on the mesh's surface instead, more of them for its area.

How fuel burns: the Fire settingsCopy Anchor Link

Under Fire in the Smoke and Fire panel:

Under them, how the burning is shaped:

The flame glows where the fuel is burning, as brightly as it burns, not wherever there's hot fuel.

The looks Campfire, Candle and Fireball set them for you: see Smoke and fire looks.

The flame's lookCopy Anchor Link

The volume's VecFlow Smoke material glows by the flame and colours it by its heat. Everything here is in the Render Look and needs no new bake, except Crease Width.

Flame

• Flame Brightness: how bright the flames glow.
• Flame Curve: reshapes the flame before it glows. It starts low, so the faint halo stays dark, and levels off at the top, so strong fire keeps its shape instead of blowing out. Pull the middle down for fewer, brighter tongues, or up for a fuller fire.
• Flame Sharpness: the flame is raised to this power after the curve (1 by default). Above 1 the edges get crisper, but strong fire blows out sooner.
• Flame Noise and Flame Noise Scale: a 3D noise that breaks up the glow, so it isn't one smooth wash. Higher Scale gives smaller patches.

Flame Colour

The heat is read as kelvin: 1,000 K, plus 1,000 K for each unit of heat, so a Smoke Source's Heat of 1 is 2,000 K. The colour ramp runs from Coolest (K) on its left to Hottest (K) on its right. The default is ember red, deep orange, orange and gold, up to a pale core. Edit the ramp freely, or pick a set from Set Colours:

• Fire: the default.
• Candle: a softer, yellower flame.
• Blue Gas and Green: gas and chemical flames.
• Blackbody: the real glow of hot matter, taken from Coolest to Hottest, so set those first. Above about 3,500 K it goes pale, so it looks less like fire on screen than Fire does.

Most of the flame's glow should sit in the middle of the ramp. If the fire looks pale, raise Hottest; if it looks dark red, lower it.

• Colour Noise and Colour Noise Scale: a 3D noise that moves each point a little up or down the ramp, so the colour varies within a flame.
• Hue Shift: turns every colour round the colour wheel (0.5 is half way round). Saturation makes it richer or greyer.
• Hue Curve and Hue Curve Scale: a hue shift that changes along the ramp. The curve says how much at each heat, and the scale how far that goes (0 for none).

Smoke

• Smoke Colour, Density, Opacity Exponent and Fuel Shadow: the smoke's colour, how thick it is, how sharply its thin edges fade, and how much the fuel shades the fire.
• Anisotropy: above 0 the smoke scatters light forward, so it glows when lit from behind.
• Absorption Intensity and Smoke Absorption: what light the smoke lets through as it absorbs, by its heat along the ramp (black lets nothing through). 0 absorbs nothing.
• Smoke by Heat: how thick the smoke is at each heat, along the same range as the flame's colour. Pull the right end down for hot gas that's clear and smoke that only shows as it cools.

Masking

• Smoke Masking: how far thick smoke dims the glow inside it, so soot folds read dark round a bright core. 0 turns it off. Start Density and End Density set where the dimming starts and where it's full, and Smoke Masking Curve how it goes between them.
• Crease Masking: how far the flame's folds are darkened, so each billow has a crisp edge. 0 leaves them. Crease Width is how wide a fold counts (cells either side). It's worked out in the bake, so it needs a new bake.

If the fire is a flat, blown-out glow, lower Flame Brightness, and raise Smoke Masking and Crease Masking.

Gas Colour: when the gas has a colour of its own (below), Gas Colour on Flame colours the flames by it, as bright as the Flame Colour ramp makes them, and Gas Colour on Smoke colours the smoke by it in place of Smoke Colour. At 0 they leave the gas's colour out. A bake without one looks just as it would without them.

Coloured smoke and flamesCopy Anchor Link

The gas can carry a colour, for coloured smoke, chemical flames and fire of several colours. It comes from two places:

• Each Smoke Source's Colour: its own, or each particle's. Give burners in different events different colours and their flames mix where they meet.
• Gas Colour in the Smoke and Fire panel changes it as the gas goes, by Colour By: its speed, heat or smoke. The gas goes toward Low Colour at Start and below, High Colour at End and above, and between them in between, Rate of the way each second. For instance, by Heat from blue to orange: smoke that's orange while hot and turns blue as it cools.

Colour is saved in the bake (the OpenVDB files' "color" grid, which other programs can read too), so changing it needs a new bake; how much the material uses it doesn't. A pure black colour counts as none. NVIDIA Flow carries no colour.

Fire lights the scene in Cycles by its glow. For more light on what's round it, add a point light inside the fire, keyframed to flicker.

ExplosionsCopy Anchor Link

• All the particles are born at once and flung out in every direction (Spread 2.5 m/s, no Speed of their own).
• Heavy Drag (3) stops them within a fraction of a second, so they leave a ball of fuel and heat. More Spread makes the ball bigger.
• The fuel burns fast and Expansion swells it outward: a fireball, which rises on its heat and leaves a cloud of smoke.

To make it yours:

• When it goes off: the Birth's Start frame.
• Bigger: more Spread for the first ball, more Expansion for the swell, and a bigger Cell Size, since the gas fills more.
• Keeps swelling as it rises: a Burn Limit (try 2), so the dense core burns over many frames instead of all at once. A flame is as bright as what burns there, so a limit also dims it; for a bright core that lasts, raise Needs Air instead (try 0.75): the fuel inside, away from the air, burns slowly while its edges blaze.
• Rolls like a film fireball: Edge Swirl (try 0.6, under Swirl's Plume) with Swirl up, so its edges billow; Shrink (try 0.5), so it draws air back in as it cools; and Soot Below Heat (try 1), so a dark skin of soot wraps the glowing core.
• Thin vertical lines in the rising fire: Fine Mixing (try 0.17, under Swirl's Plume) evens out neighbouring gas rising at different speeds.
• Denser, lumpier core: the Smoke Source's Fill on Add, so fuel and heat build up where the particles gather, with some Variation (try 0.5) so it bursts unevenly. Keep its Fuel low there: with many particles it piles up fast.
• Debris: a second event of particles thrown out with Gravity, or rigid pieces from Destruction. Debris trailing smoke carries a Smoke Source of its own.

Sparks and embersCopy Anchor Link

Sparks are particles of their own, in a second event of the same flow as the fire, so they can read its gas:

1. A Birth on In the Gas: sparks are born where the fire burns brighter than Flame Above, and only where it rushes faster than Speed Above, each with the gas's speed there.
2. A Smoke Force carries them up the plume and round its curls. Low Strength (1–3) and a little Gravity for heavy embers that lag and fall back; high (8–10) for fine ash.
3. A Set Value from Gas ▸ Heat, with Keep Doing It on, into a value such as "glow": they shine while they're in the hot gas and dim as they leave it. Read "glow" in the sparks' material for their emission.
4. A Delete by age with some variation, so they die out one by one.

Give the sparks' particles object a small Radius, and motion blur in the render for streaks. All of this reads VecFlow's own gas: with NVIDIA Flow there's nothing to read.

What goes wrongCopy Anchor Link

• No flames, only smoke. The source gives no Fuel, or Ignition is higher than the gas's heat: raise the source's Heat, or lower Ignition.
• The flames are a solid blob. Needs Air too low: raise it toward 1, so the fuel burns in sheets round its edge. Or Burn Rate is too low, so fuel piles up unburnt: raise it, or raise Flame Life for licking tongues.
• The fuel floats off unburnt. Needs Air too high for fuel that's barely stirred: lower it, add Turbulence to mix it with air, or raise Heat Spread.
• The flames end in a glowing haze. Raise Flame Cutoff a little.
• The fireball doesn't swell. Expansion too low; or the fuel burns too slowly to swell before it rises.
• The fireball glows a flat orange all through. Its fuel has piled up far past what burns: Fill on Add with thousands of particles adds up fast. Put Fill back on Up To, or give less Fuel, and raise Needs Air (try 0.5) so it burns in sheets round its edge.
• The fireball pops and is gone. Its fuel all burns in a frame or two: raise Needs Air, or set a Burn Limit, so the core burns on as it rises.
• The fireball is dull, a dim orange all through. A Burn Limit caps how bright it burns: lower it or set it to 0, and slow the core with Needs Air instead.
• The fire looks blocky. Halve the Cell Size for the final.
• The flames look soft, their edges smeared. Sharpness on Fast: Sharp keeps the thin sheets of flame crisp, Extreme more so.
• The burning edge doesn't roll or break up. Keep the curls up where it burns: Swirl In Flame or In Heat (try 1), and Front above 1 for the leading edge. Then Disturbance (try 5) to shred the flame into licks, and Turbulence In Heat with Curl noise and Detail at 1 for small eddies along it.

RelatedCopy Anchor Link

• Smoke, Smoke Source
• The Fire starter, The Explosion starter