Black Smoke From a Candle: Why It Happens and How to Stop It

By Julia
A soy candle in a glass jar with a tall flame giving off a thin wisp of smoke, beside a wick trimmer on a wooden table

Key Takeaways

  • Black smoke is soot that escaped the flame. A steady flame burns off almost all the soot it makes. A disturbed one lets some escape (Pagels et al., J Aerosol Sci, 2009).
  • Three habits deal with the usual causes. The Danish Environmental Protection Agency advises trimming the wick if it gets too long and keeping candles out of air flows that make the flame flicker (Danish EPA, 2018). Not burning for too long in one go is the third.
  • A draught matters more than you'd think. In a 2021 chamber study, all five candle types tested gave off mostly soot once moving air disturbed the flame (Andersen et al., Indoor Air, 2021).
  • White smoke after you put a candle out is something else. It's mainly unburned and part-burned wax, not soot.
  • If a trimmed candle still smokes in still air, tell the maker. It may be the candle, not you.

Black smoke from a candle means the flame is making soot faster than it can burn it off. The usual causes are a wick that's too long, a draught that makes the flame flicker, or a burn that has gone on too long. Put the candle out and let it cool. Then trim the wick to 3–5 mm, move it out of moving air and relight it.

The rest of this guide explains why that works, how to tell the causes apart, and what to do about the black marks soot leaves behind. We make soy candles, so we have a commercial interest in candle care. The advice here applies to any candle, whoever made it.

What black smoke actually is

A candle flame is a small factory for soot. Inside the flame there are regions with plenty of melted wax vapour and not much oxygen, and that's where soot forms.

Researchers at Lund University explain it this way: "In fuel rich regions of the flame, large quantities of soot particles form." The soot doesn't usually escape. "The yellow incandescent light making up the majority of the flame is the result of soot oxidation in the flame," they write (Pagels et al., 2009).

So the glow you enjoy is soot burning away. In a steady burn, "in principle all soot formed in the flame can be oxidised". The trouble starts when the flame is disturbed. Air moving past it, "for example due to an open window, may result in a flickering flame and some soot being able to escape without being oxidised". The researchers call this "sooting burn".

When they made their candles flicker on purpose, elemental carbon (the black part of soot) went up by more than an order of magnitude. That's more than ten times as much. One caution: they tested two kinds of tapered candle in a test chamber, not container candles. The same mechanism applies to a jar candle, but the exact numbers won't carry over to a jar candle on your shelf.

The four usual causes, and how to tell them apart

What you see Likely cause What to do
Tall flame, black tip, smoke rising steadily Wick too long Put it out, let it cool, trim to 3–5 mm
Flame dancing or leaning, smoke in puffs Draught Move it away from fans, vents, windows and doorways
Smoke starts after a few hours Burned too long in one go Put it out, trim, and keep burns to the care card's limit
Smokes even when trimmed and in still air The candle itself Contact the maker

1. The wick is too long

It's also the easiest cause to fix. A long wick gives a bigger flame that's harder to keep steady.

The Danish Environmental Protection Agency tested candles for a 2018 report. Its advice to consumers is to "trim the wick if it gets too long" (Danish EPA, 2018). The Lund researchers agree. In their words, "most soot emissions from candles can be avoided by using high-quality candles, trimming long wicks" and by keeping the flame from flickering (Pagels et al., 2009).

How short? The care card on our candles says to trim to 3–5 mm before every burn. That suits both our wood wicks and our cotton wicks. Always trim a cold candle. Trimming a lit wick drops hot ends into the wax.

You may also see a small black blob on the tip of the wick, often called mushrooming. It's a build-up on the end of the wick. Snip it off when you trim, and tip the bits out of the jar rather than leaving them in the wax.

2. A draught is making the flame flicker

A candle can have a perfectly trimmed wick and still smoke if the air around it is moving. That's the "sooting burn" described above.

A 2021 study in Indoor Air tested this directly. The team burned five types of pillar candle in a 21.6 cubic metre chamber, using changing air speeds to copy a candle sitting in a draught. Once the flame was disturbed, the emissions from all five candle types were mostly soot (Andersen et al., 2021).

The Danish EPA tested its candles with draught protection, because "this is what candle manufacturers strongly recommend consumers to do". Under those conditions, black carbon made up only 0.46% to 1.5% of the particle mass collected on filters (Danish EPA, 2018). Citing earlier studies, the report notes that black carbon "increases dramatically" under poor burn conditions.

In an Australian home, the usual culprits are:

  • Ceiling fans
  • Split-system air conditioners and heating vents
  • Open windows and doors, including the draught from a door opening and closing
  • Busy walkways, where people moving past push the air around

If the flame is dancing or leaning to one side, move the candle. A draught also causes uneven melting, which we cover in our guide to why candles tunnel.

3. The candle has been burning too long

As a candle burns, the wax level drops and more of the wick is exposed. A well-made wick is designed to burn its own tip away as this happens, so it stays at roughly the right length. A candle-wick study describes how this works: the exposed tip dries out, gets much hotter, and "the dry portion of the wick will be consumed" (Raju, 2007).

If a flame creeps taller late in a long burn and starts to smoke, treat it as a sign to stop. That's why our care cards set a limit.

The limit is no more than 4 hours at a time for our glass-jar candles, and 2–3 hours for our tins, which also helps keep the tin from getting too hot. If smoke starts late in a burn, put the candle out, let it cool, trim, and start again next time.

4. The candle itself

Sometimes it isn't you. The Danish EPA report puts it plainly: "High-quality candles are designed to burn with a calm, non-flickering flame". With "low-quality candles or if the combustion is disturbed", there is a risk of incomplete combustion and soot (Danish EPA, 2018).

The cause might be how the wick is matched to the jar and the wax. That isn't something you can fix at home. If a candle smokes after you've trimmed it to 3–5 mm and moved it somewhere still, contact whoever made it. A good maker will want to know.

How to stop a smoking candle right now

  1. Put it out. Use a snuffer if you have one, or blow gently.
  2. Let it cool completely. Hot wax and a hot jar both burn. Wait until the wax has fully set.
  3. Trim the wick to 3–5 mm. Use a wick trimmer, scissors or nail clippers. Remove any black blob on the tip.
  4. Tip out the trimmings. Bits of wick or matches left in the wax can catch and smoke.
  5. Wipe the rim. If there's soot inside the top of a glass jar, a dry paper towel takes off most of it once it's cold.
  6. Move it somewhere still. Away from fans, vents, open windows and doorways.
  7. Relight and watch for a minute. You want a steady, upright, teardrop-shaped flame.

If you'd like a trimmer or a snuffer for this, we stock both in our candle accessories collection.

White smoke when you blow it out is different

Most people notice a wisp of smoke right after they blow out a candle. That smoke is usually pale grey or white, and it isn't soot.

The Lund team measured it separately. They found that particles from this smouldering stage "are dominated by organic matter". An earlier study the paper cites found this is a mix of wax that evaporated and condensed again, and wax that only partly burned (Pagels et al., 2009). Their paper also reports figures from an earlier study of two candles. Putting a candle out once gave off 0.6–1.8 mg of particles. A candle in sooting burn gave off as much as 26 mg in an hour.

So a puff of white smoke at the end is normal. Black smoke while the candle is burning is the one to fix. If you'd rather skip the puff of air altogether, a snuffer does the job.

Cleaning up soot

Inside the jar. Soot often collects as a dark band around the inside of a glass jar, just above the wax. Once the candle is cold, wipe it with a dry paper towel. For a jar you've finished, see our guide to cleaning and reusing candle jars.

On walls and ceilings. A grey shadow on the wall or ceiling above where you burn candles is a sign the flame has been sooting for a while. Soot smears easily, so test any cleaning method on a hidden patch first. Then fix the cause, or the marks will come back.

Is candle soot bad for you?

Soot is worth keeping to a minimum. The Danish EPA notes that soot "can also contain problematic substances like polycyclic aromatic hydrocarbons", so "the emission should be as low as possible" (Danish EPA, 2018).

The same report found that black carbon made up only a small share of what a steady, draught-protected candle gave off. That's the practical point. The habits in this guide keep soot low, and they're the part you control. For the wider question of candles and indoor air, see our guide to whether soy candles are safe.

Frequently asked questions

Why is my candle making black smoke?

Usually because the wick is too long, the flame is in a draught, or it has been burning for too long. Each one lets soot escape the flame before it burns off. Put it out, trim the wick to 3–5 mm once it's cold, and move it away from moving air.

What does a fast-flickering candle flame mean?

Almost always moving air. A fan, vent, open window or people walking past will make a flame flicker. A flickering flame is also when soot escapes, so move the candle somewhere still.

Do some candles not make black smoke?

Any candle can smoke if the wick is long or the flame is disturbed. In the 2021 Indoor Air study, all five candle types made mostly soot once the air around them moved. How you burn a candle matters a lot, whichever one you buy, and a well-made candle should burn steadily when it's trimmed and in still air.

Do wood wicks smoke more than cotton wicks?

We haven't found good evidence either way. Both need trimming and both need still air. Our wood wick vs cotton wick guide goes through what the research does and doesn't say.

How long should you burn a candle at a time?

Follow the care card for that candle. Ours say up to 4 hours for our glass-jar candles and 2–3 hours for our tins. If smoke starts late in a long burn, put it out, trim and relight next time.

The short version

Black smoke means soot is escaping the flame. Trim the wick to 3–5 mm before every burn, keep the candle away from fans, vents and open windows, and don't burn it past its care card's limit. If it still smokes after all that, the candle may be the problem, and the maker should hear about it.

If you're lighting a new candle, our guide to tunnelling covers the first burn, which sets up how it burns from then on.

Julia hand-pours every Yarra Light candle in Melbourne. Ours are 100% soy wax with phthalate-free fragrance oils made in Australia. The Signature range uses wood wicks made in the USA, and the Weekday range uses lead-free cotton wicks. We sell soy candles, so we have a commercial interest in this topic. The steps here work for any candle, whoever made it.

Sources

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