Why Is My Candle Making Black Smoke or Soot?

A candle that makes black smoke or leaves soot on the jar is burning its fuel unevenly. The usual causes are a wick that is too long, a wick that is too big for the jar, or moving air bending the flame. The first fix is free: trim the wick to 1/4 inch (6 mm) and move the candle away from drafts. If it still smokes after that, the wick size is the problem, and the next batch needs a smaller wick. I am a candle maker, not a safety inspector, so test every change in your own jar with your own wax and wick, and never leave a burning candle out of sight.

What causes soot?

• A long or untrimmed wick. The National Candle Association (NCA) FAQ says wick length and flame disturbance are the primary factors in sooting, and that trimming the wick and keeping the candle out of drafts reduces it. Lone Star says an oversized or untrimmed wick "feeds more fuel than the flame can fully burn, so it smokes."
• A wick that is too big for the jar. Lone Star lists "wick too large, or not trimmed to the proper length" for a tall, flickering flame, and links soot to a flame too large for the wax. CandleScience says that if large plumes of soot appear often or heavy mushrooming shows up, you may need to wick down.
• Drafts. The NCA says moving air disturbs the flame, "resulting in those pesky black marks on the glass."
• Burning too long. The NCA says burning a candle for too long lets carbon build up on the wick, after which it "will start to smoke and release soot."
• A little soot is normal. The NCA calls the small amount a candle makes a normal result of incomplete combustion. CandleScience says soot "is simply the carbon buildup burning off the wick."

How do I fix it?

1. Blow out the flame and let the wick cool. Never trim a lit wick.
2. Trim the wick to 1/4 inch (6 mm). The NCA says to trim before each use to prevent high flames and soot. CandleScience says to use sharp scissors.
3. Move the candle away from fans, vents and open windows.
4. Relight and watch. Check the flame height and the jar. CandleScience says most flames are about an inch (2.5 cm) tall, and up to two inches (5 cm) is normal.
5. If it still smokes, change the wick. Lone Star says to size down one step and retest.

Worked example: a burn session and a wick test

First, how long should one session run? The NCA says to burn one hour for every inch of diameter, and not longer than four hours at a time. Say your jar is 3.5 inches (8.9 cm) wide: 3.5 inches x 1 hour = 3.5 hours, which is under the four hour limit. A 5 inch (12.7 cm) jar would work out to 5 hours by that rule, but the four hour limit wins, so you stop at 4. Over-long sessions are one way soot builds up, so write the number on your care card.

Now a test grid. These results are made up to show the method. Same wax, same oil, same 3.5 inch jar, three wick sizes, one session each:

The large wick burns above the two inch mark and soots even though it was trimmed correctly. That points at the wick size, not the trimming. Of the other two, check melt pool and jar heat before choosing. Our burn testing guide covers the full routine.

Common mistakes

• Skipping the trim because the candle is new. The NCA says to trim before each use.
• Testing in a drafty room and then blaming the wick.
• Burning a test candle for six hours "to be thorough."
• Blaming the fragrance oil or dye without testing a wick change first. Changing the load can change the wick size you need, as Lone Star notes, so retest after any change.
• Telling customers to "trim as needed" with no length.
• Going straight to a much smaller wick. Move one size and retest, so you do not trade soot for tunneling.

Questions

Is candle soot dangerous?

I cannot give medical advice, and I did not find a medical source in my research. What I can say is that smoke and heavy soot mean the candle is not burning well, so stop, trim, and fix the cause. If someone in your home has breathing problems, ask a health professional.

Does soot mean my candle is unsafe to sell?

A lot of soot in testing means the wick or the setup needs work before you sell. Fix it in the next batch. If you sell a candle that sooted in your own tests, you have no way to know how it will behave in a customer's home. Safety tests such as ASTM F2417 exist for fire safety, but I could not open the full standard, so I am not quoting its requirements.

Why is there soot only on one side of the jar?

That points to drafts or an off-center wick. Check that the wick stands in the middle of the jar and the candle is not near a vent.